RIVER RESPONSE TO LAND CLEARING AND LANDSCAPE SALINISATION IN SOUTHWESTERN AUSTRALIA
John Nikolaus Callow
B.SC (HONS) GEOGRAPHY
THIS THESIS IS PRESENTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY AT THE UNIVERSITY OF WESTERN AUSTRALIA SCHOOL OF EARTH AND GEOGRAPHICAL SCIENCES & SCHOOL FOR ENVIRONMENTAL SYSTEMS ENGINEERING PERTH, AUSTRALIA.
APRIL 2007
Declaration
Candidate Declaration
The research presented in this thesis was conducted by the candidate and remains original unless otherwise acknowledged. The candidate was lead author and presenter for all published articles and conference presentations associated with this research. K.R.J. Smettem provided advice on analysis methods and editorial review of the paper by Callow and Smettem (2007), and on the structure and scope of conference presentation and editorial review of conference abstracts (Callow and Smettem, 2004; Callow and Smettem, 2006). In the article by Callow, Van Niel and Boggs (2007), K. Van Niel provided advice on analysis methods, literature and editorial review, and G.S. Boggs provided advice on analysis methods and editorial review of the paper.
Related work includes: Callow, J.N. and Smettem, K.R.J., 2007. Channel response to a new hydrological regime in southwestern Australia. Geomorphology, 84(3-4): 254-276. Callow, J.N., Van Niel, K. and Boggs, G.S., 2007. How does modifying a DEM to reflect known hydrology affect subsequent terrain analysis? Journal of Hydrology, 332(1-2): 30-39. Callow, J.N., and Smettem, K.R.J., 2006. Variable channel responses following land clearing of a dryland catchment, Dalyup River, southwestern Australia, European Geophysical Union General Assembly, Vienna, Austria. Callow, J.N., and Smettem, K.R.J., 2004. Channel response to a new hydrological equilibrium in southern Western Australia, Joint International Geomorphology Conference, Glasgow, Scotland.
Secondary authors give their permission for published material to appear in this thesis.
© Copyright J.N. Callow 2007. This thesis may not be copied in whole or in part by any process without the prior written permission of the author.
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Abstract
Abstract
Land clearing is known to increase runoff, and in many dryland landscapes is also associated with rising saline watertables, causing increased stream salinity and degrading riparian vegetation. The limited understanding of how river morphology responds to these changes and the potential for vegetation-based strategies to offer river management options under these conditions, has prompted this research.
In southwestern Australia the severity of salinity and recent nature of land clearing provides an appropriate setting to investigate river response. A data-based, multidisciplinary methodology was applied to determine how land clearing and landscape salinisation has altered landscape sensitivity through changes in erosive potential, system connectivity and material threshold mechanisms, and how these affect patterns of river response. The study investigated the responses of morphologically similar reaches across fifty two study sites in the Kent River and Dalyup River catchments, in the south coastal rivers region of Western Australia.
Land clearing was found to have significantly altered the hydrologic regime and erosive potential in both frequency and magnitude, with flow becoming more perennial, and increased annual discharge, flood peaks and bankfull flow frequency. While sediment transport rates have also increased since land clearing, they remain low on a global scale. Human response to a reduced rainfall regime and related water security pressures has caused large hillslope areas to be decoupled from the main channels by bank and farm dam construction, and have reduced downstream transmission of change. Tools for representing known hydrologic pathways in a landscape using Digital Elevation Models are found to have limitations, particularly when replication of hydrology is required prior to subsequent terrain analyses.
Increased stream salinity has resulted in severe vegetation degradation, lowering erosive thresholds. Landscape gradient is, however, found to have a stronger control on changes in erosive potential than vegetation degradation. A reach-based geomorphic classification based on the RiverStyles® Framework was successfully applied across the eleven river styles found in the two study catchments. The severity of response was determined by channel gradient, erosivity of material surrounding the channel (bedrock, clay or sand), upstream sediment supply, and the degree of vegetation degradation (itself depending on factors such as salinity, valley width and the arrangement of the geomorphic units that affect micro topography across rivers). While many upper catchment areas have experienced severe vegetation degradation since land clearing occurred five decades ago, there has been little morphological response due to the low channel gradients and the resistivity of the clay valley fill material that underlies the
~ v ~ Abstract channels. By contrast, steeper-sloped mid-catchment areas with minimal vegetation degradation caused by salinity are associated with higher erosive potential. A more erosive response is observed in these reaches where floodplains have been cleared for agricultural purposes.
A conceptual model of vegetation growth across the salinity gradient observed in the study catchments was developed, and applied to selected river styles to assess the potential that vegetation-based strategies offer for river management. This work identifies the unsuitability of river restoration strategies, but the potential for river restoration or remediation in a saline landscape. Hydraulic modelling demonstrated that river rehabilitation strategies such as improving the vegetation condition of the riparian buffer using native or commercial species on areas elevated above saline flow can stabilise reaches. For river styles in wide and flat valleys, there is limited potential for vegetation-based river rehabilitation under the current salinity gradient. Field observation and modelling suggest that river remediation may offer geomorphic management options in salt-affected reaches through channelisation to lower watertables, and further research on this is warranted. This work found a consistent response for river styles across the two study catchments. Based on the understanding of river response and the potential for vegetation-based river management for each style, this research offers a regional-scale tool for river management in a saline landscape.
~ vi ~ Acknowledgements Acknowledgements
I have been privileged to have had the opportunity to undertake this research. Completion of this project would not have been possible without the financial, field and personal assistance of a great number of people. To the many who have assisted me, but space prevents me from specifically thanking, I remain indebted to you for your contribution to this research.
Field work was supported by postgraduate student funding from the School of Earth and Geographic Sciences and School of Environmental Systems Engineering (formerly the Centre for Water Research). Small grants and scholarships from the Department of Environment, Ernest Jackson Memorial Grant from the River Basin Management Society, and Mary Janet Lindsay of Yanchep Memorial Grant from the Faculty of Natural and Agricultural Sciences (UWA) contributed towards more extensive field work. The Mary Janet Lindsay of Yanchep Memorial Grant and postgraduate travel grant from the Australian Institute of Geographers assisted in travel to present aspects of this research at two international conferences.
Particular thanks go to Lucy Sheehy and Brad Palmer who assisted with collection of field data. Steve Janicke and Nigel Brodie are thanked for their help with the repeat photography survey. Offices from the Department of Environment (Albany), Agriculture Western Australia (Esperance) and the Salinity and Land Use Impacts Branch of the Department of Environment (Perth) are thanked for data, advice and extensive field knowledge they generously shared. Landowners are thanked for permission to access waterways, for their interest in this research and sharing their own observations and knowledge of landscape change.
The editorial advice and assistance of John Dunlop, Lucy Sheehy, Shan Callow, and my supervisors Associate Professor Keith Smettem, Dr Kimberly Van Niel and Dr Guy Boggs greatly improved the quality and clarity of this thesis. I have benefited greatly from their ideas, suggestions and varied background that my supervisors have contributed to this research, and the independence they have allowed me in exploring these rivers. I sincerely hope that this research marks the beginning of a research career which these people will be an important part of.
To the fellow Geographers who have shared the PhD experience with me on a personal and social basis, I thank all of you for making the journey one that was enjoyable, entertaining and more rounded.
Without the companionship and support of Lucy Sheehy, this research would not have been possible.
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Table of Contents Table of Contents Candidate Declaration...... iii Abstract ...... v Acknowledgements...... vii Table of Contents ...... ix List of Figures ...... xi List of Tables ...... xv List of Appendices ...... xvii List of Acronyms and Abbreviations...... xviii Chapter 1: Land Clearing, Salinisation and River Response...... 1 1.1. Introduction ...... 1 1.2. River Sensitivity to Land Clearing...... 3 1.2.1. Landscape Sensitivity...... 3 1.2.2. River Response to Land Clearing ...... 4 1.2.3. A Contemporary Example of Complex System Response ...... 6 1.3. Thesis Scope...... 9 1.4. Thesis Structure...... 9 1.4.1. Publications and Associated Works...... 11 Chapter 2: Setting and Context of River Response ...... 13 2.1. Physical Landscape...... 17 2.1.1. Climate ...... 17 2.1.2. Evolution of the Rivers – Geology, Palaeohydrology and Palaeoclimatology...... 20 2.1.3. Drainage and Hydrology...... 25 2.1.4. Soils...... 27 2.1.5. Vegetation ...... 29 2.2. Human Impact on the Landscape...... 31 2.2.1. Pre-European landscape...... 31 2.2.2. European settlement and agricultural development ...... 31 2.2.3. Present Land Use ...... 32 Chapter 3: Land Clearing and Boundary Conditions...... 35 3.1. Effects of Land Clearing on Discharge...... 37 3.1.1. Changed Rainfall in Southwestern Australia ...... 38 3.1.2. Groundwater Response to Land Clearing ...... 52 3.1.3. Changes to Discharge as a Result of Land Clearing...... 56 3.2. Effects of Land Clearing on Sediment Transport ...... 67 Chapter 4: Effects of Land Management on Landscape Connectivity ...... 81 4.1. Decoupling landscapes: the effect of farm dams and constructed banks on hydrologic connectivity in agricultural landscapes...... 82 4.2. Spatially Representing Landscape Connectivity...... 95 Chapter 5: Land Clearing, Salinity and Thresholds...... 109 5.1. Increased Stream Salinity Since Land Clearing...... 110 5.1.1. Salinity Trends in the Kent River ...... 111 5.1.2. Salinity Trends in the Dalyup River ...... 113 5.1.3. Effects of Salinity on Channel Roughness and Sediment Transport Potential ...... 117 5.2. Human Responses to Salinity: impacts on system stability...... 122
~ ix ~ Table of Contents Chapter 6: River Response to Land Clearing and Salinisation...... 129 6.1. Reach-Based Assessment of River Response ...... 130 6.2. Identifying Morphologically Similar Reaches...... 133 6.2.1. Classification Methodology...... 133 6.2.2. Reach Styles in Two South Coastal Western Australian Rivers...... 136 6.3. Changes in Morphologically Similar River Reaches...... 157 Chapter 7: Recovery and Management of Rivers in Saline Landscapes ...... 183 7.1. Introduction ...... 183 7.2. Potential for River Management in a Saline Landscape...... 184 7.2.1. Applying River Management in a Saline Landscape...... 184 7.2.2. Potential Tools for River Management in a Saline Landscape...... 186 7.3. Vegetation Regeneration and Growth in Saline Landscapes...... 187 7.3.1. Response of Vegetation in Salinising Landscapes: Downstream Trends in Salinity, Plant Growth and Regeneration...... 187 7.4. Modelling Geomorphic Effectiveness of Revegetation Options ...... 203 7.5. Discussion: River Management Potential in Saline Landscapes ...... 213 Chapter 8: Conclusion ...... 217 8.1. River Response to Land Clearing and Salinisation...... 217 8.2. Thesis Methods, Research Implications and Further Research ...... 218 8.3. Research Contribution...... 222 References ...... 223
~ x ~ List of Figures List of Figures Figure 2.1 Biogeophysical regions in the south coastal rivers region, based on boundaries from SCRPIT (2004)...... 14 Figure 2.2 Location of the Kent River study catchment...... 15 Figure 2.3 Location of the Dalyup River study catchment...... 16 Figure 2.4 Variation in average annual rainfall (mm) across the south coastal rivers region based on 1980-1999 data from CSIRO (2001)...... 17 Figure 2.5a-d Monthly average rainfall (with 10th and 90th percentile rainfall bars) and daily maximum and minimum temperature for weather stations adjacent to the study catchments in the south coast rivers region. Data from: Bureau of Meteorology (2006)...... 19 Figure 2.6 Geology of the south coastal rivers region...... 21 Figure 2.7 Location of the Jarrahwood and Stirling Range Axes, set against topographic and bathymetric data for the region...... 22 Figure 2.8 Mean daily discharge from stations 604053 (A) and 601001 (B) at the bottom of the Kent River and Young River (respectively). Data from Department of Environment (2005f; 2005d)...... 26 Figure 2.9 Soils of southwestern Australia. Source: Agriculture Western Australia (2002a)...... 28 Figure 2.10 Vegetation provinces in the south coastal rivers region, adapted from Beard (1975; 1981)...... 30 Figure 2.11 Area of land used for cropping and sown pasture in Western Australia. Data from: 1837 – 1950 data from Burvill (1979); 1966 – 1997 data taken from Western Australian Year Books 1966 – 1997 (Government of Western Australia, 1966-1997)...... 32 Figure 2.12 Land use patterns in the Kent River and Dalyup River. Source: (Beeston et al., 2001)...... 34 Figure 3.1 Change in runoff with land clearing in southwestern Australia. From: (Muirden et al., 2003)...... 38 Figure 3.2 Seasonal climate change scenarios for southwestern Australia from four Global Climate Models’s (GCM’s) using non-aerosol (“na”) and aerosol (“a”) scenarios for the periods 2010-2039, 2040-2069 and 2070-2999. Data from Allan and Hunt (1999)...... 40 Figure 3.3 Location of rain gauging stations surrounding the Dalyup River catchments used in testing spatial interpolation methods for rainfall data...... 43 Figure 3.4 Average annual rainfall for the Kent River catchment from 1921 to 2004 ...... 46 Figure 3.5 Average annual rainfall for the Dalyup River catchment from 1965 to 2004...... 49 Figure 3.6 Changes in the water balance components following land clearing...... 52 Figure 3.7 Groundwater trends in the Kent River catchment. Source: (Ryder, 2004b) ...... 54 Figure 3.8 Groundwater trends in the Dalyup River catchment. Source: (Simons and Alderman, 2004)...... 55 Figure 3.9 Location of river gauging stations and rainfall stations with the longest records in the Kent River catchment...... 57 Figure 3.10 Annual discharge and runoff coefficient for Styx Junction Gauging Station. Source: (Department of Environment, 2005f)...... 58 Figure 3.11 Estimated discharge (1957-2003) and actual discharge (1979-2003) for the upper (cleared) and lower (uncleared) Kent River catchment (based on data from: Department of Environment, 2005f; Department of Environment, 2005e).59
~ xi ~ List of Figures Figure 3.12 Rainfall runoff relationships for the Kent River catchment under different clearing rates...... 60 Figure 3.13 Decadal flow duration curve for Styx River Station from 1956-2004. Data from: Department of Environment (2005e)...... 61 Figure 3.14 Location of the Dalyup River catchment and the gauging stations in the adjacent Lort River and Young River...... 64 Figure 3.15 Flow duration curve under different clearing rates, based on data from gauging stations on the Lort River and Young River...... 65 Figure 3.16 Sediment budget for the contemporary Kent River and Dalyup River catchments based on data from the NLWRA...... 72 Figure 3.17 Downstream bedload sediment trends for the Kent River ...... 75 Figure 3.18 Downstream bedload sediment trends for the Dalyup River ...... 76 Figure 4.1 Location of the twelve basins in the upper Kent River catchment and rainfall isohyets...... 84 Figure 4.2 Investigation methodology used to incorporate the hydrologic influence of farm dams and banks into DEMs...... 86 Figure 4.3 Changes in area of the hillslope in basin (ii) retaining a hydrological connection with the main river channel without (A & B) and with (C & D) the effects of banks and farm dams incorporated into the DEM...... 87 Figure 4.4 Relationship between basin area (ha) and number of dams in 1965, 1973 and 1999...... 89 Figure 4.5 Effects of bank and farm dam construction of geomorphic statistics for basin i, ii and iv...... 93 Figure 4.6 The upper Kent River study catchment, showing how the raw DEM incorrectly routes flow out of the Kent River catchment and into the neighbouring Frankland River catchment to the west...... 98 Figure 4.7 Expected flow directions and catchment boundary from field work and people with expert knowledge...... 99 Figure 4.8 The catchment area and stream position expected from expert knowledge (a), against the results from; the original DEM data (b), unsuccessful Stream burning to 5m (c), successful Stream burning at 10m and 100m (d & e (note: similar results with different depth of trench), Agree (f), ANUDEM version 4.6.3 (g) and ANUDEM version 5.1 (h)...... 102 Figure 4.9 Residual surface showing difference in elevation between the original (pit-filled) DEM and hydrologic corrected (pit-filled) DEM created using Stream burning (a), Agree (b), ANUDEM v4.6.3 (c) and ANUDEM v5.1 (d)...... 104 Figure 4.10 Residual surface showing difference in slope between the original (pit- filled) DEM and hydrologic corrected (pit-filled) DEM created using Stream burning (a), Agree (b), ANUDEM v4.6.3 (c) and ANUDEM v5.1 (d)...... 105 Figure 5.1 Salinity trend for the Styx Junction station at the bottom of the Kent River catchment based on a box plot of discrete sampling (1956 – 1978) and a line plot of mean annual flow-weighted salinity from continuously sampled data (1979-2004). Data from: Department of Environment (2005e)...... 111 Figure 5.2 Downstream salinity trends in the Kent River before and after clearing...... 112 Figure 5.3 Predicted salinity trends for the upper Kent River Catchment at Rocky Glen Gauging Station since land clearing...... 113 Figure 5.4 Snapshot of stream salinity in the Dalyup River catchment (sampled on 22nd June 2005)...... 114 Figure 5.5 Salinity from the Munglinup and Melaluka gauging stations on the Young River, showing the effect of land clearing on salinity. Data from: Department of Environment (2005c; 2005b)...... 115
~ xii ~ List of Figures Figure 5.6 Change in vegetation across the floodplain along the reach at Wattersons Farm Gauging Station from 1946, 1965, 1972 and 1999...... 117 Figure 5.7 Changes in sediment transport potential following vegetation degradation from salinity (modified after Hjulström, 1935). Refer to Fig. 2 in Callow and Smettem (2007) for a map locating study sites (see Appendix 1A)...... 119 Figure 5.8 Downstream trends in specific stream power for the West Dalyup River, showing the modelled change in specific stream power due to reduced vegetation roughness...... 121 Figure 5.9 Deep drains in the Dalyup River catchment...... 124 Figure 5.10 The indurated ferricrete evaprolite layer formed above sediment found in deep drains in the upper Dalyup River catchment...... 126 Figure 5.11 Photo of the West Dalyup River above (A) and below (B) Boydells Road...... 127 Figure 6.1 Classification hierarchy of the RiverStyles® Framework, from Brierley et al. (2002)...... 136 Figure 6.2 Landscape Units of the Kent and Dalyup Rivers...... 139 Figure 6.3 River styles of the Kent River and Dalyup River ...... 139 Figure 6.4 River styles tree for reaches found across the Kent River and Dalyup River. The greyed box identifies new “River Styles” identified by this research...... 141 Figure 6.5 “Gorge” and “occasional floodplain pockets” river styles...... 142 Figure 6.6 “Occasional floodplain pockets” and “bedrock controlled discontinuous floodplain” river styles...... 143 Figure 6.7 “Low sinuosity planform controlled discontinuous floodplain” river style...... 144 Figure 6.8 “Planform controlled low sinuosity fine grained” and “valley fill” river styles...... 145 Figure 6.9 “Valley fill” and “channelised fill” river styles...... 146 Figure 6.10 “Channelised fill” river style...... 147 Figure 6.11 “Low sinuosity fine grained” river style...... 148 Figure 6.12 “Low sinuosity fine grained” and “low sinuosity bedrock” river styles...... 149 Figure 6.13 “Low sinuosity bedrock” and “fine grained multi channel” river styles...... 150 Figure 6.14 “Fine grained multi channel” river style...... 151 Figure 6.15 “Fine grained multi channel” and “multi channel sand bed” river styles...... 152 Figure 6.16 Repeat photography of gorge sections in the lower Kent River, originally photographed by Ian Maley in 1976. 1976 Photography reproduced with permission of the Environmental Protection Agency, Western Australia...... 158 Figure 6.17 Repeat photography of occasional floodplain pocket sections in the lower Kent River, originally photographed by Ian Maley in 1976. 1976 Photography reproduced with permission of the Environmental Protection Agency, Western Australia...... 161 Figure 6.18 Example of potential directions of change for the bedrock controlled discontinuous floodplain river style under changes in sediment load, discharge and stream salinity (vegetation roughness)...... 162 Figure 6.19 Processes of and limitation on channel adjustment at Kent Sites 17 and 18...... 164
~ xiii ~ List of Figures Figure 6.20 Avulsion channel at Site 10, illustrating the significant risk posed to agricultural land adjacent to rivers and the impact such events can have on the river geomorphology...... 165 Figure 6.21 Pools, located 500m downstream (A) and 3km downstream (b) of the avulsion at Dalyup Site 10 (see Figure 6.20). These images shows the effects of sedimentation from avulsion on pool morphology and waterbird habit...... 166 Figure 6.22 Lake Carrabundup vegetation and stratigraphy...... 168 Figure 6.23 Changes at Dalyup Site 7 from the March 2000 flood...... 170 Figure 6.24 An example of changes that have occurred in low-sinuosity fine grained river styles at Dalyup Site 18, where vegetation surrounding the wetted channel perimeter has degraded and sediment is now freely transported downstream. ...172 Figure 6.25 An example of the erosion pin networks (at Kent Site 1), used to measure erosion rates of the clay valley fill material within and between different study reaches...... 172 Figure 6.26 Erosion of lateral benches and terraces from the March 2000 flood. ...174 Figure 6.27 Stratigraphy of a mid-channel island, typical of those found in the mid and upper reaches of the Dalyup and West Dalyup Rivers. Photo taken on the West Dalyup River, 500m upstream from Spedingup Road...... 176 Figure 6.28 Evolution of the multi channel sand bed reach at Dalyup Site 13, from 1969 to after the 2000 flood...... 178 Figure 7.1 Salinity variation over a two year period at Watterson’s Farm Gauging Station (604003), April 2000 – April 2002, in comparison to regeneration and tolerance limits of Paperbark and Samphire...... 189 Figure 7.2 Downstream trend in maximum 60 day salinity, based on gauging data, with zones of response identified...... 190 Figure 7.3 Section 1: Watterson’s Farm Gauging Station (Kent Site 3)...... 192 Figure 7.4 Section 2: Poorrarecup Road to Perillup Gauging Station (Kent Sites 8 – 11)...... 193 Figure 7.5 Section 1: South Perillup Road (Kent Site 14)...... 194 Figure 7.6 Trends in downstream vegetation condition under shallow saline groundwater at Kent Sites 3, 8, 9, 10, 11, and 14...... 195 Figure 7.7 Conceptual model of vegetation recovery potential in a saline landscape...... 196 Figure 7.8 Vegetation recovery along the lower Dalyup River and West Dalyup Rivers from 2000 to 2005...... 201 Figure 7.9 Potential for stabilisation of floodplain sediment stores using tree plantations to prevent channel avulsion...... 208 Figure 7.10 Potential for rehabilitation of in-stream and lateral sediment stores using native vegetation, through fencing and stock exclusion...... 209 Figure 7.11 Use of channelisation as a river management technique to increase vegetation condition, but increasing velocity in the channel...... 210
~ xiv ~ List of Tables List of Tables Table 2.1 Characteristics of rivers in the southern coastal rivers region. From: Pen (1999)...... 25 Table 2.2 Land use in the south coastal rivers region. Source: (Beeston et al., 2001) ...... 33 Table 3.1 The various tools and methods applied in this study, based on the framework proposed by Kondorf and Piegay, 2003b...... 36 Table 3.2 Performance of different methods for spatially modelling rainfall distribution over the Dalyup River catchment in 1984...... 44 Table 3.3 Average rainfall for the Dalyup River catchment from 1984 predicted by various interpolation methods...... 45 Table 3.4 Comparison of splines and Thiessen polygons for predicting annual rainfall for the Dalyup River catchment...... 45 Table 3.5 Change in early winter (MJJ), winter (MJJAS) and summer (DJF) periods from 1913-1975 and 1976-2004...... 47 Table 3.6 Probability of extreme daily rainfall occurrence in the number of times per decade that an event in excess of that intensity over the Kent River catchment...... 47 Table 3.7 Change in early winter (MJJ), winter (MJJAS) and summer (DJF) periods from 1913-1975 and 1976-2004...... 49 Table 3.8 Probability of extreme daily rainfall occurrence in the number of times per decade that an event in excess of that intensity over the Dalyup River catchment...... 50 Table 3.9 Analysis of the discharge recorded from similar early summer events recorded at the Rocky Glen gauging station...... 62 Table 3.10 Analysis of the discharge recorded from similar winter events on a wet catchment recorded at the Rocky Glen gauging station...... 62 Table 3.11 Changes in the frequency of bankfull discharge at three sites in the Dalyup River catchment, based on predicted runoff curves (see Figure 3.15C)...66 Table 3.12 Mean sediment concentration and specific yield for the <63μm and >63μm, fractions, and total suspended sediment (TSS) yield for the Kent River catchment. Source: DoE, unpublished data and NLWRA (2002)...... 69 Table 3.13 Mean sediment concentration and specific yield for the <63μ and >63μ, fractions, and total suspended sediment yield for the Dalyup River catchment. Source: DoE, unpublished data and NLWRA (2002)...... 70 Table 3.14 Sediment transport in the Kent and Dalyup River catchments. Source: NLWRA (2002)...... 72 Table 3.15 Sediment erosion and transport data for selected rivers from the National Land and Water Resources Audit (2004)...... 73 Table 4.1 Total number of dams in each study catchment in 1965, 1973 and 1999...... 88 Table 4.2 Effects of banks and dams on catchment area ...... 90 Table 4.3 Results achieved by the original DEM and different hydrologic correction methods in replicating known hydrologic conditions and catchment parameters. The most successful method for each parameter is identified in bold...... 103 Table 5.1 Pre clearing and current salinity in the Kent River...... 113 Table 5.2 Some measured water quality data from rivers in southwestern Australia compared to various ecological tolerance values...... 116 Table 5.3 Estimated changes in channel roughness as a result of salinity, with corresponding changes in bankfull velocity, discharge and stream power...... 120
~ xv ~ List of Tables Table 6.1 Summary of the changes in landscape stability following land clearing in the study catchments...... 130 Table 6.2 Landform Units of the Kent River and Dalyup River...... 138 Table 6.3 Characteristics of the different river styles found in the Kent and Dalyup Rivers ...... 153 Table 6.4 Geomorphic evolution characteristics of the different river styles of the Kent River and Dalyup River, southwestern Australia, highlighting the changing function of river reaches from the pre-European (uncleared), to the current degraded and salt-affected condition...... 181 Table 7.1 Number of repeat photo sites where a particular species dominated (or was co-dominant) revegetation for a range of geomorphic surfaces, located next to the wetted channel and on the floodplain in the lower and mid Dalyup River. ..198 Table 7.2 Vegetation roughness (Manning’s “n”) values for different vegetation types, based on data from Chow (1959), Arcement and Schneider (2003), Ladson et al. (2003), and Lang et al. (2004)...... 203 Table 7.3 Table of the function and rehabilitation potential for different river styles found in the study catchments...... 212
~ xvi ~ List of Appendices List of Appendices
Appendices: Chapter 1 ...... A1 Appendix: 1A Article: Callow and Smettem, 2007...... A1 Appendix: 1B Article: Callow et al., 2007...... A24 Appendix: 1C Conference Proceeding: Callow and Smettem, 2006...... A34 Appendix: 1D Conference Proceeding: Callow and Smettem, 2004...... A35 Appendices: Chapter 2 ...... A37 Appendix: 2A Cyclone Tracks Affecting Southwestern Australia...... A37 Appendix: 2B Flood Hydroclimatology in Southwestern Australia ...... A38 Appendix 2B i Dalyup River – Largest Daily Rainfall and Streamflow...... A38 Appendix 2B ii Kent River – Largest Daily Rainfall from and Ex-tropical Cyclone...... A39 Appendix 2B iii Kent River – Largest Daily Streamflow...... A40 Appendix 2B iv Kent River – Largest recorded daily rainfall...... A41 Appendices: Chapter 3 ...... A45 Appendix: 3A Dalyup River Annual Rainfall Distribution...... A45 Appendix: 3B Geostatistical Models...... A45 Appendix: 3C 50mm Rainfall Isohyets for the Dalyup River ...... A46 Appendix: 3D Kent River Annual Rainfall Change ...... A47 Appendix: 3E Kent River Extreme Daily Rainfall Change...... A47 Appendix: 3F Dalyup River Annual Rainfall Change ...... A48 Appendix: 3G Dalyup Extreme Daily Rainfall Change ...... A48 Appendix: 3H Event Hydrographs for Summer Events ...... A49 Appendix: 3I Event Hydrographs for Winter Events...... A49 Appendix: 3J Runoff Data from the Lort and Young Rivers ...... A50 Appendix: 3K Dalyup River Bankfull Discharge Analysis...... A51 Appendix: 3L Kent River Suspended Sediment Data ...... A52 Appendix: 3M Dalyup River Suspended Sediment Data ...... A52 Appendix: 3N NLWRA Sediment and Erosion Data ...... A53 Appendix: 3O Sieve Data from Bedload Sediment Sampling...... A55 Appendices: Chapter 5 ...... A99 Appendix: 5A Dalyup River Discrete Salinity Sampling...... A99 Appendix: 5B Snapshot Salinity Sampling along the Dalyup River,...... A100 Appendix: 5C Areas of Southwestern Australia at Risk of Dryland Salinity....A101 Appendices: Chapter 6 ...... A103 Appendix: 6A Emerson Soil Dispersion and Slaking Test...... A103 Appendices: Chapter 7 ...... A105 Appendix: 7A Key Riparian Species of the Kent River and Dalyup River...... A105 Appendix: 7B Repeat Photography along the Dalyup River...... A110 References: A...... 207
~ xvii ~ List of Acronyms and Abbreviations List of Acronyms and Abbreviations
AgWA Agriculture Western Australia CAD Cumulative Area Distribution CSIRO Commonwealth Scientific Industry Research Organisation
D50 Mean sediment diameter DEM Digital Elevation Model DJF December, January, February DLI Department of Land Information DoE Department of Environment DoW Department of Water E. Eucalyptus EDRS Esperance Downs Research Station GDA Geocentric Datum of Australia GIS Geographic Information System IDW Inverse Distance Weighted IOCI Indian Ocean Climate Initiative IUH Instantaneous Unit Hydrograph Ka b.p. Thousands of years before present K-S Kolmogorov-Smirnov LWD Large Woody Debris Ma b.p. Millions of years before present ME Mean Error MGA Map Grid of Australia MJJ May, June, July MJJAS May, June, July, August, September MSLP Mean Seal Level Pressure NLWRA National Land and Water Resources Audit NOI Notice of Intent RMSE Root Mean Square Error SRTM Shuttle Radar Topographic Mission SST’s Sea Surface Temperatures TSS total suspended sediment WRC Water and Rivers Commission
~ xviii ~ Chapter 1: Land Clearing, Salinisation and River Response
Chapter 1: Land Clearing, Salinisation and River Response
Response of the Dalyup River to the large flood in March 2000. The road shown is the South Coastal Highway, the main road from Western Australia to South Australia. Image from Bureau of Meteorology (2005a)
1.1. Introduction Humans are one of the primary agents of geomorphic change (Hooke, 2000; Wolman, 2002). The most striking example of anthropogenic disturbance has been the effects on river systems caused by clearing native forests and woodlands for agriculture (Wolman, 1967; Walling, 1999; Brierley and Stankoviansky, 2002; Wolman, 2002; Lang et al., 2003). Rivers are dynamic systems. Both subtle and catastrophic responses in river channel morphology associated with land clearing for agriculture have been documented (Wolman, 1967; Knox, 1977; Klimek, 1987; Starkel, 1987; Starkel, 1988; Mei-e and Xianmo, 1994; Brooks and Brierley, 1997).
Changes in river morphology may impact on the economic, environmental, cultural and social values of rivers. Examples of these have included: loss of agricultural land, loss of harvestable aquatic fauna, reduction of tourism, destruction of infrastructure, habitat loss, loss of in-stream and riparian biodiversity, downstream effects on estuaries and wetlands and mobile fauna, loss of historically or culturally significant sites, and loss of recreation potential (see Dolan et al., 1974; Warner and Bird, 1988; Clark and Wilcock, 2000; Rutherfurd, 2000; Wohl, 2000b; ~ 1 ~ Chapter 1: Land Clearing, Salinisation and River Response Turpie and Joubert, 2001; Halse et al., 2003; Ibáñez and Prat, 2003; Lymbery et al., 2003; Marker, 2003).
River management has traditionally centred on regulating and controlling river functions for utilitarian exploitation and to protect human populations and infrastructure (Revenga et al., 2000; Vorosmarty and Sahagian, 2000; Downs and Gregory, 2004). More recently, a shift toward integrated and sustainable river management has occurred which considers the condition of rivers and how this affects conservation, channel hazards and water resources (Downs and Gregory, 2004). Successful river management requires understanding of the spatio-temporal patterns of anthropogenic landscape disturbance and consequent river response, functional dynamics and connectivity of systems, and how management strategies can work using these controls to influence river evolution (Downs and Gregory, 2004; Brierley and Fryirs, 2005; Hillman and Brierley, 2005).
A significant motivation behind this research has been the desire to understand how river channel morphology has responded to recent land clearing and how catchment and river management might modulate adverse impacts on the various river values described above.
Europe and North America are the focus areas of most research on, and application of, river rehabilitation and restoration (Ward et al., 2001). In these settings, research and management is concentrated on managing river responses to changing climates, impacts of mining, urbanisation, river regulation or reforestation (Nienhuis and Leuven, 2001; Downs and Gregory, 2004; Ormerod, 2004). These rivers have typically adjusted to the effects of land clearing, which occurred between several hundred and many thousands of years ago (Doolittle, 1992; Hooke, 2000; Wolman, 2002). By contrast, catchments in southwestern Australia have been subjected to extensive land clearing in only the last c. 50 years (Beresford et al., 2001; Bennett and Macpherson, 2002) and so provides an opportunity to study contemporary channel responses to land clearing as a contribution to understanding associated adjustment of fluvial systems.
While clearing of native forest for agriculture in eastern Australia began shortly after European settlement of Australia (c.1788), large portions of southwestern Australia remained uncleared until after World War II (Burvill, 1979; Beresford et al., 2001). Mechanisation of agriculture and improved farming techniques resulted in large areas of land cleared during the 1960s and 70s (Burvill, 1979). In some catchments only 3% of native vegetation remains (Beeston et al., 2001). The recent nature and scale of land clearing provides an ideal setting to investigate the role of humans in landscape change and the associated response of river channels.
~ 2 ~ Chapter 1: Land Clearing, Salinisation and River Response This chapter provides the theoretical and contextual setting of the thesis, outlining the potential for changes in landscape stability following land clearing and associated land management methods. The potential for changes in landscape stability through alteration of erosive potential, material thresholds, system connectivity and resilience, system response, and the role of land and river management are briefly discussed. These are the core themes explored in subsequent chapters of this thesis.
1.2. River Sensitivity to Land Clearing 1.2.1. Landscape Sensitivity The landscape sensitivity concept was introduced in the field of geomorphology by Brunsden and Thornes (1979). It has been widely used as a conceptual basis for investigating response of rivers (Downs and Gregory, 1993), soil erosion (Evans, 1993), mass movement (Brunsden, 2001), and applied extensively to investigating river response to perturbation (see Thomas and Allison, 1993 and the Special Edition of Catena - "Landscape Sensitivity", vol 42 (2001)). The landscape sensitivity concept considers “the likelihood that a change in the controls of a system or the forces applied to the system will produce a sensible, recognisable, sustained but complex response” (Brunsden, 2001, p. 99).
Related to system sensitivity (or the propensity to change) is landscape stability, a system state, determined by the ratio of resisting and disturbing forces, which determines its proximity to critical thresholds (Brunsden, 1990; Downs and Gregory, 1993; Brunsden, 2001; Harvey, 2001). Landscape stability is altered through variability in temporal and spatial sensitivity (Brunsden, 2001). Perturbation of extrinsic and intrinsic thresholds can shift the system towards instability (Brunsden, 2001). Extrinsic thresholds are perturbed by alteration of the magnitude and/or frequency of disturbing forces such as changes in flood magnitude, flood frequency and relaxation time, related to environmental and/or anthropogenic changes such as climate change and land clearing (Brunsden, 2001; Knox, 2001; Usher, 2001). Brunsden (1993b; 1993a; 2001) identified that numerous internal factors affect system stability, resisting changes in disturbing forces. These include; x Material strength controlling mechanical resistance to erosion; x Morphology of the landscape affecting available energy; x Structure, including the topology of the system, determining how impulses are transformed through the system, a function of location and coupling; x Filtering or the absorption and transfer of energy; and, x System state or how the system reacts and has reacted to past stress, determined by elasticity, amplitude, hysteresis and malleability.
~ 3 ~ Chapter 1: Land Clearing, Salinisation and River Response Gilvear (1999) identified four methods adopted by fluvial geomorphologists in investigating response and adjustments of river channels for the purposes of river management. These are connectivity within a fluvial system; temporal studies; landscape sensitivity; and eco- geomorphology, and while these are based on the same underlying principles of fluvial geomorphology, some are better suited to particular situations (see Gilvear, 1999 p.230). Gilvear (1999) suggests the landscape sensitivity approach is best suited to questions of response and resistance of systems, and in understanding the role of thresholds in determining the variability of spatio-temporal response to perturbations. Suitable examples of its application include: analysis of the effects of large flood events, change in land use, and climate change, using “quantitative and qualitative field techniques and modelling to identify instability” and “analysis of river channel cross-section and planform to predict future change” (Gilvear, 1999 p.231). This study applies the concept of landscape sensitivity to investigate changes in the state of system stability, and how rivers have responded to land clearing and land management. Perturbation of both disturbing forces and resistance factors are investigated, to determine why and where river reaches become unstable and undergone morphological response.
Anthropogenic impacts such as land clearing and associated impacts (e.g. the way land is managed), alter disturbing and resistive components of system stability (Thomas and Allison, 1993; Brunsden, 2001; Thomas, 2001). Because in North America and Europe such anthropogenic perturbations happened many centuries or decades ago (Doolittle, 1992; Hooke, 2000; Wolman, 2002), their role in controlling system response remains poorly understood. This study examines a recent example of massive landscape disturbance, and so presents an opportunity to observe, qualify and quantify landscape stability and morphological responses of river channels to anthropogenic perturbation. In examining a present day example of potential landscape instability, this thesis contributes toward understanding of complex system response.
1.2.2. River Response to Land Clearing Examples of human impact on river systems have been recorded from the Bronze and Iron Ages in Europe, Asia and the Middle East and are preserved in the sedimentary record, and show significant response of channel morphology to land clearing (e.g. Klimek, 1987; Starkel, 1988; Mei-e and Xianmo, 1994). Similar responses have been reported more recently in North America (Wolman, 1967; Chorley et al., 1984; Knox, 1987; De Boer, 1997; Wolman, 2002). Observations of river response has been the basis for the geomorphic axioms such as Lane’s Balance (Lane, 1957), Schumm’s “directions of response” equations (see Knighton, 1998 p.291) and his continuum of channel morphotypes (Schumm and Lichty, 1963; also see Knighton, 1999 p.291; Schumm, 1969; 1977; 1981; 1985; 1988), that summarise the potential for system instability and direction of morphological responses to land clearing for agriculture. These
~ 4 ~ Chapter 1: Land Clearing, Salinisation and River Response predict rivers’ responses to land clearing, such as channel enlargement, entrenchment, bank erosion (width adjustment) and increased sediment transport.
In eastern Australia, catchment responses to land clearing have included increased runoff, network extension, increased drainage density, and increased erosion and sediment yield in the catchment (Eyles, 1977; Prosser and Winchester, 1996; Wasson et al., 1996; Wasson et al., 1998; Prosser et al., 2001b; Olley and Wasson, 2003). Changes to channel morphology include: bank erosion, channel incision, floodplain scour, channel avulsion, increased channel capacity, shifts in gross channel morphology (from meandering to braided), increased sediment delivery ratio, channel deposition and aggradation leading to diminished flood capacity, pool sedimentation, and increased floodplain deposition (see Eyles, 1977; Warner and Bird, 1988; Erskine and White, 1996; Brooks and Brierley, 1997; Brierley et al., 1999; Brizga and Finlayson, 2000a; Brooks and Brierley, 2000; Rutherfurd, 2000; Fryirs and Brierley, 2001; Zierholz et al., 2001; Olley and Wasson, 2003; Brooks and Brierley, 2004).
Research on rivers in southwestern Australia has focused on understanding hydrologic and stream salinity responses to land clearing (Stokes and Loh, 1982; Williamson et al., 1987; Ruprecht and Schofield, 1989; Ruprecht and Schofield, 1991; Ruprecht and Stoneman, 1993; Hatton and Nulsen, 1999; Bari and Ruprecht, 2003; Hatton et al., 2003). Studies of river morphological response have focused on the Avon River, reporting channel scour, bank erosion and pool sedimentation following river “training” from 1957 - 1973 (Southwell and Wyrwoll, 1993; J Davies & Assoc and Ecoscape Pty. Ltd., 1996; Pen, 1999; Water and Rivers Commission, 1999; Sampey, 2000; Steele, 2002). While catchment groups have undertaken river condition surveys that include geomorphic aspects, these projects are spatially limited in their focus. Recent programs such as the Water Resource Recovery Catchments and Natural Diversity Recovery Catchments (Conacher, 2002; Lothian and Conacher, 2005) have been established with goals of integrated river management and river restoration, although they have not included research on river morphological response to land clearing and increased stream salinity.
The recent nature and magnitude of land clearing in southwestern Australia presents an opportunity to investigate how humans have altered landscape stability and how rivers have responded to this. Recent perturbation allows observation and direct quantification, rather than retroductive and deductive methods based on palaeo-sedimentary and qualitative investigations applied elsewhere to landscapes disturbed hundreds or thousands of years earlier (Baker, 1995; Baker, 1998; Kondolf and Piegay, 2003b). The aim of this research is to quantify how and where rivers have become unstable due to anthropogenic and environmental perturbation and to
~ 5 ~ Chapter 1: Land Clearing, Salinisation and River Response investigate management strategies which could modulate morphological responses to the controls and thresholds that now govern these rivers.
1.2.3. A Contemporary Example of Complex System Response Native forests and woodlands are more efficient users of water than agricultural crops and pasture (Zhang et al., 1999). Worldwide, the transition from woodlands and forest to agricultural pasture and cropping systems has been associated with increased river flow and soil erosion (Wolman, 1967; Trimble, 1974; Knox, 1977; Gregory and Madew, 1982; Klimek, 1987; Knox, 1987; Jacobson and Pugh, 1992; Lorup et al., 1998; Walling, 1999; Clark and Wilcock, 2000; Schreider et al., 2002; Wolman, 2002). Under an unchanging climate regime, reduced evapo-transpiration of agricultural crops results in the hydrologic balance being maintained through changes to soil water storage, and groundwater recharge and discharge (Bari and Ruprecht, 2003). Increased streamflow can move systems toward instability by increasing the frequency and magnitude of geomorphologically effective flood events and reduce the relaxation time between these events.
Land clearing initially increases the volume of water reaching the soil surface due to reduced canopy interception (Stokes and Loh, 1982). Where this flux exceeds the infiltration capacity of surface soils, the immediate response of systems is an increase in direct runoff (Stokes and Loh, 1982; Williamson et al., 1987; Ruprecht and Stoneman, 1993). If hydraulic conductivity of the soil prohibits runoff, increased infiltration initially fills available stores in the soil matrix, with excess water percolating below the root zone and increasing groundwater recharge (Nulsen et al., 1986; George, 1992; Hatton et al., 2003; Peck and Hatton, 2003). Replacement of deep- rooted perennial woodlands with shallow-rooted seasonal or annual and agricultural crops and pasture decreases transpiration of water from the soil profile. The result in southwestern Australia has been increased groundwater recharge by up to two orders of magnitude (Nulsen et al., 1986; George, 1992; Ruprecht and Stoneman, 1993; Hatton et al., 2003; Peck and Hatton, 2003).
The effects of increased groundwater recharge are not immediately observed at the soil surface, rather, response is delayed until groundwater rises sufficiently to affect runoff by expanding saturated source areas and directly discharging into streams and low-lying areas (Williamson et al., 1987; Ruprecht and Stoneman, 1993). This response continues until hydrologic equilibrium is reached, and runoff may be up to four times the pre clearing rate in southwestern Australia (Williamson et al., 1987; Ruprecht and Stoneman, 1993; Bowman and Ruprecht, 2000; Muirden et al., 2003). Progressive increases in streamflow volume, flood peaks and reduced relaxation time all increase boundary shear and move the system toward instability.
~ 6 ~ Chapter 1: Land Clearing, Salinisation and River Response Environmental variability resulting from recent climatic changes in southwestern Australia (Allan and Haylock, 1993; Yu and Neil, 1993; Ruprecht et al., 1996; Smith et al., 2000; Hatton and Ruprecht, 2001; Indian Ocean Climate Initiative, 2002), has potential to modulate the effect of land clearing on runoff. Since the 1970s, annual rainfall has decreased by 10-15% in southwestern Australia (Smith et al., 2000; Indian Ocean Climate Initiative, 2002), and this is likely to offset effects of land clearing on runoff. Furthermore, the human response to reduced annual rainfall has been an increase in water security activities (e.g. dam construction) that has further affected runoff processes and system resistance. Reduced annual runoff in a drier climate has increased the pressure for on-farm water security, and as a result, the number of farm dams has increased significantly (Callow and Smettem, 2004). Farm dams and their associated infrastructure decouple hillslopes from channels. In doing so they act similar to larger in- channel dams by trapping sediment and reducing streamflow (Beavis and Howden, 1996; Savadamuthu, 2002; Schreider et al., 2002; Teoh, 2002). Significant morphological responses to flow regulation have been reported (see Bendix and Hupp, 2000; Grams and Schmidt, 2002; Cluett and Radford, 2003; Lloyd et al., 2003), and where farm dams change flow and sediment budgets, they have potential to affect river morphology (Beavis and Howden, 1996).
Groundwater response not only affects landscape stability through hydrologic mechanisms that increase erosive potential, but vegetation degradation caused by increased stream salinity has also reduced erosive thresholds. Rising groundwater mobilises salts, bringing them to the surface and increasing streamflow salinity. In southwestern Australia, high rates of salt deposition in rainfall (Hingston and Gailitis, 1976), combined with a deep-weathered regolith and flat landscape gradient (Hookey, 1987; McFarlane and George, 1992; Salama et al., 1993a; George et al., 2001), have resulted in a landscape whose soil profile contains large stores of salt. On average 1,000 t of salt is stored below each surface hectare across the region, and values up to 10,000,000 t ha-1 have been measured (McFarlane and George, 1992; Agriculture Western Australia, 2002b). As a result, stream salinity in southwestern Australia is the highest of any dryland agricultural area in the world (based on data from du Plessis and van Vellen, 1991; Ghassemi et al., 1995; Jolly et al., 2001). Rivers in cleared catchments now have mean flow- weighted salinity that ranges from brackish (1,000 – 3,000 mg l-1) to reaches that are officially classified as brine (>35,000 mg l-1) and are saltier than seawater (Department of Environment, 2004a; Mayer et al., 2005)
While salinity was present prior to land clearing (primary salinity) (Beresford et al., 2001; Commander et al., 2001; Water and Rivers Commission, 2002a; Hatton et al., 2003; Harper and Gilkes, 2004), it has increased significantly following land clearing (secondary salinity). Salt is toxic to all plants at sufficiently high levels (Bird, 1978; Blake, 1981; Ladiges et al., 1981; van der Moezel and Bell, 1987; Mensforth et al., 1994; English et al., 1999; English et al., 2001;
~ 7 ~ Chapter 1: Land Clearing, Salinisation and River Response Munns, 2002; Barrett-Lennard et al., 2003; Loch et al., 2003; Nielsen et al., 2003), and degradation of riparian vegetation has resulted from increased stream salinity. The importance of vegetation in controlling river channel morphology and erosive thresholds, channel stability, river ecology and river rehabilitation potential is extensively acknowledged in the literature (Kouwen and Unny, 1973; Kao and Barfield, 1978; Kouwen et al., 1981; Kadlee, 1990; Trimble, 1990; Gippel, 1995; Shields Jr and Gippel, 1995; Hobbs and Norton, 1996; Hupp and Osterkamp, 1996; Piegay and Gurnell, 1997; Abernethy and Rutherfurd, 1998; Darby, 1999; Bendix and Hupp, 2000; Tabacchi et al., 2000; Abernethy and Rutherfurd, 2001; Cramer and Hobbs, 2002; Järvelä, 2002; Webb and Erskine, 2003; Järvelä, 2004). Vegetation degradation due to increased stream salinity has lowered both critical shear strength of material, and increased the erosive potential due to increased peak stream velocity and more frequent flooding. The association of vegetation degradation with increased erosive potential, highlights the capacity for channel instability in response to land clearing in this environment.
The final factor affecting system sensitivity to changes is the land management practices adopted by people in response to salinisation, drying climates, increased economic pressures and changing environmental values. These have the potential to affect landscape sensitivity and system response. Rising saline groundwater threatens low-lying land and has prompted land managers to adopt a range of strategies in an attempt to modulate impacts on often formerly productive agricultural land. Responses have included proposals to adopt landscape-scale conversion to farm-forestry, though at present this is limited to the sub-catchment scale (Kington and Smettem, 2000; George et al., 2001; Pannell and Ewing, 2004). Adoption of high water use perennial pastures has also been widely advocated as a change to farming practice that may offer a more sustainable farming future (Hamilton and Bathgate, 1996; George et al., 2001; Clarke et al., 2002; Cocks, 2003; Pannell and Ewing, 2004). Engineering works such as open deep drains in low-lying areas and pumping groundwater into arterial drainage schemes (Coyne et al., 2002), lakes and river channels have also being proposed and trialled, albeit in an ad-hoc manner, as potential solutions to the loss of agricultural land (Keen, 1998; Thomas and Williamson, 2001; Ali et al., 2004). The human response to managing landscapes affected by secondary (human-caused) dryland salinity is in its infancy. As yet, few solutions offer economic outcomes that are socially and environmentally suitable (Conacher and Conacher, 1995; Tonts, 2000; Conacher et al., 2004). All of these strategies have implications for future landscape sensitivity and channel evolution.
This thesis initially examines changes in erosive potential and thresholds following landscape disturbance and how subsequent environmental and anthropogenic changes and responses have further influenced system stability. Channel response to the altered regime is then investigated using a reach-based analysis that evaluates the evolution of morphologically similar reaches, but
~ 8 ~ Chapter 1: Land Clearing, Salinisation and River Response considered within a catchment context (see Brierley and Fryirs, 2000; Brierley et al., 2002; Brierley and Fryirs, 2005). Finally, system response is considered over human and management timescales, with reference to the potential that reach-scale management strategies offer for mitigating changes in river morphology. The effectiveness of these strategies is considered in light of the new hydrologic, social and environmental processes that now control the system.
1.3. Thesis Scope The aim of this thesis is to examine how anthropogenic and environmental perturbation has altered landscape sensitivity through erosive potential, material thresholds, system connectivity and resilience mechanisms and its relationship to spatio-temporal patterns of river response. The potential that river management strategies offer under new thresholds and forces that control the system is assessed in this context. Within this aim, a series of specific research questions are posed and tested. Each question forms the base of Chapters Three through Seven, and form the main body of the thesis: x Has land clearing altered channel boundary conditions? x How has system connectivity responded to human disturbance of the landscape and is this important for landscape stability? x What has the effect of secondary salinity been on thresholds and system resistance? x Are reach-scale responses predictable and consistent within and between catchments, and does this understanding offer a tool at the regional-scale to assess potential channel adjustments of particular reach types? x What river management strategies might be effective in mitigating adverse channel responses in light of the new set of boundary conditions, thresholds and resistance factors that now control channel evolution?
1.4. Thesis Structure Chapter Two establishes the physical and human setting of the study region, characterising climate, geology, hydrogeology, hydrology, soil, vegetation, and the human landscape. Trends across the entire regions are discussed, with particular reference to the two study catchments.
Chapter Three documents changes to water and sediment flux in the study catchments following land clearing. In investigating the changes to discharge, a water-balance approach is adopted to consider how all components of the hydrologic cycle have changed in response to land clearing. External influences such as changes in rainfall patterns are investigated for their potential to modulate effects of land clearing. Changes in sediment flux through the landscape are qualified and quantified using field data, and previous work by others in this region. From this examination, changes in channel boundary conditions following land clearing are established.
~ 9 ~ Chapter 1: Land Clearing, Salinisation and River Response These results underlie the potential for significant channel morphological response, explored in Chapter Six. Chapter Four investigates changes accompanying land clearing and modern agriculture and the effects they have had on landscape processes, specifically system connectivity. The first section of the chapter investigates how human response to a drying climate has affected hydrologic connectivity. The second section explores how hydrologic connectivity is modelled and represented in an agricultural landscape and how methods used to replicate known hydrology affect subsequent analysis of landscape processes.
Chapter Five looks at the effect of increased stream salinity on vegetation, and the influence that this had on system thresholds. Changes in vegetation condition are studied using field and analysis of remotely-collected data, together with a numeric modelling approach to estimate changes in erosive potential and thresholds. The chapter also considers the impact of human responses to secondary salinity, through the construction of open deep drains and channelisation of rivers in saline river valleys. The impact of these on vegetation and their potential for altering erosive potential, thresholds and system resistivity is discussed.
Chapter Six uses a reach-based geomorphic classification method (RiverStyles® Framework – see Brierley and Fryirs (2000; 2005) and Brierley et. al. (2002)) to investigate the response of morphologically similar reaches to land clearing in the 1950s and 60s within the study catchments. Change is investigated in representative reaches, but is considered in a catchment context, placing the evolution of the reach in terms of type and severity of perturbation, influence of upstream processes, and downstream impact. Temporal variation in channel morphology is investigated with references to changes in erosive potential in the system and changes in erosive thresholds.
Chapter Seven considers the potential that river management strategies based on soft- engineering (i.e. vegetation based) may have in a salinising landscape. An understanding of particular species that could be used for river management is developed in combination with a model of channel hydraulics, to determine whether vegetation-based strategies can influence the evolution of particular river styles. This is considered under the altered boundary conditions, erosive thresholds, upstream processes and salinity gradients that now control river management potential and processes in these rivers.
Chapter Eight discusses the local and global implications of this research. The potential for management strategies to offer solutions in a landscape affected by severe anthropogenic and environmental perturbation is considered. The implications for dryland river systems elsewhere,
~ 10 ~ Chapter 1: Land Clearing, Salinisation and River Response river response to perturbation and the role of temporal variation in thresholds are discussed. Recommendations for further research are also highlighted.
1.4.1. Publications and Associated Works A number of published articles, conference presentations, and a manuscript in preparation are related to work presented in this thesis. The candidate was lead author on all of these publications, and conducted the overwhelming majority of research, analysis, writing, and/or presentation of the material contained in the papers, and presented in this thesis. Please refer to the “Candidate Declaration” on page ii for specific details of the minor contribution of secondary authors to these works.
Contribution to Presented in Thesis Full Peer-Reviewed Publications: Callow, J.N. and Smettem, K.R.J., 2007. Channel response to Chapter 3.1.3; Appendix 1A a new hydrological regime in southwestern Australia. Chapter 5.1.1; Geomorphology, 84(3-4): 254-276. *1 Chapter 5.1.2; Chapter 6 Callow, J.N., Van Niel, K. and Boggs, G.S., 2007. How does Chapter 4.2 Appendix 1B modifying a DEM to reflect known hydrology affect subsequent terrain analysis? Journal of Hydrology, 332(1-2): 30-39.
Unsubmitted Manuscripts: Callow, J.N. and Smettem, K.R.J., in prep. Decoupling Chapter 4.1 Chapter 4.1 landscapes: the effect of farm dams and constructed banks on connectivity in agricultural landscapes.
Conference Proceedings: Callow, J.N. and Smettem, K.R.J., 2006. Variable channel This presentation See Appendix responses following land clearing of a dryland catchment, contributes to 1C for Abstract Dalyup River, southwestern Australia. European Geophysical sections of Chapter Union General Assembly, Vienna, Austria. 6.3 Callow, J.N. and Smettem, K.R.J., 2004. Channel response to This presentation is See Appendix a new hydrological equilibrium in south western Australia. the basis of: 1D for Abstract Joint International Geomorphology Conference, Glasgow, Callow and Smettem Scotland. (2007) *1 Note that this paper has been split into several sections to suit the structure of this thesis. These sections expand the work presented in Callow and Smettem (2007) which focused on seven sites in the upper Kent River, to present data from across the 52 sites throughout the Kent River and Dalyup River investigated in the thesis.
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~ 12 ~ Chapter 2: Setting and Context of River Response
Chapter 2: Setting and Context of River Response
Looking down the Kent River from Mallawillup Road, at the Lake Carabundup crossing. To the left a pine plantation borders the floodplain and to the right salt-affected, dead floodplain vegetation gives way to stands of remnant native jarrah-marri woodland.
Overview
The south coastal rivers region of Western Australia (see Figure 2.1), is a stable and ancient landscape. Its coastal Mediterranean climate is strongly seasonal, grading into a more extreme and episodic semi-arid climate in the interior. Indigenous Australians have managed the land for at least the last 40,000 years (Turney et al., 2001) through frequent, small and organised burns (Jones, 1969; Hallam, 1975). Following European settlement from 1826, deep-rooted native species have been replaced by seasonal cropping and pasture. The majority of clearing has occurred in the last 50 years (Burvill, 1979), with increased recharge causing groundwater to rise bringing salts accumulated in the soil profile to the surface (Hatton et al., 2003). Across the region, less than 40% of the native vegetation remains (Beeston et al., 2001).
River response is influenced by: variations in climate, geology, hydrogeology, hydrology, soil, vegetation, and human activity. This chapter discusses the nature and variability of these over the region, with specific reference to the Kent and Dalyup River catchments. The spatial patterns are examined with reference to six regions; Kent-Frankland, Albany Hinterland, Pallinup-North Stirlings, Fitzgerald Biosphere, Esperance Sandplain and Esperance Mallee, adapted from sub-regions identified by the South Coast Regional Initiative Planning Team (SCRIPT) (see Figure 2.1).
~ 13 ~ Chapter 2: Setting and Context of River Response
Figure 2.1 Biogeophysical regions in the south coastal rivers region, based on boundaries from SCRPIT (2004).
This research focuses on two catchments in the south coastal rivers region: the Kent River and Dalyup River. The Kent River (catchment area 2,040 km2) is approximately 350 km south of Perth. Agricultural activity is concentrated in the upper catchment, west of the town of Cranbrook (pop. 270) (Australian Bureau of Statistics, 2001). The smaller townships of Frankland, Rocky Gully and Tenterden are also adjacent to the catchment (see Figure 2.2). The upper catchment is serviced by numerous sealed and unsealed roads. Much of the mid and lower catchment is native forest and is unpopulated. There are no sealed roads though the mid catchment, with limited access to the river by unsealed roads, tracks and fire breaks. The South Coast Highway crosses the lower catchment, passing from the former logging town of Walpole (pop. 300) located 30 km west of the lower Kent River to the tourist centre of Denmark (pop. 2,450) 30km east of the Kent River (Australian Bureau of Statistics, 2001). The only major tributaries, Nile Creek and Styx River join the main river in the lower catchment as the river flows past rural and recreational lots, before ending in Owingup Swamp (see Figure 2.2).
The Dalyup River is 725 km from Perth. The 850 km2 catchment is 35 km west of Esperance (pop. 9,400) (Australian Bureau of Statistics, 2001), a large rural centre and port servicing the region. The catchment is almost entirely cleared, and with land use dominated by broadacre cropping, with grazing of sheep and cattle. There are small, recreational lots, hobby farm and a vineyard along the lower catchment (below the South Coastal Highway). There are numerous unsealed roads through the catchment, and good access via farmland along the length of the Dalyup River. The West Dalyup and Dalyup Rivers are the major channels, meeting several kilometres south of the South Coastal Highway, before the terminating at Lake Gore, a Ramsar- listed wetland of international significance (see Figure 2.3).
~ 14 ~ Chapter 2: Setting and Context of River Response
Figure 2.2 Location of the Kent River study catchment
~ 15 ~ Chapter 2: Setting and Context of River Response
Figure 2.3 Location of the Dalyup River study catchment
~ 16 ~ Chapter 2: Setting and Context of River Response 2.1. Physical Landscape 2.1.1. Climate Southwestern Australia lacks large inland mountains which restrict the geographical extent of Mediterranean climates elsewhere in the world (di Castri and Mooney, 1973; Dallman, 1998). There is a strong climate gradient across the region from the wetter and cooler southwest to the warmer and semi-arid northeast. The maritime influence moderates the highly seasonal Mediterranean climate in the southwestern corner, which is classified as a Köeppen’s temperate climate with distinctly dry and warm summers and cold to mild winters with moderate rainfall (Figure 2.4) (Gentilli, 1972; Bureau of Meteorology, 1995). The semi-arid interior is characterised by more extreme temperature variations and significantly lower annual rainfall with high inter-annual variability (see Figure 2.5a-d). Throughout the region evaporation is in excess of rainfall for all but the winter months (Bureau of Meteorology, 1995).
Figure 2.4 Variation in average annual rainfall (mm) across the south coastal rivers region based on 1980-1999 data from CSIRO (2001).
Regional Climate Drivers
The strong seasonality that characterises this region is linked to migration of the global-scale Hadley and Ferrell Cells (Hadley, 1735; Sturman and Tapper, 1996; Dallman, 1998). The Hadley Cell originates in the tropics, forming a belt of low pressure (called the intertropical convergence zone (ITCZ)) in which the warm and moist tropical air ascends and travels poleward, eventually cooling and descending at higher latitudes to form the subtropical high pressure belt (Sturman and Tapper, 1996). The position of the ITCZ is associated with the region of maximum insolation, and migrates north-south with the seasons (Sturman and Tapper, 1996).
~ 17 ~ Chapter 2: Setting and Context of River Response During the southern hemisphere summer, the ITCZ is located south of the equator and the subtropical high pressure belt is generally located to the south of the study region. This results in warm and dry prevailing easterly winds from the arid, dry interior of the Australian continent. In winter the subtropical high pressure belt moves north of the study region, bringing onshore air flow. This allows cold fronts embedded in westerly moving low pressure cells to reach further north, bringing rainfall to the region (Sturman and Tapper, 1996).
Northwestern cloudbands can bring moist, warm tropical air from the Indian Ocean in a southeasterly direction over the region, leading to more intense and widespread precipitation, particularly when interacting with colder air from frontal systems (Sturman and Tapper, 1996; Wright, 1997; Telcik and Pattiaratchi, 2001). These two mechanisms generate most of the annual rainfall during the winter months (Wright, 1997; Telcik and Pattiaratchi, 2001). Local thunderstorms can deliver heavy but typically localised falls, particularly during summer months. Less common, but associated with the greatest hydrometeorological potential are ex- tropical cyclonic lows that occasionally track over the region during the cyclone season (November – April) (Wright, 1997; Telcik and Pattiaratchi, 2001). Cyclone tracks across southwestern Australia over the past four decades are shown in Appendix 2A.
Average Climatic Conditions
Rainfall is greatest in the south western corner, decreasing inland and eastwards as the moist air is precipitated (refer to Figure 2.4). Summer is particularly dry, with occasional frontal rainfall mainly in coastal areas. Local thunderstorms and ex-tropical cyclonic lows deliver rainfall during these months, but these are highly unpredictable. During winter, fronts generally reach the entire region, with strong climatic seasonality in rainfall, as shown in Figure 2.5a-d. Research into climate variability in southwestern Australia has found a 10-15% decrease in annual rainfall since the 1970s, driven by decreased May-July (early winter) rainfall (Indian Ocean Climate Initiative, 2002; Bari and Ruprecht, 2003).
Temperatures are moderated in coastal regions, becoming more extreme (both maximum and minimum temperatures) in semi-arid inland regions (see Figure 2.4 and Figure 2.5a-d). Average daily maximum temperatures are 25-300 C in summer, and 16-18oC during winter. Minimum temperatures vary from 12-14oC in summer, to 5-8oC in winter (see Figure 2.5a-d).
~ 18 ~
Chapter 2: Setting and Context of River Response
Average Temperature (deg C) (deg Temperature Average Average Temperature (deg C) (deg Temperature Average 35 30 25 20 15 10 5 0 35 30 25 20 15 10 5 0 Denmark daily minimum mean PO Salmon Gums meandaily maximum Denmark PO mean daily maximum daily mean PO Denmark Salmon Gums mean daily minimum daily Gums mean Salmon Average climatic conditions for Frankland Vineyard weather station weather Vineyard Frankland for climatic conditions Average Average climatic conditions for Salmon Research Gums Station weather station (error bars on rainfall data indicate 10th and 90th percentiles for monthly rainfall variability) rainfall monthly for percentiles 90th 10th and indicate data on bars rainfall (error (error bars on rainfall data indicate 10th and 90th percentiles for monthly rainfall variability) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Denmark PO mean monthly rainfall (annual average 1000mm) average (annual rainfall monthly mean PO Denmark Salmon Gums.Salmon mean monthly rainfall (annual average 342mm) 0 0 50
50
300 250 200 150 100
300 250 200 150 100 Monthly Rainfall (mm) Rainfall Monthly Monthly Rainfall (mm) Rainfall Monthly d b
Average Temperature (deg C) (deg Temperature Average Average Temperature (deg C) (deg Temperature Average 35 30 25 20 15 10 5 0 35 30 25 20 15 10 5 0 Denmark PO mean daily minimum daily mean PO Denmark Esperancemaximumdaily Res. mean Denmark maximum daily mean PO Esperance Res. minimum daily mean 90th percentiles for monthly rainfall variability) rainfall monthly for percentiles 90th Average climatic conditions for Denmark PO weather PO station weather Denmark climatic conditions for Average Average climatic conditions for Esperance Downs Research Station weather station weather Station Research Downs Esperance for conditions climatic Average (error bars on rainfall data indicate 10th and 90th percentiles for monthly rainfall variability) rainfall monthly for percentiles and 90th 10th data indicate on rainfall bars (error (error bars on rainfall data indicate 10th and Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Denmark PO mean monthly rainfall (annual average average (annual Denmark 1000mm) rainfall mean PO monthly Esperance Res. mean monthly rainfall (annual average 497mm) 0 0 50
50
300 250 200 150 100
300 250 200 150 100 Monthly Rainfall (mm) Rainfall Monthly Monthly Rainfall (mm) Rainfall Monthly a c
Figure 2.5a-d Monthly average rainfall (with 10th and 90th percentile rainfall bars) and daily maximum and minimum temperature for weather stations adjacent to the study catchments in the south coast rivers region. Data from: Bureau of Meteorology (2006)
~ 19 ~ Chapter 2: Setting and Context of River Response
Flood Hydroclimatology
Frontal systems are responsible for most of the annual rainfall in the study area. Large, slow moving or multiple frontal systems may generate moderate flood events, particularly when they interact with moist and warm tropical air originating from north-west cloud bands. Flood potential is maximised when the rainfall activity occurs on wet catchments where runoff from saturated source areas is greatest. While thunderstorms deliver high intensity rainfall, their limited geographic extent and the high infiltration capacity of the sandy surficial soils often modulates their flood generating potential. Ex-tropical cyclonic lows are capable of delivering the largest volume of rainfall, though these events are restricted to the cyclone season (November to April), when catchments are dry and runoff potential is lower.
Antecedent catchment conditions are important for generating large floods in this environment. For example, although ex-tropical cyclone Errol delivered 120 mm on 21st January 1982 over the dry upper and middle catchment of the Kent River (Bureau of Meteorology, 2005b), the greatest recorded streamflow resulted from a large cold frontal system (26th June 1988) that delivered a similar rainfall over three days, but onto a wet catchment (Bureau of Meteorology, 2005b). This resulted in a peak discharge of 120 m3 sec-1, compared to a peak discharge of 27 m3 sec-1 for the cyclonic event (Department of Environment, 2005f). The largest recorded daily rainfall in the study area, was in March-April 2005. This was caused by interaction between a north-western cloud band and a cold front and delivered 30-50 mm on the 31st March and 100- 130 mm on the 1st April over the upper Kent River catchment (see Appendix 2Biv). A backlog in processing gauging records means that the data for this event was not available for this thesis. In the more arid and ungauged Dalyup River catchment, oral history suggests that the largest recent flood occurred when ex-tropical cyclone Steve delivered the highest recorded daily rainfall (11th March 2000) on an catchment that was already wet from summer storms, resulting in a flood that had a peak discharge estimated at 300 m3 sec-1 (velocity 3.9 m sec-1) and 200-250 m3 sec-1 (velocity 3.5-3.8 m sec-1) where the Dalyup and West Dalyup Rivers respectively cross the South Coastal Highway (Water and Rivers Commission, 2002a). Appendix 2B gives more detailed information on the meteorological patterns responsible for flooding in the study catchments.
2.1.2. Evolution of the Rivers – Geology, Palaeohydrology and Palaeoclimatology
Geology
Geology of the south coastal rivers region is dominated by the hard crystalline rocks of the Archaean Yilgarn Craton, bordered by high-grade metamorphic Proterozoic Albany-Fraser Province to the south and bounded by the Perth (west), Bremer (far south) and Eucla (east)
~ 20 ~ Chapter 2: Setting and Context of River Response sedimentary basins (Figure 2.6). Tectonic stability, a wetter palaeoclimate with deep weathering and laterisation, and recent aridification have preserved elements of the ancient surface, and now have a strong influence on physical processes across the study region (Pen, 1999; Commander et al., 2001; Twidale, 2004).
Figure 2.6 Geology of the south coastal rivers region The Yilgarn Craton is the dominant tectonic feature of southwestern Australia, with the Albany Fraser Orogen to the south (comprising the Stirling Range Formation, Mt Barren Group, Biranup and Nornalup Complexes). This is bounded by the Perth (west) Bremmer (south) and Eucla (east) Basins.
The Yilgarn Block was a nucleus of the Gondwanan Supercontinent, with drainage from south (Antarctica) to north (Australia) prior to separation (van de Graaff et al., 1977; Ollier, 1988; Clarke, 1994). The wide valleys of palaeorivers, carved and scoured by northward movement of glaciers and glacial meltwater during the Permian (c. 290 – 250 Ma b.p.) are evident (Johnstone et al., 1973; Finkl and Fairbridge, 1979; Ollier, 1988; Clarke, 1994; Goodreid, 2000; Eyles and de Broekert, 2001). Rifting between Australian and Antarctica began in the Jurassic (c.150 Ma b.p.), with separation completed in the Late Palaeocene or Early Eocene (c.53-43 Ma b.p.) (Johnstone et al., 1973; Powell et al., 1988; Middleton, 1991; Veevers et al., 1991).
Crustal thinning around the Late Jurassic/Early Cretaceous (c.135-96 Ma b.p.) (Johnstone et al., 1973; Powell et al., 1988; Middleton, 1991; Veevers et al., 1991; White, 1994; Commander et al., 2001; Lawver and Gahagan, 2003) caused drainage to reverse (Clarke, 1994; Beard, 1999; Beard, 2003), causing rivers to flow south and east in the east of the southern coastal rivers region, and northwest in the west (Clarke, 1994; Beard, 1999; Commander et al., 2001; Beard, 2003). Southerly drainage was disrupted by epeirogenic uplift of the Jarrahwood Axis in the Oligocene, which formed a hinge line running 80-150 km inland of the present coastline (Cope, 1974; Cockhain and Hocking, 1990; Beard, 1999; Beard, 2003). Uplift of the axis also formed the south facing surface called the Ravensthorpe Ramp, which most rivers in the contemporary ~ 21 ~ Chapter 2: Setting and Context of River Response south coastal rivers region now flow across, with the northern watershed boundary associated with the Jarrahwood Axis (Cope, 1974; Beard, 1999; Beard, 2003) (Figure 2.7).
Figure 2.7 Location of the Jarrahwood and Stirling Range Axes, set against topographic and bathymetric data for the region. The location of the Jarrahwood Axis (Cope, 1974) and its proximity to the northern watershed of the south coast rivers region is evident, along with the secondary Stirling Range Axis (Beard, 1999; Beard, 2003) in the west that divides the Frankland and Kent River catchments. Palaeodrainage complexes are also evident from the salt lake chains (and topography) to the north (particularly the northeast) of the south coastal watershed. Clarke (1994) amended the position of the Jarrahwood Axis in the east, where it now divides the northern Lefroy and southern Cowan palaeorivers.
Smaller secondary axes parallel to the Jarrahwood axis have been identified, and are significant in determining current catchment boundaries. Beard (2003) named the Stirling Range Axis, which runs parallel to, and 50 km south of the Jarrahwood axis for 200km. Beard considered it responsible for diversion of the Gordon River to the west, into the Frankland River, rather than to the south across the ramp and so discharging into the Kent River. Ferdowsian & Greenham (1992) identified a third axis, the “Perillup Line”, which they concluded as responsible for rerouting of a single, large south flowing Eocene river to the present position of the Denmark River and Kent River watersheds. This feature also formed the large palaeo-lakes and sedimentary features in the mid Kent River catchment (refer to mapping by Myers (1995) and Smith (1997)).
~ 22 ~ Chapter 2: Setting and Context of River Response Palaeoclimatology and River Evolution
Across the southern coastal region, marine transgressions during the Eocene (c.45 Ma b.p.) deposited the Plantagenet Group sediments (Pallinup Siltstone over Werillup Formation) on top of the granite and gneiss basement (Muhling et al., 1985), filling valley floors up to about 100 km inland, with the Eocene shoreline now at around 300 m above present day mean sea level (Finkl and Fairbridge, 1979; Cockhain and Hocking, 1990; Ferdowsian and Ryder, 1997; Commander et al., 2001). Retreat of water levels and uplift has left ancient valley floors with low gradients and sluggish drainage where rivers now flow. The Tertiary sediments and palaeochannels affect contemporary catchment hydrology and salinity processes (Clarke et al., 1998; Goodreid, 2000; Commander et al., 2001; Abbott, 2002; Clarke et al., 2002), and are discussed in greater detail in the hydrogeology section below.
Following glacial retreat, the local climate was wet with rainforest covering most of southwestern Australia until the Eocene (c.45 Ma b.p.) (Kemp, 1978; Hopper, 1979; Cockhain and Hocking, 1990; White, 1994). The seasonal and pluvial regime caused laterisation of in-situ weathered soil profiles and marine sediments (Johnstone et al., 1973), leaving a landscape capped with a duricrust. From the Oligocene (c.37-30 Ma b.p.) climate became more arid (Kemp, 1978) and during the Miocene (c.17-10 Ma b.p.), a major shift in climate occurred when sea surface temperatures cooled, glaciation in Antarctica commenced, and rainfall decreased in southern Australia (Bowler, 1976; Kemp, 1978). Aridification has contracted the zone of laterisation to the far south-west coast, where annual rainfall exceeds 750 mm (Johnstone et al., 1973). Aridity reached its present extent around 80 Ka b.p. (Wyrwoll, 1979), with an arid phase from 20 – 10 Ka b.p. resulting in the formation of dune fields and saline playa lakes (Bowler, 1976; Wyrwoll, 1979; Harper and Gilkes, 2004).
Beard (2003) considers that the latest phase of uplift of the Darling Scarp in the Late Miocene- Early Pliocene (c. 10-5 Ma b.p.) resulted in increased tilt of the Ravensthorpe Ramp surface and the latest period of drainage rejuvenation, diversion, river headcuting and beheading (Ferdowsian and Greenham, 1992; Commander et al., 2001; Beard, 2003). The region has been tectonically stable since the Pliocene uplift of the Darling Scarp, with sea level and climate varying with glacial cycles during the Quaternary (last 1.8 Ma b.p.). The latest phase of river incision in coastal areas occurred during the last glacial maximum (20 Ka b.p.) when sea levels were over 100 m lower than their present levels (Hodgkin and Clark, 1988; Olsen and Skitmore, 1991). As sea levels rose, valleys were flooded around 8 – 6 Ka b.p. (Hodgkin and Clark, 1988; Olsen and Skitmore, 1991) and have filled rapidly with marine and riverine sediments (Hodgkin and Clark, 1988; Hodgkin and Clarke, 1989; Olsen and Skitmore, 1991; Hodgkin and Hesp, 1998). Between 4 – 3 Ka b.p., many of the estuaries became sufficiently shallow for bars to form across their mouths (Hodgkin and Hesp, 1998). Most rivers are now seasonally or
~ 23 ~ Chapter 2: Setting and Context of River Response permanently barred from the ocean, only opening after extreme flows or due to human interference. The Kent River empties into the Owingup swamp which drains into the Irwin Inlet via groundwater discharge (Semeniuk and Semeniuk, 2001) and the Dalyup River terminates at Lake Gore, a Ramsar-listed wetland of international significance located 12 km inland (Agriculture Western Australia, 2000).
Hydrogeology
The geological stability together with subdued relief and low erosion rates means that many relict features are present and are significant to present day landscape processes. On a regional scale, subterranean features including dolerite dykes that have extensively intruded granitic country rock and tertiary palaeochannels (discussed previously), affect the horizontal flux of groundwater and thus surface salinity (Engel et al., 1987; Salama et al., 1993b; George et al., 1997; Clarke et al., 1998; Goodreid, 2000; Clarke et al., 2002).
Hydrogeology of the upper Kent River is dominated by a series of Tertiary palaeochannels (mapped by Smith (1997)), orientated perpendicular to the main Kent River channel. Ferdowsian & Greenham (1992) and Salama et al., (1997) identified these features as palaeo rivers that flowed to the west, and filled with marine sediment during the late Eocene (Salama et al., 1997). Abbott (2002) modelled the basement topography, concluding that these groundwater systems discharge westward into the neighbouring Frankland River catchment.
Groundwater levels, trends and quality vary with landscape elevation. In the lower landscape, groundwater lies between 0-1m from the surface, rising an average 0.05 m yr-1 (Ryder, 2004b). Mid slope aquifers are variable, affected by climatic variation and land use, with long term monitoring showing some falling (<0.1 m yr-1), while most continue to rise (§ 0.1 – 0.2 m yr-1) (Ryder, 2004b). Water levels are typically less than 5-10 metres from the surface, with strong seasonal fluctuation linked to development of a perched water table in the duplex soils. In the upper landscape, groundwater tables are typically less than 10 metres below the surface, with most measured sites showing upward trends (0.1 m yr-1) (Ryder, 2004b). Groundwater quality varies with annual rainfall, with lower rainfall regions (<500 mm) having high salinity (>13,750 mg l-1) and pH (4.6). In the higher rainfall zone (>700 mm), salinity is lower (7,150 mg l-1) and pH less acidic (5.8) (Ryder, 2004b).
The hydrogeology of the Dalyup River system is different to the Kent River, due to its setting on top of predominantly sandy Tertiary deposits and the lack of relict drainage features. Because the soils are sandier, the aquifers are more reactive. Groundwater trends show mixed but typically upward trends, varying between 0 – 0.55 m yr-1 rise (Agriculture Western
~ 24 ~ Chapter 2: Setting and Context of River Response Australia, 2000; Simons and Alderman, 2004). Bores in mid slope positions, and those in more reactive aquifers with higher hydraulic conductivity, show greater response to rainfall variation and land management changes. Groundwater quality is generally poor across both study catchments, particularly in clayey alluvial soils in low landscape positions where water is saline (less than 16,000 mg l-1), though there are localised perched aquifers that contain brackish water (less than 2,000 mg l-1) (Simons and Alderman, 2004).
2.1.3. Drainage and Hydrology The geological setting has led to evolution of southern coastal catchments that have many similarities. Table 2.1 highlights similar catchment area, and the gradual west to east gradient of rainfall and annual discharge (higher to lower), with worsening water quality under the influence of the drier climate and increasing trends in the amount of cleared land.
Table 2.1 Characteristics of rivers in the southern coastal rivers region. From: Pen (1999). Rivers are listed from west to east. Of note is the decreasing rainfall gradient and accompanying decrease in water quality. There is also a trend of increased clearing to the east. Only the perennial rivers in the high rainfall zone have sufficient discharge to maintain an open link with the ocean. Catchment Receiving Length Area Median Clearing Annual Water coastal water of main (km2) Rainfall (%) Flow Quality body or channel (mm) Ml wetland (km) Donnelly Ocean 60 1,670 1200 11 310,000 Fresh Warren Ocean 150 4,310 850 35 380,000 Marginal Gardner Ocean 35 530 1420 16 125,000 Fresh Shannon Broke Inlet 47 610 1320 10 145,000 Fresh Deep Nornalup Inlet 120 1,000 1120 3 140,000 Fresh Frankland Nornalup Inlet 400 4,650 600 56 200,000 Marginal/ Brackish Kent Irwin Inlet 100 2,040 780 42 123,000 Marginal Denmark Wilson Inlet 60 690 850 15 450,000 Fresh/ Marginal Hay Wilson Inlet 80 1,280 760 60 78,000 Marginal/ Brackish Kalgan Oyster Harbour 140 2,560 600 70 52,000 Brackish Pallinup Beaufort Inlet 150 4,970 410 80 36,000 Saline Bremer Wellsted Inlet 70 720 465 75 14,000 Saline Gairdner Gordon Inlet 130 1,770 430 40 10,700 Saline Fitzgerald Fitzgerald Inlet 80 1,610 420 35 5,400 Brackish Phillips Culham Inlet 50 1,940 400 35 2,000 Saline Jerdacuttup Jerdacuttup 65 2,320 415 30 8,800 Saline Lakes Oldfield Oldfield Inlet 95 2,480 440 30 8,100 Brackish/ Saline Young Stokes Inlet 120 1,610 400 75 5,880 Brackish/ Saline Lort Stokes Inlet 100 2,800 375 60 6,000 Saline Dalyup Lake Gore 35 660 450 80 11,000 Brackish Bandy Mullet Lakes 30 1,380 465 85 6,400 Brackish/ System Saline Note- Water Quality Criteria: Fresh (0 – 500 mg l-1), marginal (500 – 1500 mg l-1), brackish (1500 – 5000 mg l-1), Saline (> 5,000 mg l-1).
~ 25 ~ Chapter 2: Setting and Context of River Response The catchment area of the Kent River is 2,040 km2, but a series of lakes in the upper catchment that only occasionally overflow results in an effective catchment area that is much smaller. This is a feature common to many catchments in southwestern Australia, with the contributing area expanding by up to three or four times during high rainfall events (e.g. see Appendix O in Mayer et al., 2005). The Nile Creek and Styx River that join in the lower part of the catchment are the most significant tributaries, with a series of minor tributaries, mostly in the upper catchment. The catchment has been gauged since 1956 and the movement of water and salts through the catchment is relatively well understood, with the upper catchment contributing only one third of the annual discharge but two thirds of the salt (Kington and Pannell, 1999; Kington and Smettem, 2000; Winter, 2000). Flow is perennial, with peak flow at the end of winter (Figure 2.8). The water quality is classed as “Marginal”, with dissolved salts averaging 1700 mg l-1 and is increasing at 14 mg l-1 per annum (although the rate of increase has slowed in the last 10 years) (Department of Environment, 2004a).
The Dalyup River has two main tributaries, the West Dalyup and Dalyup which drain similar areas of the 660 km2 catchment, joining only 8 km upstream of Lake Gore. The upper catchment is characterised by low gradients and deep sandy soils, with drainage lines becoming discontinuous with sporadic runoff. The catchment watershed is not clearly defined and is dynamic, dependent on rainfall distribution and intensity (N. Middleton, pers. comm.). Data from a neighbouring catchment (Figure 2.8) shows that flow is greatest during winter (June), but not as seasonal as the Kent River. This is due to this semi-arid catchment being affected by infrequent and very large flow events such as ex-tropical cyclones during summer that skew the data (e.g. the largest observed flow in the Dalyup River in March 2000, associated with an ex- tropical cyclone (Water and Rivers Commission, 2002a)).
Mean daily discharge (Ml) from Styx Junction Mean daily discharge (Ml) from Neds Corner A Gauging Station (604053), Kent River. B Gauging Station (601001), Young River. (Catchment area above gauging station 1,862 km2) (Catchment area above gauging station 1,893 km2) 800 800 45 Monthly Discharge (Primary Axis) 700 700 40 Monthly Discharge (Secondary Axis) 35 600 600 30 500 500 25
400 400 20 15 300 300 10 200 200 5 Mean Daily Discharge (Ml) Discharge Daily Mean Mean Daily Discharge (Ml) Discharge Daily Mean Mean Daily Discharge (Ml) 100 100 0 0 -5 0 y il r r l r r e ry i e ly er er rch July b n ust b be b rua Apr May June mbe m Apr May Ju g nuary Ma e rua Ju u m a August te v b March A te J October o January p Octo Feb Fe e ovember Sep N December S N Decem Figure 2.8 Mean daily discharge from stations 604053 (A) and 601001 (B) at the bottom of the Kent River and Young River (respectively). Data from Department of Environment (2005f; 2005d). The seasonality of flow is highlighted in the Kent River (A), where mean daily discharge peaks in August. For the Young River (B), flow is significantly lower, for a catchment with almost identical catchment area to the Kent River. The influence of extreme events linked to ex-tropical cyclones is evident from increased flow in January to March. These trends are likely to be representative of flow characteristics in the Dalyup River.
~ 26 ~ Chapter 2: Setting and Context of River Response 2.1.4. Soils Soil development in the study region is largely controlled by the geological parent material, tectonic processes and climatic regime described above (Churchward et al., 1988; McArthur, 1991). Soil landscapes of the agricultural regions in Western Australia were first mapped in a comprehensive manner by the Department of Agriculture (Teakle, 1938) and then advanced by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) (Smith, 1952; McArthur and Bettenay, 1960; Mulcahy, 1967). More recently the Western Australian Department of Agriculture has mapped the region at larger scale, filling gaps and updating previous surveys using a unified soil classification scheme (Schoknecht, 1999; Schoknecht et al., 2004). This work concentrated on mapping land resources and capabilities for agricultural development as well as identifying environmental management problems such as sodic and acid-sulphate soils (Nicholas et al., 2003; Schoknecht et al., 2004).
Figure 2.9 identifies broad trends in soil characteristics across the region. Loamy-duplex and gravelly lateritic soils are dominant in the higher rainfall zone in the south west corner. Towards the Esperance Mallee and Esperance Sandplain regions, soils grade to sandy duplexes, deeper sandy duplexes and deep sands with decreasing fertility. “Duplex” or “texture-contrast” soils are a feature of southwestern Australia (Agriculture Western Australia, 2002a). They are characterised by sandy topsoil with high infiltration capacity that overlies clay subsoil (B- horizon) with significantly lower infiltration capacity. As a consequence, throughflow is the dominant flux path from hillslope to stream, with saturation-excess overland flow in concave depressions and low landscape positions (Ruprecht and Schofield, 1989; Ruprecht and Schofield, 1991; George and Conacher, 1993; Ruprecht and Stoneman, 1993). The high infiltration capacity of surface soils limits potential for Hortonian overland flow (Ruprecht and Schofield, 1989). The soil survey maps at 1:100,000 and 1:250,000 scale and accompanying reports by various authors (Burvill, 1935; Burvill, 1988; Churchward et al., 1988; Nicholas and Gee, in prep; Overheu, in prep; Stuart-Street, in prep), provide a more comprehensive guide to soil landscapes and properties across the southern coastal rivers region.
The soil patterns in the Kent River catchment are typical of those in the western sector, where rejuvenated drainage has incised the lateralised landscape. The valley floors are typically wide (often residual palaeo drainage lines), filled by fine-grained alluvial and colluvial sequences overlying tertiary sediments (Kelly, 1995; Salama et al., 1997; Pen, 1999; Agriculture Western Australia, 2004a). The surrounding undulating country is characterised by duplex soils, clay loams and duplex gravely sand soils, topped with remnant lateritic ridges (Agriculture Western Australia, 2004a). In the mid and lower catchment the landscape is more incised, with gravely sands and loamy duplex on laterite, and loamy earths developed from gneiss and granite in the higher rainfall zones (Agriculture Western Australia, 2004a). Exposed granite outcrops and
~ 27 ~ Chapter 2: Setting and Context of River Response
Figure 2.9 Soils of southwestern Australia. Source: Agriculture Western Australia (2002a). The soil landscape of Western Australia varies from the gravely and sandy duplexes derived from the incised lateritic profiles in the south west corner, to shallow duplexes further inland and deeper sands in the east.
~ 28 ~ Chapter 2: Setting and Context of River Response preserved duricrust breakaways are also a common feature from the mid catchment (Kelly, 1995). In the coastal zone, marine sediments overlie the bedrock which is sporadically exposed across the plain, but common along the coast. Churchward et al., (1988) and Stuart-Street (in prep) provide more detailed soils information for the Kent River.
Soils in the Dalyup River catchment have developed on top of tertiary marine sediments. Shallow and deep grey sandy duplex and pale deep sands are the most dominant soil groups, occupying 80% of the catchment (Agriculture Western Australia, 2000; Water and Rivers Commission, 2002a). Soils become increasingly clayey in the valley floors and the areas surrounding Lake Gore (Agriculture Western Australia, 2000; Water and Rivers Commission, 2002a). For more detailed information on soil landscapes in the Dalyup River, consult Nicholas & Gee (in prep) or section 2.1 of Agriculture Western Australia (2000).
2.1.5. Vegetation The south coast rivers region is within the South-West Botanical Province, spanning the Roe, Eyre and Darling Districts (Beard, 1975; Beard, 1980) (refer to Figure 2.10). The area is internationally recognised for its biodiversity, rich in species that have evolved in an old and nutrient-poor landscape (Hopper and Gioia, 2004). Progressive aridification from the Oligocene/Miocene saw rainforest taxa die out by the Pleistocene, to be replaced by sclerophyllous shrubs, trees, and herbs growing in eucalypt forests, woodlands and mallee, and heathlands, shrublands and herbfields (Hopper, 1979; Hopper and Gioia, 2004). Present day vegetation patterns reflect the complex evolutionary biogeography, decreasing rainfall gradient from the south west to east-north-east (see Section 2.1.1), and the changes in the soil-landscape across the region (Gardner, 1941; Beard, 1975; Hopper, 1979; Beard, 1981; Hopper and Gioia, 2004). Vegetation surveys cover the entire region at 1:250,000 scale (Beard, 1972; Smith, 1972; Beard, 1973b; Beard, 1973a; Beard, 1979) and 1:1,000,000 scale (Beard, 1975; Beard, 1981).
Beard (1979; 1980; 1981) mapped the Kent River area which spans the Warren and Menzies botanical provinces (see Figure 2.10). An open Jarrah-Marri forest dominates the upper and mid catchment, which lies in the Menzies sub-district of the Darling district. Eucalyptus occidentalis (Yate) and Meleleuca sp. (Paperbark) are common in the low-lying and waterlogged flats (Beard, 1979; Beard, 1980; Beard, 1981). Sedges (predominantly Gahnia sp., Juncus sp.) form dense and continuous clumps at the channel margins in the higher rainfall areas (Beard, 1979; Beard, 1980; Beard, 1981). As rainfall decreases eastwards and soils become sandier, vegetation communities become progressively more open and mallee forms become dominant (Beard, 1979; Beard, 1980; Beard, 1981). In more arid regions and on poorer, sandy soils, open and low mallee woodlands, give way to diverse Banksia-Melaleuca-Dryandra scrub-heath (Beard, 1979; Beard, 1980; Beard, 1981).
~ 29 ~ Chapter 2: Setting and Context of River Response The Warren sub-district (south western coastal strip) contains the only area of tall wet sclerophyll forest, characterised by Karri (Eucalyptus diversicolor) on fertile red earth soils and mixed Jarrah (Eucalyptus marginata) and Marri (Eucalyptus calophylla) open forest on shallow duplex red and yellow loamy earths, grading to Jarrah-Marri woodland in poorer sandy duplex soils (Beard, 1979; Beard, 1980; Beard, 1981). The seasonally waterlogged valley-floors are characterised by Paperbark and sedges (Gahnia sp., Juncus sp.) (Beard, 1979; Beard, 1980; Beard, 1981). On the lateritic, ironstone and deep sands, the Marri-Jarrah forest grades into stands of Sheoak (Eucalyptus Casuarina) (Beard, 1979; Beard, 1980; Beard, 1981). Toward the coast, the Jarrah-Marri woodlands grade to a mixed Banksia-Eucalypt low woodland, with scrub-heath on the deeper aeolian and marine sands of the coastal plains, and with paperbark and sedges dominant in low, waterlogged landscape positions (Beard, 1979; Beard, 1980; Beard, 1981).
Figure 2.10 Vegetation provinces in the south coastal rivers region, adapted from Beard (1975; 1981). The vegetation provinces grade from the Warren system with the tall Eucalypt forests in the high rainfall south western corner, through the Menses system with Jarrah-Marri low forest and woodlands to the more arid Avon, Eyre and Roe Provinces, where shrubby heath and low mallee woodlands dominate.
Beard (1973a; 1973b; 1975) identified four vegetation systems within the Dalyup River. The Fanny’s Cove system is characterised by Banksia scrub-heath on deep aeolian sands of the undulating coastal dunes found between Lake Gore and the coast. Inland, the Esperance system covers the mid-catchment and features a mosaic of Eucalyptus mallee-heath with a diverse Banksia and Dryandra understorey in sandy duplex soils, which grades to scrub-heath in deeper sands. In wetter areas, Swamp Yate woodland dominates, with more permanently waterlogged areas characterised by Melaleuca and mixed sedge species (Agriculture Western Australia, 2000). The upper catchment is dominated by the Eucalyptus mallee-scrub and scrublands of the Ridley and Lort systems (Beard, 1973a; Beard, 1973b; Beard, 1975). ~ 30 ~ Chapter 2: Setting and Context of River Response 2.2. Human Impact on the Landscape 2.2.1. Pre-European landscape Southwestern Australia has been managed by Indigenous Australians for at least the last 40,000 years (Turney et al., 2001). Their land management practices involved the maintenance of grassland and thinning of woodlands and forests using regular, controlled and organised small- scale “firestick agriculture” methods (Jones, 1969; Hallam, 1975; Olsen and Skitmore, 1991). These management techniques had both short-term impacts due to increased runoff following firing and long term impacts which may have contributed to aridification of the interior of Western Australia (Olsen and Skitmore, 1991). These practices persisted over long timescales, with Olsen & Skitmore (1991) describing them as “consistent with the current and popular concept of sustainable development”. It can reasonably be assumed that the catchment was in equilibrium with indigenous land management by the time of European settlement.
2.2.2. European settlement and agricultural development Europeans settled in Albany (then Fredrickstown) in 1826 and for the first 60 years population growth was slow as settlers struggled to grow crops or find large tracts of suitable land (Burvill, 1979). Clearing of native vegetation for crops was slow, and generally restricted to the more fertile river valleys with alluvial soils (Burvill, 1979). An economic boom associated with a gold rush in the 1890s increased population and corresponded with a time when fertilisers and other methods to increase agricultural productivity became more widely available (Burvill, 1979). Failed gold miners provided a labour source, with land released under the “Conditional Purchase” agreements at discounted rates, conditional on land being cleared and cropped immediately (Burvill, 1979; Beresford et al., 2001).
Conditional Purchase and the later “Group and Soldier Settlement Schemes”, where land was offered in ballots (with the same conditions as conditional purchase) to returned servicemen following World War I, saw the area of cropping increase. Land to the east of Perth was extensively cleared, before rates of land development slowed during the depression in the late 1920s (Beresford et al., 2001). After World War II the availability of civilian and ex-military heavy machinery, expanded use of fertilisers, and recognition of trace-element deficiencies in sandy soils saw agriculture push further east and along the south coast into increasingly marginal land (Burvill, 1979; Beresford et al., 2001). In the 1950s and 60s, large profits from high wool prices stimulated continued expansion and land clearing (Burvill, 1979), with the boundaries of the “wheatbelt” (region of broadacre agricultural in southwestern Australia) at its current limits by the 1970s (Beresford et al., 2001). Since then clearing of alienated land (land that was allocated but left uncleared) within these boundaries has continued, with clearing in southwestern Australia estimated to average 6,000 ha yr-1 (Australian Conservation Foundation, 2001).
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2 Area of crops and pastures (millions ha) (millions and pastures ofArea crops
0 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 Year Figure 2.11 Area of land used for cropping and sown pasture in Western Australia. Data from: 1837 – 1950 data from Burvill (1979); 1966 – 1997 data taken from Western Australian Year Books 1966 – 1997 (Government of Western Australia, 1966-1997). The agricultural sector was stagnant from the time of settlement (1829) until large scale release of land to ex-goldminers in the 1890s. Soldier settlement schemes following World Wars I and II saw increases in cleared land under crop. The rapid expansion during the 1960s and 70s can be attributed to advances in technology and machinery which saw expansion into the eastern wheatbelt, with limited expansion since the 1980s.
Parts of the Kent River were grazed shortly after settlement, but land was not cleared for cropping until the 1870s (Kelly, 1995). Even then, clearing was slow, with better cropping land cleared by hand and the remainder of the forest grazed. Most broad-scale clearing occurred after the 1940s (Webb, pers. comm.) and continued until clearing restrictions were imposed in 1978 (Kelly, 1995). In the Dalyup River catchment, land was allocated from the 1890s, with small areas of land cleared along the lower Dalyup River, and sporadic grazing activities elsewhere. Much of the land was abandoned during the depression (1929) and only reoccupied in the 1950s and 60s. Recognition of the trace element deficiency of these soils, and the advent of broadacre cropping machinery, and high commodity prices spurred massive land clearing in the area during the 1960s and 70s (Burvill, 1979; Water and Rivers Commission, 2002a).
2.2.3. Present Land Use Data from the National Land and Water Resources Audit (Beeston et al., 2001), shows that land use is dominated by broadacre cropping and mixed cropping with sheep/cattle grazing farming systems across the south coastal rivers region (Table 2.2). Agricultural use on cleared land accounts for 56.1% of the area, with 38.6% of vegetation in natural or near-natural condition, 2.6% grazed pastoral leases and 2.4% plantation forestry.
The Kent River study catchment has a larger proportion of remnant vegetation (45%) predominantly in the lower part of the catchment (Figure 2.12), with agriculture dominating land use in the upper catchment (39% of total catchment area). Plantation forestry, which accounts for 12% of total catchment area (Beeston et al., 2001), is also concentrated in the upper ~ 32 ~ Chapter 2: Setting and Context of River Response catchment, with most plantations established in the last decade (Kelly, 1995; Kington and Smettem, 2000). The Dalyup River catchment is 97% cleared for agriculture, with the only other land use being a reserve surrounding the outlet of Lake Gore and an Aboriginal reserve in the upper catchment.
Table 2.2 Land use in the south coastal rivers region. Source: (Beeston et al., 2001) Cropping (3.2 million ha (54.7%)) is the predominant land use in the 5.9 million ha south coastal rivers region, with and additional 77,866 ha (1.4%) cleared for other agricultural activities. Native vegetation classified under various land uses occupies 2.3 million ha (38.7%). Land Use Area (ha) Percent of total area Broadacre Agriculture 3218964 54.7% National Park 764122 13.0% Remnant vegetation 759048 12.9% Managed for resource protection 580002 9.9% Reserves for nature conservation, 157017 2.7% species or habitat protection Livestock grazing (rangeland) 150132 2.6% Plantation forestry 142784 2.4% Grazing sown pastures 47077 0.8% Intensive agriculture 30789 0.5% Water body 12151 0.2% Traditional indigenous use 8265 0.1% Other minimum intervention use 8015 0.1% Residential 686 0.0% Services (roads) 457 0.0%
Land use patterns in parts of the region are dynamic. Growing involvement in the landcare movement since the 1980s has seen some areas regenerated or replanted with native species (Conacher and Conacher, 1995; Kington and Pannell, 1999; Kington and Smettem, 2000). Commercial timber plantations (predominantly Tasmanian Blue Gum (Eucalyptus globus) and Pinus radiata) have become increasingly important, particularly in moderate rainfall districts (600+mm) (Kelly, 1995; Kington and Pannell, 1999; Bartle et al., 2000; Kington and Smettem, 2000; Mercer and Underwood, 2002). Plantations have been successful in lowering mid-slope aquifers, and have been widely advanced as a potential solution to the salinity problems facing the region (Stolte et al., 1997; Bell, 1999; Hatton and Nulsen, 1999; Bartle et al., 2000; George et al., 2001; Pannell, 2001; Mercer and Underwood, 2002; Pannell and Ewing, 2004). Changes in land management practices and moves to change traditional farming systems to adopt high- water using crops (and pasture) show that land use and management philosophies are dynamic at present and the future is difficult to predict.
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Figure 2.12 Land use patterns in the Kent River and Dalyup River. Source: (Beeston et al., 2001). A large proportion of the lower Kent River is remnant vegetation, with the upper catchment dominated by cropping, with some remnant bushland pockets and plantation timber. The Dalyup River is a stark example of clearing of wheatbelt catchments, with 97% of the land cleared for cropping.
Summary
The landscape of southwestern Australia has a long history which impacts on present day geomorphic processes. The geologic and tectonic evolution of the region has a large influence on the contemporary drainage pattern, hydrogeology and salinity. Within the study catchments, river patterns and processes are variable, constrained by boundary conditions and independent channel controls area linked to both relict and contemporary landscapes.
While European settlement began 180 years ago, widespread clearing of native vegetation has occurred in only the last 50 years. Given the lag period between clearing and when groundwater reached the surface, man made hydrologic transformation of the landscape has only occurred in the last few decades. Land clearing may have altered channel boundary conditions (sediment and water flux), factors fundamental to the maintenance and change of river geomorphology. The extent of change in these is investigated in the next chapter, and their potential to affect landscape stability considered.
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Chapter 3: Land Clearing and Boundary Conditions
Looking up the Young River (40 km west of the Dalyup River), close to the West Point Road crossing at one of the last large areas of mallee scrubland left in the Esperance Mallee region. This is the same vegetation landscape that once covered most of the Dalyup River catchment.
Introduction
Identification of rivers as dynamic systems sensitive to flux of water and sediment has brought with it the realisation that human activity can change rivers. Human impact takes the form of direct intervention (such as channel engineering) or indirect actions that inadvertently affect river processes (Warner and Bird, 1988). Within this latter class, a distinction can be made between point-source (e.g. dam construction) and diffuse impacts such as land clearing. Across the world, there is evidence that transformation in river morphology is associated with land clearing for agriculture that has altered water and sediment movement through landscapes (Wolman, 1967; Knox, 1977; Klimek, 1987; Starkel, 1987; Starkel, 1988; Mei-e and Xianmo, 1994; Brooks and Brierley, 1997).
Inductive (i.e. investigation and formulation of after the fact hypotheses to explain specific observed phenomena), deductive (i.e. hypotheses testing), and retroductive (i.e. inferring current or future processes and responses from geomorphic features generated by preserved past events) investigation methods are applied here in investigating channel response to perturbation (Baker, 1995; Kondolf and Piegay, 2003a). This study uses deduction to investigate whether the landscape sensitivity concept applied at the reach-scale is able to consistently and predictably describe the response of channel morphology to land clearing (and associated processes). This research attempts to understand why certain changes in channel morphology have occurred, and whether this understanding might suggest future changes. This chapter establishes the magnitude and temporal scales of change in the discharge regime and sediment flux since European settlement and land clearing.
~ 35 ~ Chapter 3: Land Clearing and Boundary Conditions The range of available and appropriate research tools is limited by the research aim (Kondolf et al., 2003), actual river geomorphology (i.e. whether processes create and preserve particular geomorphic features) and external factors such as project time-lines and available resources such as project budget and size of the research team. Data limitations (e.g. gauging records, scale and frequency of aerial photography), a short project lifespan, limited financial resources, lack of previous research on river geomorphology in both study catchments, the independence of this research, and remote field locations that are between four and ten hours away from Perth by car and are largely inaccessible during moderate rainfall events, has affected the tools available for determining landscape changes and river response to land clearing in this study.
Kondolf and Piégay (2003b) identified five “frameworks” of investigation. The range of tools and methods used for this thesis fall within each of these frameworks, as outlined in Table 3.1. Schumm et al. (1987) suggested that various scales of investigation that collate, compare and review information from a variety of sources (system approach) are more likely to be successful in understanding fluvial geomorphic processes. This thesis adopts this philosophy and uses a range of tools and methods in applying the landscape sensitivity concept to investigate changes in boundary conditions, system resistivity and connectivity, thresholds and system response to perturbation to land clearing. This chapter firstly investigates the hydrologic response (Section 3.1) to land clearing using a water-balance approach. The second section (Section 3.2) investigates changes in sediment transport in response to land clearing in the study catchments.
Table 3.1 The various tools and methods applied in this study, based on the framework proposed by Kondorf and Piegay, 2003b Historical Framework x Archival aerial photography x Exploration journals x Oblique re-photography (photographing sites where oblique photos that document changes in river condition over time) Spatial Framework x Reach-based approach (RiverStyles® Framework) that investigate the reaction of specific river styles within two rivers to land use change and increased salinity. The reaction of specific reaches in catchment context and propagation of change through the system. x Aerial photography Chemical, Physical and Biological Framework x Analysis of changes in stream salinity from gauging records x Vegetation mapping and investigation of the changes in the stream salinity and effects on vegetation Process and Form Analysis x Channel morphological analysis – channel surveying x Analysis of changes in discharge from gauging records Future Framework - Modelling and Simulation x Using Digital Elevation Models (DEMs) for simulating spatial variability in processes x Numerical modelling of channel adjustment potential and future directions of change x Conceptual and numerical analysis approach to simulating changes in land management, vegetation and channel erosive potential
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Sections of this chapter are based on Callow and Smettem (2007) 1
3.1. Effects of Land Clearing on Discharge Across all climate and rainfall zones, woodlands and forests are more efficient users of water than cultivated crops and pastures (Zhang et al., 1999). In dryland regions this difference is even greater as deep-rooted vegetation has evolved under high water stress conditions, and has adapted by developing deep root systems that extract water stored in the soil profile during the many months of water deficit. Agricultural crops and pasture by comparison, have shallow rooting zones and are seasonal rather than perennial. The replacement of native vegetation systems by agricultural crops has altered water flux in favour of increased groundwater recharge and runoff to maintain water balance. Studies investigating changes in the discharge regime following land clearing have consistently found that river flow has increased following land clearing, often with serious geomorphic consequences (Trimble, 1974; Knox, 1977; Gregory and Madew, 1982; Klimek, 1987; Knox, 1987; Jacobson and Pugh, 1992; Lorup et al., 1998; Walling, 1999; Zhang et al., 1999; Clark and Wilcock, 2000; Schreider et al., 2002).
Similar trends have been reported from gauged catchments in southwestern Australia (Wood, 1924; Peck, 1978; George and Conacher, 1993; Ghassemi et al., 1995; Clarke et al., 2002; Bari and Ruprecht, 2003; Hatton et al., 2003; Mayer et al., 2005). The magnitude of these changes is summarised in Muirden et al. (2003) (see Figure 3.1), though trends are generalised. Specific response of catchments to land clearing is affected by the spatio-temporal variation of clearing through catchments and by the physical landscape. Revegetation and growth of commercial tree plantations has reduced cleared area (Kington and Smettem, 2000; Beeston et al., 2001; Webb, pers. comm.), while reduced rainfall in southwestern Australia since the 1970s (Allan and Haylock, 1993; Ruprecht et al., 1996; Smith et al., 2000; Indian Ocean Climate Initiative, 2002) may modulate changes in discharge. The complex interaction between land use, climatic change, landscape and potential system response across the study region (see Chapter 2), required investigation of the response of boundary conditions in the Kent River and Dalyup River following land clearing.
Callow, J.N. and Smettem, K.R.J., 2007. Channel response to a new hydrological regime in southwestern Australia. Geomorphology, 84(3-4): 254-276. (A copy of this paper is presented in Appendix 1A) ~ 37 ~ Chapter 3: Land Clearing and Boundary Conditions
Figure 3.1 Change in runoff with land clearing in southwestern Australia. From: (Muirden et al., 2003) Research from control and cleared catchments in southwestern Australia shows a clear link between clearing and the rates of runoff. For low to medium rainfall catchments, clearing can increase annual runoff by between 2-5 times.
The aim of the following sections is to examine the specific changes in streamflow following land clearing. Changes to annual discharge, flow duration (perenniallity), flood responses, bankfull discharge are investigated as indicators of the changes in hydrologic boundary conditions that have the potential to affect geomorphological stability. The following sections investigate changes in catchment hydrology from a water balance perspective, studying changes to precipitation (Section 3.1.1), groundwater recharge (3.1.2) and streamflow (Section 3.1.3) that have resulted from land clearing (alteration of the evapo-transpiration component of the water balance).
3.1.1.Changed Rainfall in Southwestern Australia Annual rainfall has decreased across southwestern Australia since the mid 1970s (Allan and Haylock, 1993; Ruprecht et al., 1996; Smith et al., 2000; Indian Ocean Climate Initiative, 2002). Decreased early winter (May-July or MJJ) rainfall is largely responsible for the 10-15% reduction in annual rainfall over southwestern Australia (Indian Ocean Climate Initiative, 2002). Decreased rainfall has coincided with increased atmospheric pressure (Allan and Haylock, 1993; Smith et al., 2000), higher sea surface temperatures (SSTs) (Smith et al., 2000), and with fewer frontal depressions reaching southwestern Australia (Simmonds and Keay, 2000); though no causal link has been conclusively established. Ruprecht et al. (2005) investigated changes in extreme rainfall, reporting no changes in winter extreme rainfall but an increased frequency of summer extreme rainfall, but cautioned that determining trends for rare events was particularly difficult given the short record length. The Indian Ocean Climate Initiative (IOCI) has identified the impact of reduced rainfall on annual streamflow in Perth’s water supply catchments. In uncleared water supply catchments, long term average annual inflow is now 41% lower than the pre 1970 period (Indian Ocean Climate Initiative, 2002).
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Bates et al. (2001) found that of the six climatic states in southwestern Australia, the frequency of “state 5”, in which high pressure systems bring dry conditions across the southwest, had increased. Occurrences of “state 3”, in which low pressure troughs in the westerly stream bring rainfall across the region and heavy falls in coastal areas, have decreased. The increased prevalence of state 5 at the expense of state 3 appears linked to increased mean atmospheric pressure as described by Smith et al. (2000). Simmonds & Keay (2000) reported a decreased number of cold fronts passing over the region, but increased atmospheric pressure and SSTs in the Indian Ocean (Smith et al., 2000) have increased activity of northwestern cloudbands, which bring moist tropical air from the Indian Ocean over large areas of southwestern Australia (Gentilli, 1972; Sturman and Tapper, 1996). This process may explain increased annual and summer rainfall in inland areas (Indian Ocean Climate Initiative, 2002).
Climate change models for southwestern Australia show some scatter in the magnitude of predicted change in rainfall (see Figure 3.2) (Allan and Hunt, 1999; Indian Ocean Climate Initiative, 2002). Most predict that changes in winter rainfall will either remain at current levels or decrease further over the next 100 years, with summer rainfall predicted to either remain the same or increase slightly, though the magnitude of this change would be insignificant compared to reduced winter rainfall (Allan and Hunt, 1999; Indian Ocean Climate Initiative, 2002). While little data is available for the future changes in extreme daily rainfall events, the same models used in Figure 3.2 predict SSTs increasing by 2-4.5oC by 2100 (Allan and Hunt, 1999). Walsh and Katzfey (2000) predict increased poleward track, intensity and longevity of tropical cyclones with higher SSTs in the Indian Ocean suggesting that ex-tropical cyclones could become more frequent and significant contributors to annual rainfall and flood probability in southwestern Australia.
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-10 -10 Change in DJF Rainfall DJF in Change -30 inChange MAM Rainfall -30 (in comparison to 1961-200 average) 1961-200 to comparison (in 2010 - 2039 2040 - 2069 2070 - 2099 average) 1961-200 to comparison (in 2010 - 2039 2040 - 2069 2070 - 2099 -50 -50 CCC-na CCC-a HADCM2-na HADCM2-a GFDL-na CCC-na CCC-a HADCM2-na HADCM2-a GFDL-na a GFDL-a MPI-na MPI-a CSIRO-na CSIRO-a bGFDL-a MPI-na MPI-a CSIRO-na CSIRO-a
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-10 -10 Change in JJA Rainfall Change in SON Rainfall SON in Change -30 -30 (in comparison to 1961-200 average) 1961-200 to comparison (in 2010 - 2039 2040 - 2069 2070 - 2099 average) 1961-200 to comparison (in 2010 - 2039 2040 - 2069 2070 - 2099 -50 -50 CCC-na CCC-a HADCM2-na HADCM2-a GFDL-na CCC-na CCC-a HADCM2-na HADCM2-a GFDL-na c GFDL-a MPI-na MPI-a CSIRO-na CSIRO-a dGFDL-a MPI-na MPI-a CSIRO-na CSIRO-a
Figure 3.2 Seasonal climate change scenarios for southwestern Australia from four Global Climate Models’s (GCM’s) using non-aerosol (“na”) and aerosol (“a”) scenarios for the periods 2010-2039, 2040-2069 and 2070-2999. Data from Allan and Hunt (1999). GCM’s used for climate change modelling included: Canadian Climate Center (CCC), Hadley Centre for Climatic Prediction and Research (HADCM2), Max-Planck Institute (MPI), Geophysical Fluid Dynamics Laboratory (GFDL), and CSIRO (the CSIRO Mark 2 GCM). These show that for winter and spring rainfall, all climate models predict decreased rainfall. Results for summer and autumn modelling are more variable, with the average of all eight models predicting little change for summer, and slight decrease in autumn rainfall.
Under the climate change scenarios depicted in Figure 3.2 reduced annual rainfall and unchanged or slightly increased extreme rainfall is most likely to persist over the next 100 years (see also Simmonds & Keay (2000), Bates et al. (2001) and Smith et al. (2000)). Research on changing rainfall in Western Australia has concentrated on the area south of 300S and west of 1200E (Wright, 1974; Nicholls et al., 1999; Smith et al., 2000). The Kent River catchment falls within this zone, but the Dalyup River lies some 150 km to the east. Previous research has also identified some spatial variability within the 300S, 1200E study region (Ruprecht et al., 1996; Indian Ocean Climate Initiative, 2002). Differences between the Dalyup River and Kent River may arise due to the relative importance of different rainfall generation mechanisms between these two catchments (see Chapter 2). Given that the Dalyup River catchment receives a greater proportion of rainfall from ex-tropical depressions and northwestern cloudband activity (Agriculture Western Australia, 2000), trends in the southwestern corner may not hold for central and eastern parts of the south coastal rivers region. There is a need to further investigate the specific changes in annual, seasonal and extreme daily rainfall in the study catchments. The following section investigates changed annual, seasonal and extreme daily rainfall in the Kent River and Dalyup River catchments.
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Spatially Interpolating Rainfall Data
Rigorous analysis of changes in catchment rainfall patterns requires accurate methods for interpolating the sparse rainfall data available for the study catchments. Different methods for spatial interpolation have been used for various gauge densities and environmental settings, with little consistency in conclusions as to the best method to apply (e.g. Hutchinson and Gessler, 1994; Goovaerts, 1999; Hartkamp et al., 1999; Croke et al., 2001). Ebert and Weymouth (2003) found for low gauge densities, that scarcity of data rather than technique, was responsible for most of the error in rainfall interpolation. While this may be true, there is still a need to test interpolation methods for low gauge densities in this environment, particularly given the variable performance of the different techniques (see Tabios and Salas, 1985; Hutchinson and Gessler, 1994; Goovaerts, 1999; Hartkamp et al., 1999; Croke et al., 2001). Amongst these, no single interpolation method has consistently outperformed the others; therefore, six different methods used by the researchers identified above were tested here. These are: linear regression (using elevation and distance from the coast); Thiessen polygons; splines; inverse distance weighting (IDW); ordinary kriging, and cokriging. Cokriging with elevation, used by Govaerts (1999) was tested, and cokriging with distance from the coast was also tested due to the pronounced rainfall gradient inland. Preliminary data analysis using linear regression, suggests that distance from the coast (r2 = 0.54), may be a better predictor of rainfall than elevation (r2 = 0.49).
Rainfall distribution has traditionally been interpolated using a Thiessen polygon method (Thiessen, 1911; Tabios and Salas, 1985; Lynch and Schulze, 1995; Gómez-Hernández et al., 2001). The purpose of this investigation was to determine whether more complex techniques significantly improved the interpolation of rainfall data. Western Australia has particularly low gauge densities, with the 1050 km2 Dalyup River catchment having only 7 rain gauges in the catchment boundary and the Kent River Catchment (2,400 km2) only 15 stations. While some gauging stations have long and consistent records collected from automated stations or by meteorologists, much rainfall data comes from farmers and large gaps in records from months to years are present for some stations (e.g. some stations have no data for December/January of every year when people go on holidays). Additional to these there are many stations that have closed after a few years or decades and which can skew results when comparing long-term changes in rainfall patterns between different periods.
The Dalyup River catchment was selected to investigate different interpolation techniques as it has a lower gauge density than the Kent River. Daily rainfall data from the Bureau of Meteorology 2005 Daily Rainfall Data CD was extracted for every station within 50 km of the
~ 41 ~ Chapter 3: Land Clearing and Boundary Conditions Dalyup River catchment. Stations with less than 30 years of data, and those with long (i.e. more than three years) periods of data missing during the record or numerous consistent gaps (such as the problem with December/January data mentioned above) were excluded, leaving 18 stations. Where rainfall recordings had accumulated over several days, accumulated rainfall was distributed as a percentage for each day according to distribution of rainfall from the nearest station with data for that period, a method commonly applied elsewhere (see Sinclair Knight Merz, 2000a; Department of Environment, 2004b). Annual rainfall was calculated for each station, with those stations missing data excluded for that particular year.
Analysis of rainfall data from 18 stations within 50 km of the Dalyup River catchment found only four years when data from all stations was available; 1973, 1984, 1985 and 1987. Analysis showed that 1987 was a dry year (460 mm), though 1985 (504 mm), 1984 (506 mm) and 1973 (514 mm) were close to the mean annual rainfall. 1984 had a spatial distribution of rainfall most similar to the annual rainfall isohyets defined by the Bureau of Meteorology and CSIRO for the region (CSIRO, 2001) and was therefore selected for analysis.
Exploratory data analysis found that the Dalyup Park station was a significant outlier (see Appendix 3A). While the rainfall data collected in this station is real data, this station has abnormally high rainfall, some 300 mm greater than the average rainfall isohyets for its location based on rainfall isohyets from CSIRO (2001). Low gauge density combined with data from this station caused a significant overestimate of annual rainfall for the lower catchment and therefore this station was excluded from all further analyses. Inclusion would have also violated the assumption of normality, which is required for interpolation using kriging techniques (with Dalyup Park, Kolmogorov-Smirnov (K-S) significance = 0.019 (a K-S Sign. < 0.05 violates normality), but once removed normality was not violated (K-S Sign. = 0.200)).
The 1984 record was divided into a “training” set of 10 records (a minimum of 10 records was required by ArcGIS Geostatistical Analyst for the interpolation dataset) comprising stations with the longest record as these were the stations most likely to be present when estimating rainfall in subsequent analysis. A “testing” or interpolation dataset of the remaining 7 stations was used for cross-validation of the interpolated results. Figure 3.3 identifies that despite a non- random selection of stations for training and testing datasets, both have a good spatial distribution across the catchment.
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Figure 3.3 Location of rain gauging stations surrounding the Dalyup River catchments used in testing spatial interpolation methods for rainfall data. Rainfall stations with the least number of years of record were separated into a validation dataset, with those with the longer records selected for spatial interpolation.
Data analysis and interpolation was conducted using ArcGIS 9.1 Geostatistical Analyst. Thin plate splines were selected as the spline interpolation method (as used by Hutchinson and Gessler, 1994), with power values, window size, anisotropy and search direction optimised using the “on-the-fly” Mean Error (ME) (Equation 3.1) and Root Mean Square Error (RMSE) (Equation 3.2) estimated from cross validation in ArcGIS. Standardised root mean square error (Equation 3.3) was also used to assess the model performance. Kriging and cokriging models were optimised using the same criteria, with all using a spherical model with no nugget (semivariogram fit and model parameters are shown in Appendix 3B). Thiessen polygons were created using the thiessen command in ArcInfo workstation and area calculated for the projected data (GDA94, MGA Zone 51) to derive Thiessen multipliers required for calculating average catchment rainfall. 1 n Mean Error (ME) = * (3.1) ¦[z(ui ) z (ui )] n i 1
n Root Mean Square Error = 1 * 2 (3.2) ¦[z(ui ) z (ui )] n i 1
§ª 1 n º · ¨ [z(u ) z * (u )]2 MIN ¸ ¨« ¦ i i » ¸ « n i 1 » Standardised RMSE = ©¬ ¼ ¹ (3.3) MAX MIN
~ 43 ~ Chapter 3: Land Clearing and Boundary Conditions where n is the number of samples, z(ui) is the actual rainfall value, z*( ui) is the predicted value, MAX is the maximum value, and MIN is the minimum value. RMSE was a more robust measure of model performance, evident from the results of linear regressions (Table 3.2), therefore only RMSE was used to rank model performance.
Linear regression was the poorest method for spatial interpolation, as indicated by the standardised RMSE scores, with distance from the coast a better predictor than elevation (in line with the r2 correlation statistics). Thiessen polygons and IDW produced reasonable results, but did not perform as well as the group that included splines, kriging or cokriging using either elevation or distance from the coast. Overall, splines were the best performer, with more complex geostatistical techniques (i.e. kriging and cokriging) not improving interpolation of rainfall.
Table 3.2 Performance of different methods for spatially modelling rainfall distribution over the Dalyup River catchment in 1984. Splines were the best performing technique, followed by cokriging and kriging methods. While IDW and Thiessen polygons performed reasonable well, linear regression was particularly poor for spatial interpolation of rainfall patterns when cross-validated against eight testing rainfall stations. Method Mean Root Mean Error Standardised RMSE Error Squared RMSE Rank Linear Regression with -0.01 73.39 1.00 8 Elevation Linear Regression with 0.00 66.88 0.84 7 Distance from Coast Thiessen Polygons -3.21 42.01 0.22 6 Spline -0.92 33.07 0.00 1 IDW -1.85 39.71 0.16 5 Ordinary Kriging -2.72 34.00 0.02 2 Ordinary Cokriging with -1.64 35.23 0.05 4 Elevation Ordinary Cokriging with -1.44 34.96 0.05 3 Distance from Coast
Table 3.3 presents results of different methods for interpolating annual rainfall for the Dalyup River catchment in 1984. While Thiessen polygons and IDW did not perform as well as splines in cross validation testing of point data interpolation, Thiessen polygons produced the same result for the catchment rainfall as splines, with IDW the next best performer (+4 mm). Ordinary kriging under predicted (-12 mm) and both cokriging methods over predicted by 9 mm. Results of the different interpolation techniques in plotting 50 mm isohyets over the catchment are presented in Appendix 3C.
~ 44 ~ Chapter 3: Land Clearing and Boundary Conditions Table 3.3 Average rainfall for the Dalyup River catchment from 1984 predicted by various interpolation methods. The different interpolation techniques showed little variation between the best interpolation technique based on cross validation (splines) and the commonly used Thiessen polygon method. The more computationally complex methods such as ordinary kriging which underestimated rainfall and cokriging overestimated rainfall, both by 10 mm showed little improvement over more simple methods. Method Average Catchment Rainfall (mm) Thiessen Polygons 471 Spline 471 IDW 475 Ordinary Kriging 459 Ordinary Cokriging with Elevation 480 Ordinary Cokriging with Distance from Coast 480
The Thiessen polygon method is the most commonly used for estimating spatial rainfall quantity (Tabios and Salas, 1985; Lynch and Schulze, 1995; Gómez-Hernández et al., 2001), due to its computational simplicity which allows trends to be calculated using a spreadsheet program, whereas spatial interpolation (e.g. splines, IDW or kriging) requires interpolation for every year of interest and then amalgamation of results. Given that the Thiessen polygon method produced an identical result to the spline method for 1984 (Table 3.3), further testing was conducted to see if Thiessen polygons consistently produced precise results. Average catchment rainfall was interpolated for the three remaining years where every station had rainfall data (1973, 1985 and 1987). Table 3.4 shows close agreement between the two methods, and validates adoption of the Thiessen polygon method to determine annual catchment rainfall totals. Splines were selected for spatial interpolation of point data to determine spatial trends across the catchments for annual and extreme rainfall.
Table 3.4 Comparison of splines and Thiessen polygons for predicting annual rainfall for the Dalyup River catchment. For the three additional years that were tested (1973, 1985 and 1987), splines and the simpler Thiessen polygon technique produced similar results. Method 1984 1973 1985 1987 Spline 471.4 503.4 487.4 477.0 Thiessen Polygon 471.1 507.2 487.8 476.4
Changed Rainfall Patterns in the Kent River
Rainfall data from the 24 rainfall stations within 50km of the Kent River catchment were selected for analysis (based on the same criteria for long and consistent records applied above). Annual rainfall for three periods were analysed; before 1920, 1921-1975, and 1976-2005; these are the same periods used by IOCI (2002) and the Water and Rivers Commission (Ruprecht et al., 1996) in their analyses of changes in rainfall patterns. Average annual rainfall has decreased by an average of 7.1 % since 1975 across the 24 stations (see results in Appendix 3D), and is statically significant at the 99% level (N.B. all statistical analysis of change discussed herein is based on a two-tailed t-test at the 99% or 95% confidence interval). The direction of change (i.e.
~ 45 ~ Chapter 3: Land Clearing and Boundary Conditions decrease) was relatively consistent throughout the catchment, with only two stations showing localised variation (see Appendix 3D). For average catchment rainfall, the stepped decreased in annual rainfall of 10.5% (77 mm) from the early to mid 1970s in comparison to the 1921-1975 is shown in Figure 3.4. The eight-year moving average trend indicates that annual rainfall now oscillates around a new mean of 658 mm (average rainfall 1921-1975 was 735 mm) since the late 1970s or early 1980s.
Figure 3.4 Average annual rainfall for the Kent River catchment from 1921 to 2004 This chart shows the stepped decrease in rainfall in the early 1970s, from an average from 735 mm (1921-1975) to 658 mm (1976-2004). The 8r moving average shows the stepped decrease (10.5%), and establishment of what appears to be a new, mean annual rainfall since the 1970s
Changes in seasonal rainfall at four stations with the longest and most consistent rainfall record that represent trends across the lower (Denmark Post Office), middle (Wonnenup) and upper (Arundel, and Kendenup) catchment were analysed. Early winter (MJJ) rainfall has decreased by an average 14.9% since 1975, a decrease that was statistically significant at the 99% level for three stations and at the 95% level for the fourth station (Wonnenup) (Table 3.5). Winter rainfall (MJJAS) decreased by an average 11.4%, statically significant at the 99% level for two stations (Demark PO and Kendenup), 95% level for one station (Arundel) and not significant at the remaining station (Wonnenup). When the changes in later winter rainfall (AS) were examined, no stations showed a statistically significant change, indicating that reduced early winter rainfall is driving the overall reduction in total winter rainfall. All stations recorded increased summer (DJF) rainfall, though despite an average 20.3% increase (range +7.0% to +32.7%, see Table 3.5), changes were not significant at either the 99% or 95% level for any stations.
~ 46 ~ Chapter 3: Land Clearing and Boundary Conditions Table 3.5 Change in early winter (MJJ), winter (MJJAS) and summer (DJF) periods from 1913-1975 and 1976-2004. Arundel Denmark PO MJJ MJJAS DJF MJJ MJJAS DJF 1907-1975 253.7 392.8 57.5 573.4 821.5 39.9 1976-2004 219.0 348.4 61.9 469.8 709.5 53.5 Change -13.7%** -11.3%* 7.7% -18.1%** -13.6%** 34.0%
Kendenup Wonenup MJJ MJJAS DJF MJJ MJJAS DJF 1907-1975 247.2 383.2 59.1 242.0 359.5 39.8 1976-2004 209.8 333.0 63.2 211.5 331.8 52.9 Change -15.2%** -13.1%** 7.0% -12.6%* -7.7% 32.7% * indicates a statistically significant change at the 95% ** indicates a statistically significant change at the 99%
Table 3.6 Probability of extreme daily rainfall occurrence in the number of times per decade that an event in excess of that intensity over the Kent River catchment. Average number of days per decade when Average number of days per decade when daily rainfall exceeds a given total (all daily rainfall exceeds a given total stations analysed) (stations with a long record analysed *1) Change Daily Pre 1975- Change 1975- Overall Overall Pre 1975 pre1975 Rainfall 1975 2004 pre1975 to 2005 Average Average Average to post (mm) Average Average post 1976 Average 1976 >30 22.9 22.4 23.6 5.0% 15.6 17.1 13.2 -23.1% >40 8.5 8.3 8.9 7.3% 5.9 6.4 5.1 -19.9% >50 3.9 3.8 4.0 4.4% 2.7 2.9 2.3 -22.5% >60 1.9 1.8 1.9 6.2% 1.3 1.5 1.1 -25.9% >70 1.0 1.0 1.1 8.6% 0.7 0.7 0.6 -18.6% >80 0.6 0.5 0.7 52.5% 0.3 0.3 0.3 2.4% >90 0.3 0.2 0.4 84.5% 0.2 0.1 0.3 78.4% >100 0.2 0.1 0.3 160.2% 0.1 0.1 0.2 238.2% *1 Criteria used to judge a long record was having at least 7,500 days of record from each period (pre 1975 and 1975-2005).
Analysis of daily rainfall (Table 3.6), shows that the probability of daily rainfall up to 70 mm, has changed very little for all rainfall stations, although when considering only stations with a long record (more than 7500 days of record before and after 1975), a reduction of around 20% since 1975 was found. The spatial trends for daily rainfall exceeding 30 mm indicates a greater reduction in the upper catchment, with a moderated decrease or slight increase in the lower catchment (refer to Appendix 3E). An explanation for this could be reduced inland penetration of cold frontal systems identified by Simmonds & Keay (2000), which are a common contributor to moderate rainfall totals. More extreme rainfall events (greater than 80 mm per day) have become more common since 1975. The spatial patterns shows the greatest increases in the upper catchment, possibly consistent with increased northwestern cloudband activity (Allan and Haylock, 1993; Ruprecht et al., 1996; Smith et al., 2000; Indian Ocean Climate
~ 47 ~ Chapter 3: Land Clearing and Boundary Conditions Initiative, 2002), which can deliver high rainfall, particularly over the upper catchment, as evidenced by the April 2005 event (see Chapter 2 and Appendix 2B.iv).
The decreasing trend in rainfall for the Kent River appears relatively consistent across the catchment, reducing by 10.5% in the early 1970s, and currently oscillates around a new average annual rainfall of 658 mm. The results for changes in seasonal and annual rainfall in the Kent River catchment are almost identical to those reported for southwestern Australia in other research (Allan and Haylock, 1993; Ruprecht et al., 1996; Smith et al., 2000; Indian Ocean Climate Initiative, 2002). Early winter rainfall is on average, 51 mm lower across the catchment (total winter rainfall is 59 mm lower), while increases in summer rainfall have not offset decreases (average 9 mm higher). There is some evidence that the occurrence of extreme daily rainfall events (>80 mm) has increased since the 1970s, but with a reduction in occurrence of moderate daily rainfall events up to 70 mm per day. The trends and spatial patterns observed over the Kent River correspond with other reports on rainfall change in southwestern Australia
Changed Rainfall Patterns in the Dalyup River
Overall, annual rainfall has changed little over the Dalyup River catchment in the past few decades (Figure 3.5), with no statistically significant change since 1975. Annual average rainfall for the Dalyup River catchment shows that while there was a downward trend from the 1970s through to the mid 1980s, recently it has returned to levels approximating those of the late 1960s and early 1970s (Figure 3.5). Average annual rainfall for the Dalyup River catchment from 1965-1975, was 491 mm per year, with a reduction of 2.2% to 481 mm since 1976. Records before 1965 are too few to allow the average catchment rainfall to be calculated with any certainty, although analysis of data from the gauges with the longest records suggests that while inter-decadal variations in rainfall are apparent, longer term rainfall has been stationary for the entire twentieth century. Spatial trends show an easterly gradient, with decreased rainfall in the west (toward the Kent River and areas investigated by most previous research), balanced by minor increases in the east (see Appendix 3F). North of the catchment (around Salmon Gums), annual rainfall may have increased slightly since the mid 1970s. This area is further inland and receives a greater proportion of annual rainfall from northwestern cloudbands and ex-tropical cyclonic events, which may explain this trend.
~ 48 ~ Chapter 3: Land Clearing and Boundary Conditions
Figure 3.5 Average annual rainfall for the Dalyup River catchment from 1965 to 2004. This chart shows the minor decline in rainfall from the 1970s that persisted until the mid 1980s, but a recent (post-1998) return to annual rainfall around the levels of the 1960s and 70s. Average rainfall decreased from 491 mm (1965-1975) to 481 mm (1976-2004).
Previous research carried out on changes in rainfall seasonality to the west of this catchment (e.g. Wright, 1974; Nicholls et al., 1999; Smith et al., 2000; Indian Ocean Climate Initiative, 2002), and the results reported above for the Kent River, do not support results from research in the Dalyup River catchment. Data from the three stations with the longest rainfall records that are in or directly adjacent to the Dalyup River catchment (Scaddan, Speddingup, Scaddan West), show no statistically significant change in rainfall over any period (Table 3.7). Results differed somewhat between stations, with no discernible spatial trend to explain the variability. Both Speddingup and Scaddan West showed a slight increase in DJF rainfall, though Scaddan Post Office recorded a decrease. Decreased MJJAS rainfall was not necessarily caused by lower early winter (MJJ) rainfall, as in the Kent River and reported elsewhere for southwestern Australia. Trends in summer rainfall are not statistically significant, but suggest slightly increased rainfall.
Table 3.7 Change in early winter (MJJ), winter (MJJAS) and summer (DJF) periods from 1913-1975 and 1976-2004. Scaddan Post Office Speddingup Scaddan West MJJ MJJAS DJF MJJ MJJAS DJF MJJ MJJAS DJF 171.2 285.4 61.3 135.4 224.2 20.0 109.8 183.1 54.2 1913-1975 170.7 276.7 48.1 126.8 204.4 23.4 112.7 181.3 67.3 1976-2005 -0.3% -3.1% -21.4% -6.3% -8.8% +16.9% +2.7% -1.0% +24.2% Change N.B. DJF rainfall counts data for December of the previous year, to February of the reporting year, e.g. DJF 1975 rainfall is accumulated from December 1974 to February 1975. N.B. none of these changes were statistically significant at either the 95% or 99% level
Analysis of changes in extreme daily rainfall for all stations suggests the occurrence of moderate falls between 30 – 50 mm per day have changed little since 1975 (Table 3.8). Extreme
~ 49 ~ Chapter 3: Land Clearing and Boundary Conditions daily rainfall events have become more frequent since 1975, with an increase in the number of days when stations recorded more than 100 mm per day (Table 3.8). A strong east (increased frequency) to west (decreased) gradient is apparent for the probability of daily rainfall exceeding 70 mm, suggesting that rare events in more arid areas have increased over the past three decades (refer to Appendix 3G). These trends can be explained by, and are consistent with the changes in weather patterns and rainfall mechanisms since 1975.
Table 3.8 Probability of extreme daily rainfall occurrence in the number of times per decade that an event in excess of that intensity over the Dalyup River catchment. Average number of days per decade when Average number of days per decade when daily rainfall exceeds a given total for all daily rainfall exceeds a given total for stations stations with a long record*1 Change Daily Pre 1975- Change Pre 1975- Overall Overall pre1975 Rainfall 1975 2004 pre1975 to 1975 2005 Average Average to post (mm) Average Average post 1976 Average Average 1976
>30 10.27 10.68 9.86 -7.6% 12.51 12.76 12.17 -4.6% >40 3.89 3.93 3.86 -1.8% 4.99 4.83 5.20 +7.7% >50 1.85 1.82 1.89 +3.8% 2.26 2.15 2.42 +12.5% >60 1.11 1.09 1.14 +4.2% 1.36 1.28 1.49 +16.6% >70 0.67 0.61 0.73 +18.9% 0.78 0.67 0.93 +38.4% >80 0.42 0.39 0.45 +16.6% 0.51 0.54 0.46 -13.5% >90 0.24 0.18 0.30 +65.2% 0.31 0.27 0.37 +38.4% >100 0.15 0.09 0.21 +136.0% 0.16 0.13 0.19 +38.4% *1 Criteria used to judge a long record was having at least 7,500 days of record from each period (pre 1975 and 1975-2005).
There is large variation in annual rainfall for the Dalyup River (refer to Figure 3.5), strongly influenced by extreme events such as ex-tropical cyclones that are associated with the largest hydrometeorological potential and are disproportionately significant events in the catchment. Based on data presented in Table 3.7 and Figure 3.5, annual rainfall totals and the seasonal distribution of rainfall has remained unchanged in the Dalyup River catchment since the time of major land clearing, but the occurrence of extreme events greater than 90 mm per day has increased since the mid 1970s. These trends differ to those reported above for the Kent River and by other research for the southwestern corner of Western Australia. The data presented here identify the Dalyup River catchment as the centre of the transition from the trends of decreased annual rainfall in the southwestern corner, with increased rainfall in inland areas.
Summary of Changes in Rainfall
Rainfall and weather patterns have changed over southwestern Australia since the 1970s, though these changes have been spatially variable. As a result the impact of land clearing on catchment hydrology varies accordingly. Annual rainfall has decreased in the Kent River by 10.5% since
~ 50 ~ Chapter 3: Land Clearing and Boundary Conditions 1975, representative of broader changes reported elsewhere for the southwestern corner. Further to the east, there has been no significant change to annual rainfall patterns in the Dalyup River during the same time period. Spatial variability around the Dalyup River catchment suggests that it lies close to a zone of transition over which annual rainfall has increased in semi-arid, inland area since the mid 1970s. Other research has associated reduced annual rainfall in the southwestern corner with reduced northerly extension of cold fronts as a result of increased atmospheric pressure (Simmonds and Keay, 2000; Smith et al., 2000; Bates et al., 2001). Increases in SSTs have increased the activity of northwestern cloudbands, bringing higher rainfall to the inland, semi-arid pastoral areas (such as the north of the Dalyup River). The results reported for both catchments in this investigation are consistent with these mechanisms and their spatial pattern.
Predictions of climate change for southwestern Australia over the next century (see Figure 3.2, or Allan and Hunt, 1999) consistently indicate current trends will continue or become more pronounced. Annual rainfall in the southwestern corner is likely to remain lower than rates recorded during the early and mid twentieth century, driven by reduced early winter rainfall. Predicted increase in SSTs (Allan and Hunt, 1999) suggests that increased moderate to extreme daily rainfall totals in both catchments will persist or increase as they are apparently linked to increased northwestern cloundband activity (Smith et al., 2000), and increased intensity, longevity and poleward track of tropical cyclones (Walsh and Katzfey, 2000; Walsh and Ryan, 2000).
Implications of these changes in annual, seasonal and extreme rainfall are significant for river geomorphology. Seasonal rainfall and low runoff under vegetated catchment conditions have resulted in rivers with ephemeral or perennial streamflow regimes (Hatton et al., 2003). Decreased annual rainfall in water supply catchments have resulted in a four-fold decrease in annual streamflow in high-rainfall water catchments, which Bari and Ruprecht (2003) recognised would be significantly worse for similar changes in lower rainfall catchments. With changing rainfall patterns, it can be expected that annual streamflow would have decreased by at least 30-40% under static land use conditions since the early 1970s, based on the work of Muirden et al. (2003) (refer to Figure 1.3). The result presented here for the Kent River suggests that despite significant land clearing and the effects that this may have on streamflow, reduced annual rainfall has the potential to significantly moderate the impacts on increased streamflow. Data presented in this section indicates little change to annual rainfall in the Dalyup River catchment. Given the low natural runoff rates that would be expected from this low-rainfall catchment and the extent of land clearing (96%), a significant hydrologic impact can be expected in this catchment. For both catchments, the increased frequency in extreme rainfall
~ 51 ~ Chapter 3: Land Clearing and Boundary Conditions events and corresponding increased potential for generation of large flood events has significant potential for channel response, even under reduced annual rainfall trends.
3.1.2. Groundwater Response to Land Clearing Native vegetation systems are efficient users of water, surviving water deficit over at least eight months of the year (Bureau of Meteorology, 1995) by transpiring rainfall stored in the soil over these months. Replacement of deep-rooted native vegetation systems with shallow rooted seasonal crops and pastures has significantly reduced the evapo-transpiration component of the water balance (see Figure 3.6). George (1992) found that groundwater recharge in low-rainfall wheatbelt catchments was around 0.02 – 0.14 mm yr-1, increasing to 6 – 30 mm yr-1 under annual agriculture. The consequence of land clearing and increased recharge has been groundwater tables rising at rates of up to 0.3 m yr-1 (George, 1992; Salama et al., 1993a; Hatton and Ruprecht, 2001; Bari and Ruprecht, 2003; Hatton et al., 2003; Peck and Hatton, 2003; Ryder, 2004b; Simons and Alderman, 2004).
a b Figure 3.6 Changes in the water balance components following land clearing.
Groundwater systems in southwestern Australia are described in terms of a perched ephemeral aquifer and a deeper permanent system (Bari and Ruprecht, 2003). Perched groundwater tables are a feature of the texture contrast soils in southwestern Australia, where a seasonally saturated zone develops over clayey B-horizon soils. In undulating areas, water discharges laterally under the hillslope before emerging at a break of slope or directly into streams. This process creates the seasonally saturated zone from which overland runoff is generated. Hortonian overland flow is very rare across this region due to high hydraulic conductivity of surficial soils (Ruprecht and Schofield, 1989).
The deeper, regional aquifer is usually separated from the perched system. Infiltration below the B-horizon and root zone of the native vegetation (at rates typically less than 1 mm per year George, 1992; Hatton et al., 2003) recharges the deep groundwater system. This system was typically well below the soil surface, although saline lakes and some low-lying stream channels ~ 52 ~ Chapter 3: Land Clearing and Boundary Conditions were discharge areas before land clearing (Hatton et al., 2003; Harper and Gilkes, 2004). Reduced plant evapo-transpiration following land clearing has increased recharge, with regional trends showing that groundwater has risen significantly, and is continuing to rise, since clearing most landscape positions (Ferdowsian and Crossing, 2004; Lillicrap, 2004; Ryder, 2004b; Ryder, 2004a; Simons and Alderman, 2004).
Where deeper systems have reached the surface, they are now discharging larger quantities of groundwater into streams, resulting in longer periods of baseflow. Rising watertables have also brought salts dissolved in groundwater and previously stored in the unsaturated zone to the surface. As a result, stream salinity has increased. In addition to the direct contribution to channel of water and salts, rising groundwater has expanded the saturated source areas, leading to increased overland flow contribution to channels. This has implications for river geomorphology with potential for increased sediment loads eroded by increased overland flow on hillslopes, generated from source areas proximal (i.e. highly coupled) to river channels and floodplains.
Groundwater Changes in the Kent River Catchment
Prior to clearing of the upper Kent River catchment, groundwater was typically more than 10 - 15 m below ground level higher in the landscape and within a few metres on valley floors (Ferdowsian and Ryder, 1997; Salama et al., 1997; Ryder, 2004b). At present, monitoring bores in the Kent River catchment show continued rising trends of 0.2 – 0.3 m yr-1 in most landscape positions (Figure 3.7), except for areas of tree plantations, where groundwater tables appear to be static or falling slightly (Ferdowsian and Ryder, 1997; Salama et al., 1997; Ryder, 2004b). Aquifers higher in the landscape show great inter-year variability, with significant rises during wet years, and either little change or falling levels during dry years or following conversion of cropping and pasture grazing to tree plantations (Figure 3.7 D). In these areas, watertables are typically within 5 – 10 m of the surface (Ryder, 2004b). Along valley-floors, the water table is close to, or at, the surface, resulting in groundwater discharge across the floodplain and surrounding low-lying areas.
Ryder (2004b) reported that groundwater was slightly acidic (pH 4.6) in the lower rainfall upper catchment and neutral to slightly acidic in the wetter mid catchment (pH 5.6), and salinity typically varied between 7,150 mg l-1 to 13,750 mg l-1. Gauging data has recorded significantly higher streamflow salinity (values in excess of 60,000 mg l-1 during flow events). This suggests higher rates than those measured in bores by Ryder (2004b). Elsewhere in the wheatbelt, values between 60,000 mg l-1 and 110,000 mg l-1 have been measured (de Broekert and Coles, 2004).
~ 53 ~ Chapter 3: Land Clearing and Boundary Conditions A B
C D
Figure 3.7 Groundwater trends in the Kent River catchment. Source: (Ryder, 2004b) Groundwater trends show a general rising trend under cropping and grazing land uses (Figure 17A-C), with the lower slopes (17-A) showing a water table at ground level, that seasonally fluctuates. The mid-slope aquifer (17-B) is still rising (~20cm yr-1), with seasonal fluctuations and also affected by longer-term rainfall trends. The upper slopes show less seasonal variation, and generally rise under cropping land uses (17-C). For sub-catchments where higher water-use farming systems have been adopted, like tree plantations (17-D), water tables have fallen.
Groundwater Changes in the Dalyup River Catchment
The Dalyup River is dominated by the Scaddan and Esperance soil landscape systems, which are both characterised by deep, sandy soils with high transmissivity. Consequently, these aquifers are more responsive to large-scale land use and land management changes and seasonal and longer-term variation in rainfall. The upper catchment is dominated by the Scaddan soil landscape, in which groundwater levels were typically greater than 10m below the surface, but have risen (and continue to rise) at rates between 0.05 – 0.025 m yr-1 (Figure 3.8 A & B) following clearing. Many groundwater tables are close to, or at the surface in lower lying areas surrounding rivers, minor watercourses and saline lakes, turning these locations into groundwater discharge sites. Measured groundwater salinity in bores averages 30,800 mg l-1, but is as high as 77,000 mg l-1 (approximately twice as salty as seawater), with groundwater pH variable between neutral (pH 7.0) to highly acidic (pH 2.9). Discrete data collected as part of this research found salinity between 19,900 mg l-1 and 92,800 mg l-1 (see data presented in Figure 5.4 and Appendix 5B).
~ 54 ~ Chapter 3: Land Clearing and Boundary Conditions A B
C D
Figure 3.8 Groundwater trends in the Dalyup River catchment. Source: (Simons and Alderman, 2004) Groundwater trends show a general rising trend under cropping and grazing land uses (Figure 17A-C), with the lower slopes (17-A) showing a water table at ground level, that seasonally fluctuates. The mid-slope aquifer (17-B) is still rising (~20cm yr-1), with seasonal fluctuations and also affected by longer-term rainfall trends. The upper slopes show less seasonal variation, and a general rise under cropping land uses (17-C). For sub-catchments where higher water-use farming systems have been adopted, like tree plantations (17-D), water tables have fallen.
Summary of Groundwater Response to Land Clearing
Groundwater is still rising in both catchments in response to a new hydrologic regime following landscape-scale clearing of high water using native vegetation for seasonal agricultural crops. While some areas will reach hydrologic equilibrium in the next 10 years, water tables in other catchments are predicted to continue rising for the next 50-70 years under the current land use (Ryder, 2004b; Simons and Alderman, 2004). Predictions that up to 30% of the Kent River (Caccetta et al., 1995; Ferdowsian et al., 1996; Salama et al., 1997) and 25% of the Dalyup River (Short et al., 2000) catchments are at risk of shallow (within 2m of the surface) water tables once hydrologic equilibrium is reached, around 2050. The implication of a continuing rise is that saturated source areas will continue to expand and runoff as a percentage of rainfall would be likely to increase further. Data from other research presented in this section confirms ~ 55 ~ Chapter 3: Land Clearing and Boundary Conditions that groundwater tables have risen in response to land clearing, and are likely to have a significant impact on discharge patterns in the study catchment.
3.1.3. Changes to Discharge as a Result of Land Clearing Chapter 1 introduced research from a range of monitored catchments throughout southwestern Australia, where increases in streamflow have been observed following land clearing (Stokes and Loh, 1982; Williamson et al., 1987; Ruprecht and Schofield, 1989; Ruprecht and Schofield, 1991; Ruprecht and Stoneman, 1993; Pen, 1999; Bari and Ruprecht, 2003; Hatton et al., 2003; Muirden et al., 2003). These investigations provide a background to responses and trends across the region. Specific changes in catchment hydrology in the Kent and Dalyup Rivers are investigated in this section, with reference to how the variability of land clearing rates, annual rainfall and groundwater response has affected the perturbation of hydrologic boundary conditions and its effect on landscape stability.
This section investigates a number of discharge components that are significant for river geomorphology, specifically; annual discharge, flow duration (perenniallity or hydroperiod), discharge responses to specific rainfall events and changes in bankfull frequency. Previous research in this region found that recharge increased following land clearing, causing increased lateral sub-surface flow, expressed as increased baseflow and flow perenniality (Stokes and Loh, 1982; Williamson et al., 1987; Ruprecht and Schofield, 1991; Ruprecht and Stoneman, 1993; Hatton et al., 2003). Direct groundwater discharge into stream channels further increases baseflow, causing a transition from ephemeral towards perennial discharge patterns. Expansion of saturated source areas results in greater overland flow from saturated areas proximal to the river channel (Stokes and Loh, 1982; Williamson et al., 1987). In the Kent River, changes in discharge are investigated using discharge data from four gauging stations. While one has operated since the 1950s (corresponding to the time of major land clearing), another has just over two decades of flow data while the two remaining stations have only been operating for a few years. In the ungauged Dalyup River catchment changes in discharge are investigated using data from adjacent gauged catchments, supplemented with more qualitative sources. The section investigating changes in the Kent River catchment is from the paper by Callow and Smettem (2004).
Discharge changes in the Kent River Catchment
Flow data have been collected at the bottom of the Kent River catchment from the Styx Junction Station (604053) since 1956 (refer to Figure 3.9 for locations), coinciding with the time of major land clearing. The data reveal an increase in the annual runoff coefficient for the first decade of the record (see Figure 3.10). However, a lack of data prior to clearing and phases of
~ 56 ~ Chapter 3: Land Clearing and Boundary Conditions very low runoff from 1969-72 and 1985-87 make it difficult to determine whether this is due to land clearing or higher rainfall during this period (see data in Figure 3.4).
Figure 3.9 Location of river gauging stations and rainfall stations with the longest records in the Kent River catchment.
~ 57 ~ Chapter 3: Land Clearing and Boundary Conditions
Figure 3.10 Annual discharge and runoff coefficient for Styx Junction Gauging Station. Source: (Department of Environment, 2005f) Annual flow shows a trend of increasing runoff from the start of the record (1957) for the first decade, however, a lack of data prior to major land clearing which started in the 1940s and 50s means that the effects of land clearing cannot be quantified.
Rocky Glen (604001) gauging station was established at the bottom of the upper catchment during 1978. This allows calculation of the annual flow contribution for the lower catchment from 1979 – 2003, using the difference between gauges 604053 and 604001. Rainfall-runoff plots were used to estimate annual runoff curves using the Tanh rainfall-runoff method (Grayson et al., 1996, p. 81, and equation 3.4 below) for the upper and lower catchments from 1979 - 2003: ª P L º Q P L F *Tanh (3.4) ¬« F ¼» where Q is annual runoff (mm) and P is the annual rainfall (mm), L is the notional loss (mm) and F is the notional infiltration (mm). Curves were fitted to the data in Microsoft Excel using the “solver” add-in to optimise notional infiltration and loss values based on RMSE between predicted and actual runoff. Using these curves in combination with upper and lower catchment rainfall data, the relative contributions of the upper and lower catchment to the flow recorded at Styx Junction from 1957-1978 were estimated and tested against 1979 – 2003 data. Estimates of discharge closely match actual 1979 - 2003 data (Fig. 8A & B), with an R2 correlation of 0.931 for the lower catchment and 0.902 for the upper catchment.
Graphical analysis of trends in annual discharge for the upper and lower catchment showed no clear response to land clearing (upper catchment) or to decreased annual rainfall (lower catchment). Analysis of the data using trend analysis techniques such as residual mass and double mass curve methods (as suggested by Grayson et al. 1996) were also insensitive to the complex interaction between rainfall, land use and runoff, with no clear trend in annual discharge patterns for either upper and lower catchment.
~ 58 ~ Chapter 3: Land Clearing and Boundary Conditions
Figure 3.11 Estimated discharge (1957-2003) and actual discharge (1979-2003) for the upper (cleared) and lower (uncleared) Kent River catchment (based on data from: Department of Environment, 2005f; Department of Environment, 2005e).
To better investigate change in the rainfall-runoff relationships due to land clearing under a drying climate regime, the Tanh methods was used. The upper (Rocky Glen), whole (Styx Junction) and lower (difference between Styx Junction and Rocky Glen) catchments were grouped by the percentage of cleared land (10-30%, 30-50%, 50-70%) and rainfall-runoff curves plotted. Values for “notional infiltration” (see Grayson et al. 1996) were found to be similar for all three groups, and, given the dominance of sandy surficial soils with low infiltration across the catchment, adopting a uniform value is a valid assumption. The average value of 519.65 mm was adopted, which allowed the “notional loss” figure to account for vegetation, a proxy for the effect of land clearing on the rainfall-runoff relationship. Figure 3.12A shows the strong relationship between percentage of land cleared and runoff, with a linear relationship between the percentage clearing and the notional loss figure (Figure 3.12B). Rainfall-runoff relationships across all clearing rates (including uncleared) were estimated using the relationship defined in Figure 3.12B (refer to Figure 3.12C for results). Runoff from the upper catchment before clearing and with average annual rainfall of 602 mm (1921-1975 average), was 6.3 mm yr-1, equating to a runoff coefficient of 0.010 (i.e. runoff is only 1.0% of rainfall). Under current clearing conditions and with reduced upper catchment annual rainfall of 545 mm (1976-2004 average), runoff has increased to 14.0 mm with a runoff coefficient of 0.026 While the gauging record does not extend prior to land clearing, there is sufficient evidence based on the results of the model presented above to conclude that runoff has increased in the cleared upper catchment and that the magnitude of this increase has been between two and three times the pre-clearing rate, despite a 10% reduction in annual rainfall.
~ 59 ~ Chapter 3: Land Clearing and Boundary Conditions
Figure 3.12 Rainfall runoff relationships for the Kent River catchment under different clearing rates.
In the Kent River, flow data from Styx Junction show increased baseflow following land clearing in the upper catchment (Figure 3.13). Flow records from 1956-1964 show significantly lower baseflow than for all subsequent periods (1965-2004). Increased baseflow is almost certainly due to land clearing and increased flow duration in the upper catchment, as land use in the lower catchment has changed little, and the 10% reduction in annual rainfall since the mid 1970s would shift the flow duration curve in favour of reduced discharge. Baseflow data for the upper catchment can only be calculated since 1979, and no trend in baseflow was found for that record period. This suggests that the most significant (and observable) changes occurred within 10-20 years of land clearing, as was found in the discharge record at the bottom of the catchment (Figure 3.13).
Baseflow and quickflow were separated using the Lynne and Hollick filter (see Grayson et al. (1996)), to analyse changes in the proportion of baseflow to quickflow for the whole catchment (Styx Junction) and upper catchment (Rocky Glen) from 1978 to 2004. Baseflow proportion for the whole catchment was constant through time and showed no response to land clearing (Mean: 83.7%, Std Dev: ±1.97%), with this trend also found for the upper catchment, although baseflow proportion was lower (Mean 46.8%, Std Dev: ±2.49%). It was expected that the proportion of total flow from quickflow would have increased as a result of expanding saturated source areas in the upper catchment, though the increased baseflow period due to groundwater direct discharge may be masking any increases in quickflow.
~ 60 ~ Chapter 3: Land Clearing and Boundary Conditions
Figure 3.13 Decadal flow duration curve for Styx River Station from 1956-2004. Data from: Department of Environment (2005e).
Changes in discharge response to large rainfall events were investigated to determine whether peak discharge has changed for early-season events on a dry catchment and mid to late winter events on a saturated catchment. Analysis was restricted to the cleared upper Kent River catchment and conducted on the discharge record from the Rocky Glen Gauging station. The criteria used to select events for early season rainfall on a dry catchment were: x it must occur before June, x less than 20 mm of rainfall over the preceding 30 days, x no more than 50 mm since the start of that year, x event with rainfall greater than 50 mm. For large winter events on a wet catchment: x it could occur between May and September x at least 300 mm of rainfall that year was required, x at least 50 mm in the previous 30 days, x event rainfall greater than 50 mm. Hydrograph characteristics were analysed for events with near-identical antecedent and event conditions to determine whether the hydrologic response to large rainfall events has increased over the last two decades.
Three events that fulfilled the criteria for each scenario were analysed. Table 3.9 and Table 3.10 identify that antecedent conditions and event rainfall were very similar and allowed comparison of the discharge response. For the early summer event (Table 3.9), peak discharge increased by five times, from 3 to 15 m sec-1, and time to peak discharge was reduced. The hydrograph had a significantly steeper rising limb, indicating that storage limits are reached more quickly (refer to Appendix 3H for hydrographs). For a winter event on a saturated catchment, discharge responses also increased, though the magnitude of the changes was not as significant as for the
~ 61 ~ Chapter 3: Land Clearing and Boundary Conditions summer events. (see Table 3.10, and Appendix 3I for hydrographs). The results of this analysis suggest that for summer events of 55-65 mm, peak discharge has increased by five times, and the time to peak discharge reduced by 30-40% in the last 20 years. For the winter events, time to peak discharge decreased by around 20%, with a 60% increase in peak discharge from a rainfall event of 80 – 90 mm on a wet catchment.
Table 3.9 Analysis of the discharge recorded from similar early summer events recorded at the Rocky Glen gauging station. 8/04/1985 – 17/03/1993 - 16/04/2002 – Criteria 6/04/1985 27/03/1993 30/04/2002 3 Day Rainfall 0.6 mm 0.6 mm 0.1 mm 30 Day Rainfall < 20 mm 6.2 mm 2.0 mm 12.8 mm Year-to-Date Rainfall < 50 mm 43.1 mm 11.7 mm 21.9 mm Event Rainfall > 50 mm 55.8 mm 63 mm 63.3 mm Time to Peak Discharge 3 days: 21 hours: 4 days: 17 hours: 2 days: 13 hours: 55 minutes 35 minutes 20 minutes Peak Discharge 2.7 m3 s-1 2.2 m3 s-1 15.2 m3 s-1
Table 3.10 Analysis of the discharge recorded from similar winter events on a wet catchment recorded at the Rocky Glen gauging station. 20/07/1990 – 26/08/1998 – 19/08/2003 – Criteria 1/8/1990 2/09/1998 01/09/2003 3 Day Rainfall 5.5 6.3 5.7 30 Day Rainfall > 50 mm 84.8 80.4 77.9 Year-to-Date Rainfall > 300 mm 381.9 472.8 320.6 Event Rainfall > 50 mm 91.6 92.9 79.9 Time to Peak Discharge 5 days: 9 hours: 4 days: 5 hours: 4 days: 17 hours: 05 minutes 05 minutes 55 minutes Peak Discharge 38.4 m3 s-1 31.8 m3 s-1 53.3 m3 s-1
These results highlight how the discharge response to similar rainfall events is continuing to increase as a proportion of rainfall, despite little evidence from total annual discharge data, or from the proportion of flow generated from quickflow. Bowman and Ruprecht (2000) found that as the area of salt-affected land doubled, tripled and quadrupled, increases in peak discharge were directly proportional (i.e. double salt affected land, double peak discharge for a given rainfall event). In the upper Kent River catchment, high saline watertables were estimated to cover 3.1% of the catchment in 1977, 6.1% in 1988, 15.3% by 1994, with the potential to affect 34% of the catchment area (Evans et al., 1995; Ferdowsian et al., 1996). The expansion of saturated area has lagged behind land clearing due to the depth of the watertable before clearing and the rates at which regional aquifers have risen. Continued rising groundwater tables in the Kent River (Figure 3.7) will cause saturated areas to expand, and runoff response will continue to rise until hydrologic equilibrium is reached.
Despite decreased annual rainfall, discharge responses of the (cleared) upper Kent River are now significantly greater than prior to land clearing. Annual discharge is around three times the pre-clearing rate, and flow is more perennial. The runoff response continues to increase as the
~ 62 ~ Chapter 3: Land Clearing and Boundary Conditions catchment becomes wetter from rising groundwater, with implications for river geomorphology and landscape sensitivity. Analysis of specific rainfall events under similar antecedent conditions found that peak discharge has increased by around 60% for large winter events and is now five times greater for summer events. Given that the occurrence of extreme rainfall events has increased slightly in a drying climate, the continued expansion of saturated areas under a rising groundwater gradient is likely to increase erosive potential from increased overland flow, further altering landscape stability.
Discharge changes in the Dalyup River Catchment
The Dalyup River is not gauged, and flow records have not been collected over any extended period of time anywhere in the catchment (Water and Rivers Commission, 2002). Therefore, no direct calibration of regional data to the Dalyup River catchment is possible to help quantify changes in discharge resulting from land clearing. Pen (1999) suggests that flow has increased by 2-4 times across the entire south coastal region (a figure that matches data for the Kent River). Reports on the Dalyup River catchment (Agriculture Western Australia, 2000; Water and Rivers Commission, 2002a), refer to a personal communication with Pen that quotes this increase as being applicable to the Dalyup River catchment. Pen (1999) states the annual discharge of the Dalyup River is 11,000 Ml yr -1, equating to a runoff coefficient of 2.6.
Extrapolating the data of Muirden et. al., (2003) (Figure 3.1), runoff from the uncleared Dalyup River would have been approximately 5 mm yr -1, with a runoff coefficient of 0.9 and an annual discharge of 3,800 Ml yr-1 before clearing, and would predict an increase to 35 mm yr -1 or seven times the original value, with a current runoff coefficient of 7 and annual discharge of 29,500 Ml yr-1. Muirden et. al. (2003) have identified significant variation in the curves, and few stations included in the data set drain catchments similar to the Dalyup River, with low rainfall, sandy soils and almost 100% clearing. Runoff coefficients suggested by the “Muiriden” curves compared to gauging data available from other catchments indicate that these runoff figures are well in-excess of expected values.
The gauged Lort River and Young River are 50-80km west of the Dalyup River (see Figure 3.14). As an initial comparison, contemporary runoff is only 0.91% and 0.90% respectively, from catchments that are 60% cleared compared to 7% runoff predicted from the Muirden curve. In the Young River catchment, two paired control catchments: the cleared Munglinup catchment (601006 – 11.52 km2) and the uncleared Melaleuka catchment (601600 – 3.47 km2) can be used to investigate changes in runoff processes under cleared and uncleared conditions (see Appendix 3J). The cleared Munglinup catchment has a runoff coefficient of 1.95% and the
~ 63 ~ Chapter 3: Land Clearing and Boundary Conditions uncleared Melaleuka catchment has runoff coefficient of 0.03%. Though these catchments are small, the magnitude of change is relatively consistent across the catchment.
Figure 3.14 Location of the Dalyup River catchment and the gauging stations in the adjacent Lort River and Young River.
Figure 3.15A shows the flow duration curve data adjusted to millimetres per year to account for variation in catchment area, from all five gauges in the Lort and Young systems. By calculating the flow duration curve from gauging data over decadal increments according the amount of land clearing (similar as for the Kent River), changes in the discharge response of the catchment were estimated for the proportion of land clearing (see Figure 3.15B). Applying these curves to the Dalyup River catchment, the estimated flow duration curve at the bottom of the catchment (catchment area 841 km2) highlights the transformation of discharge regime and the frequency with which larger events are predicted to occur, based on data from an adjacent catchment (Figure 3.15C). Peak discharge is significantly higher and flow pereniallity has increased from ~ 64 ~ Chapter 3: Land Clearing and Boundary Conditions 20% of the year to 80% of the year. While this represents the best estimate for the changes that have occurred in the Dalyup, scaling responses from these catchments (including the small cleared and control catchments) may not hold at the catchment scale for the Dalyup River. In particular, the changes in the higher end of the discharge spectrum would normally be expected to come closer together under cleared and uncleared condition.
Figure 3.15 Flow duration curve under different clearing rates, based on data from gauging stations on the Lort River and Young River. Flow data for each stations in the Lort River and Young River (A), were used to determine average flow duration curves under various rates of clearing (B). these were used to predict a flow duration curve for the bottom of the Dalyup River (area 841 km2) prior to clearing and under its present condition (96% cleared) based on the data presented from Figure 3.15A and B.
As an indication of the potential for river response to these changes in catchment hydrology, the probability of bankfull discharge being achieved for typical reaches in the upper, middle and lower catchment was estimated, using data from Figure 3.15. Peak daily discharge was plotted against mean daily discharge, and a strong relationship found across all (41,821) data points (peak discharge = 1.655* mean daily discharge, R2 = 0.928 – see Appendix 3K). Using this relationship, the curve from Figure 3.15B was recalculated to estimate the flow probability of peak discharge, and then applied at four sites surveyed throughout the catchment where the bankfull discharge was predicted using the area-slope method (based on field survey data, Table 3.11). Frequency of bankfull discharge has increased significantly following land clearing. At most locations, bankfull discharge was achieved only in 1:10 to 1:20 years, whereas data from adjacent catchments suggests that bankfull discharge is now achieved at least annually, highlighting the implications of the new boundary conditions for channel adjustment.
~ 65 ~ Chapter 3: Land Clearing and Boundary Conditions Table 3.11 Changes in the frequency of bankfull discharge at three sites in the Dalyup River catchment, based on predicted runoff curves (see Figure 3.15C). Location Survey Bankfull Catchment Average Recurrence Interval Site Discharge Area (km2) Natural Condition Present (m3 sec-1) Condition Upper catchment Site 21 0.42 27.25 1:20 Years 3.5 times per year Mid catchment Site 11 2.25 180.28 1:10 years 4 times per year Lower catchment, Site 5 8.15 244.41 Greater than the 2 times per incised channel record (> 1:40 year) year
The results from a regionalised, quantitative approach closely match more qualitative measures of the response of catchment hydrology to land clearing. Evidence of changes in annual discharge has been in the increased area of Lake Gore, a terminal basin for the Dalyup River system (evident from historic and recent aerial photography). Lake area has increased significantly in the last decade, resulting in extended periods of flooding, covering playa shorelines and inundating the delta at the mouth of the Dalyup River (Massenbauer, pers. comm.; Wilson, pers. comm.). Oral history cited within a report on the Dalyup River (Water and Rivers Commission, 2002a) include description of flooding and inundation of Lake Gore. Reports of severe flooding begin in the 1940s, but since the mid 1960s floods have become more frequent and damaging, with large floods in 1968/69, 1979, 1989, 1999 and 2000. Before the flood of 1968/69, Lake Gore was significantly drier, but since then it has become progressively more inundated. Oral history for the Dalyup River matches results from gauging data from adjacent, gauged catchments to highlight the magnitude of changed hydrology since land clearing.
Summary of Hydrologic Responses to Land Clearing
Both study catchments have experienced significant changes in hydrology since deep-rooted native vegetation was cleared. The response of both catchments has been an increase in total discharge in a drying climate, as a result of increased runoff as a proportion of rainfall. Analysis of the discharge trends and mechanisms in the Kent River indicates that while the proportion of total annual flow generated from quickflow has not increased, floods generated from a given rainfall event produce higher flood peaks and “flashier” hydrographs. Bankfull discharge now occurs more frequently. Landscape stability has been significantly altered due to the hydrologic response of these catchments to land clearing, acting through both the erosive potential or power, and by an increased frequency of events. Despite a drying climate, the erosive potential will continue to increase until hydrologic equilibrium is reached.
~ 66 ~ Chapter 3: Land Clearing and Boundary Conditions 3.2.Effects of Land Clearing on Sediment Transport Measuring the movement of sediment through landscapes is difficult given the temporal and spatial scales over which sediment transport rates vary and consequently, over which data must be collected (Hicks and Gomez, 2003). The association of flood events with the highest rates of sediment transport (Nelson and Benedict, 1950) places further impediments on direct measurement of sediment transport rates. These and other real-world limitations, such as the inherent geomorphology of this study area (e.g. river processes operate such that a stratigraphic floodplain approach cannot be used), project timeframe and budgetary limitations, affect the methods for measuring and modelling sediment transport. Therefore this study adopts a holistic or systems approach using a range of tools to investigate sediment transport.
In the study catchments, field data were collected during several campaigns and included: grab samples of bedload sediment, surveying of channel dimensions, measurement of sediment bodies, analysis of stratigraphy where possible, and geomorphic evidence of erosion and deposition. Event-based sediment sampling was not suitable, due to the distance to the catchments (4 and 10 hours drive away from Perth), unpredictability of rainfall, and roads that become impassable during heavy rainfall. Data for event sampling of suspended sediment transport collected sporadically by the Department of Environment since the 1970s, was used to complement bedload sampling undertaken as part of this project, and to compare with modelled data. Data from modelling of changes in sediment transport rates since European settlement of Australia, undertaken as part of the National Land and Water Resources Audit (NLWRA) (Hughes et al., 2001; Lu et al., 2001; Moran et al., 2001; Pickup and Marks, 2001; Prosser et al., 2001a; National Land and Water Resources Audit, 2002), were used for both catchments. Modelling outputs for suspended and bedload sediment transport (from the NLWRA) were also compared to data from this and other research projects, which include a number of projects identifying changes in estuarine sedimentation rates across the south coastal rivers region (Radke et al., 2004; Wilson, 2005).
Background
Removal of native vegetation systems is not only associated with increased discharge, but also with increased sediment transport. Removal of native woodlands leaves the surface exposed to raindrop impact (Young and Wiersma, 1973; Hairsine and Rose, 1991; Sharma et al., 1993) and combined with animal trampling (Trimble and Alexander, 1995; James et al., 1999) and agricultural activities (i.e. tillage and machinery movement), soil erosion rates are significantly higher under agriculture (Storey et al., 1964; Trimble, 1974; Pimentel et al., 1987; Walling, 1999; Wolman, 2002). Changes in sediment transport following land clearing have traditionally been associated with significant adjustments in river channel morphology (Storey et al., 1964;
~ 67 ~ Chapter 3: Land Clearing and Boundary Conditions Wolman, 1967; Trimble, 1974; Knox, 1977; Klimek, 1987; Knox, 1987; Jacobson and Pugh, 1992; Walling, 1999; Clark and Wilcock, 2000; Brierley and Stankoviansky, 2002; Wolman, 2002; Lang et al., 2003).
Research on river systems where clearing has occurred within the last few hundred years, such as in eastern Australia, has found increased hillslope erosion, expansion of gully networks, and increased channel capacity through bank and bed erosion. These have resulted in higher flux of sediment from hillslope to channel and through rivers (Warner and Bird, 1988; Prosser and Winchester, 1996; Wasson et al., 1998; Rutherfurd, 2000; Fryirs and Brierley, 2001; Prosser et al., 2001b). High system resistivity (i.e. landscape insensitivity) has also been reported in many Australian catchments due to poor sediment connectivity at the hillslope-channel and reach- scales, evidenced by low sediment delivery ratios and high rates of internal storage at the channel margins and on the floodplain (Wasson et al., 1996; Wasson et al., 1998; Fryirs and Brierley, 2001; Prosser et al., 2001b; Olley and Wasson, 2003).
Downstream transmission of changes from land clearing can be buffered through connectivity mechanisms in many environments (Walling, 1999; Harvey, 2001; Harvey, 2002). Given the low gradient catchments of southwestern Australia and the dominance of sandy-duplex soils with low overland runoff rates (Ruprecht and Schofield, 1989; Agriculture Western Australia, 2002a), system buffering potential may be high. Thus it may modulate impacts of recent and extensive land clearing on sediment transport rates. A factor further affecting the impact of land use change is the temporal variation in sediment availability. Sediment stores directly coupled to the channel are eroded rapidly and unsustainably, resulting in high initial sediment transport rates followed by progression toward lower post-clearing equilibrium rates, which are lower than the post-clearing peak. Wasson et al. (1998) highlighted this transition, where sediment transport rates increased by two-orders of magnitude following land clearing, but dropped to a post-clearing equilibrium only several times higher than the original rate.
Suspended Sediment Transport Sampling
The collection of suspended sediment data by the Department of Environment for these catchments, has been largely unplanned. While sampling covers a range of discharges, there is insufficient sampling to develop a sediment rating curve. There is however, sufficient data to assess mean sediment transport concentrations and to compare these with modelling of suspended sediment transport rates from the NLWRA. Data collected by the Department of Environment was classified into fine (less than 63μm) and medium (greater than 63μm), with 63μm marking the boundary between silt- and sand-sized particles on the Wentworth scale (Wentworth, 1922).
~ 68 ~ Chapter 3: Land Clearing and Boundary Conditions Kent River
Data from the Kent River covers a limited time period, from April 1978 to May 1981. These were collected from seven stations located throughout the catchment, with sampling in both the main channel, minor and major tributaries, and from cleared and uncleared areas. Data have been collected across a range of flows (up to 100 m3 sec-1), but no relationship between discharge and sediment concentration is discernable (see Appendix 3L). Mean values for suspended sediment loads for both the <63μ and >63μ fractions are typically low, averaging 6.1 mg l-1 for the <63μ, and 5.2 mg l-1 for the >63μ fractions (see Table 3.12 and Appendix 3L). Plotting suspended sediment concentration against the instantaneous discharge over time for the fine (<63μm) and medium to coarse (>63μm) material shows no relationship between suspended sediment and discharge, or any temporal trend (Appendix 3L).
Table 3.12 Mean sediment concentration and specific yield for the <63μm and >63μm, fractions, and total suspended sediment (TSS) yield for the Kent River catchment. Source: DoE, unpublished data and NLWRA (2002). TSS <63μ TSS >63μ TSS Catch Ave Mean Yield (kg Mean Yield (kg Yield Area Discharge (mg l-1) km-2 (mg l-1) km-2 (kg km-2 (km2) (l day-1) day) day) day) Station Name Muirs 6621 73043 33 0.0037 5 0.0005 0.0042 6041005 Highway Sth Coast 16521 1042562 3 0.0020 2 0.0010 0.0030 6041006 Highway Watershed 591 7615 3 0.0004 6 0.0008 0.0012 6041007 Tributary Mulcahy 9661 60924 5 0.0004 0.3 0.0000 0.0004 6041008 Farm 1 6041018 Nile Creek 177 113242 5 0.0030 30 0.0190 0.0220 2 604001 Rocky Glen 1125 78406 6 0.0005 2 0.0002 0.0006 Styx 18312 226009 4 0.0006 1 0.0002 0.0007 604053 Junction 1 Catchment area estimated from hydrologic modelling. 2 Catchment area from Department of Environment data.
Data for all stations are typically low (<10 mg l-1), except for the <63μm figure (33 mg l-1) for Muirs Highway (Station 6041005), and >63μm (30 mg l-1) for Nile Creek (Station 6041018). These figures stand out, particularly for Nile Creek, which drains a flat and completely forested sub-catchment, but which has only three data points. The result for the Muirs Highway Station is skewed by the one extreme reading (559 mg l-1), whereby the mean concentration is 22.2 mg l-1 after this extreme value is removed. Despite this apparent variability (Table 3.12), when units are adjusted to the number of significant figures used in other studies (e.g. Maybeck et al. (2003) group all values below 20 mgl-1 into one class), variability is low. When the results for the Kent River catchment are compared to data from around the world (Maybeck et al., 2003), rates are exceptionally low. Maybeck et al. (2003) classified rivers with a mean flow weighted suspended sediment load between 5-20 mg l-1 as very low on an international scale. Suspended sediment data collected here shows little difference between cleared and uncleared catchments. This result was not unexpected given field observations and aerial photograph analysis that ~ 69 ~ Chapter 3: Land Clearing and Boundary Conditions failed to identify areas of easily erodible lithologies, extensive gully networks, or reworking of fine-grained alluvial floodplain material within the catchment.
Dalyup River
No streamflow and sediment rating data are available for the Dalyup River, therefore data from the Lort River and Young River are used in this analysis. Data were collected between February 1983 to September 1995 from one station on the Lort River and four stations in the Young River (see Figure 3.14 for locations). Data span a range of flows, but show no relationship of discharge to suspended sediment loads for either the <63μm or >63μm, and show no temporal change (see plots of sediment transport against discharge velocity and temporal variation in Appendix 3M). Mean concentrations are higher (approximately two orders of magnitude) than those for the Kent River, but are still low on an Australian- and global-scale (National Land and Water Resources Audit, 2002; Maybeck et al., 2003), averaging 740.3 mg l-1 for the <63μm 161.2 mg l-1 for the >63μm fractions.
Data (Table 3.13) show significantly higher mean concentrations for both size fractions compared to the Kent River, but still with low specific yield (due to lower runoff, refer to Section 3.1). The uncleared Melaleuka (Station: 601600) catchment has higher mean sediment concentrations for both size fractions, compared to the paired Munglinup (Station: 601006) catchment that has been cleared, though the specific yield of suspended sediment from the Munglinup catchments was lower due to lower discharge. Using the classification of Maybeck et al. (2003), the mean sediment concentrations put this catchment in the “high” category (average suspended sediment concentration is ~ 950 mg l-1), although rates fall within the “very low” category for average daily sediment yield, due to the low discharge rates in this climate (less than 10 kg km-2 day-1 is “very low”, average for these stations is 0.009 kg km-2 day-1).
Table 3.13 Mean sediment concentration and specific yield for the <63μ and >63μ, fractions, and total suspended sediment yield for the Dalyup River catchment. Source: DoE, unpublished data and NLWRA (2002). TSS <63μ TSS >63μ TSS Catch Ave Mean Yield (kg Mean Yield (kg Yield Area Discharge (mg l-1) km-2 (mg l-1) km-2 (kg km-2 Station River Name (km2) (l day-1) day) day) day) 601001 Young Neds Corne 18931 18690 728 0.0072 158 0.0016 0.0088 601004 Lort Fairfield 29011 17059 1093 0.0064 243 0.0014 0.0078 601005 Young Cascades 88.91 1579 519 0.0092 96 0.0017 0.0109 601006 Young Munglinup 11.51 245 631 0.0135 77 0.0017 0.0152 601600 Young Melaleuka 3.51 1 915 0.0003 274 0.0001 0.0004 1 Official Department of Environment (DoE catchment area figures)
The results of discrete suspended sediment samples collected by the Department of Environment since the 1970s, shows some departure from the expected spatial and temporal ~ 70 ~ Chapter 3: Land Clearing and Boundary Conditions rates of suspended sediment transport, with little difference measured between cleared and uncleared catchments. No changes for either catchment were detected through time, despite evidence of rising groundwater and increased overland flow and runoff over the period of sampling. Sediment transport concentrations were two orders of magnitude higher in the more arid and sandy Dalyup River catchment, although the total sediment loads was less due to significantly lower discharge. Overall, both of these rivers carry little suspended sediment, evidenced by this data and the geomorphology of the study catchments. Both rivers lack current floodplain developed by overbank deposition of fine-grained sediments, and no relict evidence of these features. Because of the low number of data samples, and data collection of low flow rates, the transport rates during high flow may be underestimated. Data from McKergow et al. (2003) from the Kalgan River catchment (adjacent to the Kent River catchment), indicate that sediment transport is strongly event driven. While the rates for suspended sediment transport under flood conditions may be underestimated, the stratigraphy and geomorphology of these rivers suggests that even during flood events, large sediment loads are not entrained, transported or deposited, even after land clearing.
Changes in Sediment Transport Following Land Clearing
One focus of the National Land and Water Resources Audit was to model changes in erosion and sediment transport following European settlement of Australia. Development and calibration of the model concentrated on data from catchments in eastern Australia, although it included some data from Western Australian catchments (Hughes et al., 2001; Lu et al., 2001; Moran et al., 2001; Pickup and Marks, 2001; Prosser et al., 2001a). Subsequent work has also compared model results to cores of estuarine sedimentation, finding close agreement (Radke et al., 2004). Figure 3.16 presents an annual sediment budget for the Kent River and Dalyup River, based on the data from the NLWRA. The catchments have been divided into the major tributaries, with internal and external sediment transport paths in terms of sources and sinks identified. Of note for the Kent River is the connectivity that the NLWRA model identified between the Poorrarecup tributary, where a series of lakes intercept flow with only minor sediment contribution for the lower part of this tributary that actually connects with the main channel. Rates for specific yield of bedload sediment are significantly higher than those recorded for suspended sediment. Owingup Swamp, the receiving basin for the Kent River, receives 20 429 t yr-1, equating to a specific yield of 23.3 kg km2 yr-1, with Lake Gore receiving 11 508 t yr-1 with a specific yield of 37.5 kg km2 yr-1.
~ 71 ~ Chapter 3: Land Clearing and Boundary Conditions
Figure 3.16 Sediment budget for the contemporary Kent River and Dalyup River catchments based on data from the NLWRA. Note that the arrows are proportional to the sediment loads.
Table 3.14 Sediment transport in the Kent and Dalyup River catchments. Source: NLWRA (2002). Erosion Deposition Down- Sediment Ratio pre- Total Flood- stream delivery to post- input Hillslope Gully Bank Bed plain output ratio European Nunijup 8926 2931 5248 747 3809 1500 3617 40.5% 1264 Wamballup 4604 1384 2462 758 1748 600 2255 49.0% 101 Carrabumbup 12470 1311 3461 1826 2643 2400 7427 59.6% 857 Porrarecup 7980 1781 5629 570 3740 1600 2640 33.1% 281 Forested 26658 4169 9446 2975 0 4000 22657 85.0% 489 Nile Creek 502 407 95 0 0 0 502 100.0% 2 KENT RIVER Styx River 3296 807 2241 248 0 300 2996 90.9% 20 Lower Kent 34043 723 5133 2031 11713 1900 20429 60.0% 48 Upper West 8652 3690 3756 1206 2101 900 5652 65.3% 36 Dalyup Upper Dalyup 6040 2555 2475 1010 999 700 4342 71.9% 49 West Dalyup 10935 2026 2184 1073 0 1100 9835 89.9% 105 River
DALYUP Dalyup River 8040 1682 1545 471 2085 800 5155 64.1% 17 Lower Dalyup 19521 1964 2240 327 5313 2700 11508 59.0% 36 River Note: Erosion and deposition values in tonnes per year.
Based on the data from the NLWRA, gully erosion is the predominant sediment source (59.8%), followed by hillslope erosion (24.0%) and bank erosion (16.2%). Of the internal storage mechanisms, floodplain deposition accounts for 40.2% while bed erosion dominates with
~ 72 ~ Chapter 3: Land Clearing and Boundary Conditions 59.8%. These results are highly spatial variability, with floodplain deposition dominant in the upper reaches, and bed sedimentation dominant in the lower catchment.
Modelling of sediment supply and transport processes by the NLWRA, found similar spatial trends across both study catchments to those patterns described above. In the Kent River, suspended sediment concentrations match those from discrete sampling (Appendix 3N - A). Erosion rates are predicted to be greater in the upper catchment, with high average bank erosion 0.02-0.03 m yr-1, compared to an average 0.005- 0.01 m yr-1 in the lower, uncleared catchment. Post-European sediment transport rates are significantly higher in the upper catchment, due to high hillslope and gully erosion rates. In the Dalyup River, rates of bank erosion are slightly higher on the West Dalyup (matching field observations and aerial photography records), though are relatively consistent, varying between 0.01 – 0.015 m yr-1. Deposition and storage is higher in the mid catchment and also lower reaches, with higher erosion from hillslope and gully through the mid and upper catchment. Overall, the NLWRA predicts that the sediment transport rates have increased by 512 and 45 times in the Kent and Dalyup Rivers respectively (see Table 3.15).
Table 3.15 Sediment erosion and transport data for selected rivers from the National Land and Water Resources Audit (2004). Mean depth of In- Mean annual Mean annual channel Ratio of suspended sediment load sedimentation current to pre- sediment reaching estuary since European European River Region load (kt yr-1) (t ha-1 yr-1) settlement (m) sediment yield Rivers in NLWRA 82.9 0.2 0.4 85 All NLWRA (not in WA) 95.3 0.2 0.4 72 Murray Darling Basin 109.1 0.08 0.5 67 All NLWRA (not MDB or WA) 82.9 0.03 0.3 76 South Coast - WA 3.6 0.06 0.4 98 Kent River 9.8 <0.01 0.5 512 Dalyup River 2.5 <0.01 0.2 45
Field investigation of gully networks in steeply-sloped sub-catchments and analysis of aerial photography show only minimal extension of gully networks since the first aerial photography (1946 on the Kent River, and 1969 on the Dalyup River, both corresponding to the start of major land clearing). Based on field investigation of likely sediment source areas identified from modelling of catchment slope and aerial photo analysis, the predominant sources of sediment in both catchments has been from in-channel sources, rather than hillslope or gully erosion, though no quantitative data was collected. In-stream sediment has been sourced primarily from degradation of mid-channel islands and the stream bed, evidenced by the exposed tree roots that indicate up to 15 cm of incision. Bank erosion is a very minor process in the upper part of both catchments as the river is not confined within steep banks or incised into a floodplain or sedimentary fill. This becomes a more significant process further downstream,
~ 73 ~ Chapter 3: Land Clearing and Boundary Conditions particularly on the Dalyup River catchment. More detail of sediment transport processes and degradation of in-stream sediment stores in the context of channel response is provided in Chapter Five.
Bedload Transport Data as a Sediment-Process Indicator
The conventional model of sediment transport describes sediment as being sourced from an upland catchment and transported downstream, leading to a downstream fining sequence under the influence of greater flow competence for finer sediments and increasing storage at the channel margins including the floodplain (see Church, 2002 Fig.1, p.543). Using this model, the mapping of downstream trends in sediment calibre can be used as an indicator of downstream sediment connectivity and changing sediment sources through catchments. For the Kent River,
Figure 3.17 shows the downstream variation in median (D50) grain size increasing, particularly in the lower catchment (Figure 3.17A). This is an atypical result, showing that sediment becomes coarser downstream. Breaking the distribution into gravel, sand (very coarse, coarse, medium, fine and very fine) and mud fractions (Figure 3.17B) shows that sediment characteristics vary significantly through the catchment and are related to changes in storage or sources through the catchment (see Figure 3.17C). Four to five distinct sedimentary zones were interpreted along the Kent River and Dalyup River, based on an understanding of the unique geomorphology of catchments in this region, and adapting the model of Church (2002).
In contrast to the typical catchment with a steeply-sloped upper catchment, zone one drains a flat landscape and sources fine to medium sand from areas proximal to the channel. In the lower catchment, rainfall and low channel gradients result in poor downstream transport competence and consequently in downstream connectivity. Zone two identifies areas as the catchment enters the more dissected terrain; from here lateral sub-catchments are highly coupled to the main channel and can contribute sediment. There is significant downstream variability due to the low landscape gradient and poor downstream connectivity. Zone three represents the steeper sloped and confined mid catchment, where channel gradients are higher, downstream competence is increased and sediment variability is reduced. This zone is dominated by the medium to coarse sand that is found at the channel margins, with high storage through these reaches. In zone four, coarse sand and gravels from weathering of the exposed country rock increases sediment calibre, before sediment is stored in the lower reaches as the channel emerges onto the coastal plain.
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Figure 3.17 Downstream bedload sediment trends for the Kent River The downstream changes in bedload sediment characteristics suggest a series of sediment zones. These relate to changes in sediment sources and storage through the river.
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Figure 3.18 Downstream bedload sediment trends for the Dalyup River The downstream changes in bedload sediment characteristics suggest a series of sediment zones, which relate to changes in sediment sources and storage through the river.
In the Dalyup River catchment, mean sediment diameter (D50) is similar to the upper reaches of the Kent River catchment, although downstream increases in sediment calibre are not as
~ 76 ~ Chapter 3: Land Clearing and Boundary Conditions pronounced. Downstream trends in this catchment also reflect the sediment transport competence related to the low gradient upper catchment with poor downstream sediment connectivity, a steeper sloped mid-catchment, and the depositional and storage environment of the lower catchment on the flat coastal plain. Four zones at corresponding locations on the Dalyup River and West Dalyup River were found. Zones one and two are similar to the corresponding zones in the Kent River i.e. the far upper catchment with little sediment being transported and poor downstream connectivity. Sediment calibre increases in zone three, but lacks the unweathered mineral particles that cause such significant increases in sediment calibre in the Kent River. In the Dalyup River, this zone is associated with mobilisation of sediment stored at the channel margins and on the floodplain. Zone four corresponds to zone five in the Kent River, with sediment deposited in downstream reaches.
Data from Modelling, Sediment Cores and other Studies.
Following land clearing, estuarine sedimentation rates have typically increased by between ten and eighty times the pre European rate (Prosser et al., 2001a; Radke et al., 2004; Wilson, 2005), with annual sedimentation rates now between 4 and 20 mm yr-1 (Hodgkin and Clarke, 1989; OzEstuaries, 2006). Radke et al. (2004) compared results of the NLWRA model, to rates based on 210Pb analysis of sedimentation rates, and found that, while there was some variation due to overestimation of gully erosion, measured results were close to model predictions. Across the south coastal rivers region, specific yield varied from 0.17 kg km-2 yr-1 to 10.8 kg km-2 yr-1. Other research has identified the episodic nature of sediment transport, with large events able to transport and deposit large quantities and coarse material, resulting in the equivalent of years of deposition during single events (Western Australian Water Resources Council, 1992; McKergow et al., 2003). The episodic nature of sediment transport is supported by field observations and background literature from the study catchments.
Little data is available for sedimentation rates in the Owingup Swamp/Irwin Estuary (Kent River). Semeniuk & Semeniuk (2003) investigated the geomorphology and stratigraphy of Owingup Swamp, extracting cores from the modern delta of the Kent River and across the swamp. Stratigraphic logs presented in their report have insufficient detail or stratigraphic features to differentiate pre-and post European deposition layers, with no quantitative or qualitative estimate or data on changes in sedimentation rates presented in their report. Work in progress by Wilson using pollen and 14C analysis, has found that rates of sedimentation have increased in Lake Gore by 50 times the pre European rate (Wilson, 2005), while NLWRA modelling predicts a 33 fold increase.
~ 77 ~ Chapter 3: Land Clearing and Boundary Conditions Summary of Changes in Sediment Transport Patterns Following Land Clearing
Field data and observations data from the NLWRA, as well as data from other researchers, show that sediment transport rates have increased significantly compared to the pre-clearing regime. A variety of techniques and methods have been used to assess changes in sediment transport regime. A relatively consistent picture emerges from the variety of quantitative and qualitative primary sources, and field-based and modelling data from secondary sources. Sediment transport rates in the Kent River have increased, with NLWRA suggesting a greater than 500 times increase. Despite the magnitude of increase, mean (suspended and bedload) sediment transport rates remain very low. Predictions for increased sediment transport for the Dalyup River from the NLWRA match data from Wilson (2005) and report similar trends. While the increase in sediment transport relative to the pre-clearing rates has been large, the volume of sediment moved through these landscapes is still low.
Patterns of sediment transport differ somewhat from typical catchments due to the “backwards” arrangement in this landscape, with low gradient upper catchment and comparatively steeper- sloped mid catchment. Consequently, sediments in the upper catchment are fine to medium sands, sourced from areas proximal to the river channel. The sedimentary response to land clearing has been a stripping of sediment from in-channel sources, rather than from hillslope sources or extension of gully networks as has been reported elsewhere in Australia (e.g. Wasson, 1991; Prosser and Winchester, 1996; Wasson et al., 1996; Callow, 2000; Hughes et al., 2001; Prosser et al., 2001b). Observations of sediment stripping and present sediment availability in the upper portion of both the Kent River and Dalyup River catchments suggests that trends found by Wasson et al. (1998) have been repeated here, with much of the coarse sediment removed from these reaches, leaving a more resistant clay valley fill. Further downstream, sediment is sourced from lateral highly-coupled sub-catchments and floodplain and bank erosion as the channels become confined.
Results of the NLWRA model suggest the dominance of gully and hillslope erosion as sediment sources. Field-observations, aerial photo analysis and research by others identify that while the magnitude is correctly predicted, the majority of the increased sediment yield has come from highly-coupled, in-stream sources rather than hillslope and gully erosion. Overall, downstream sediment transport competence appears poor, largely due to the low channel gradients, and storage of sediments at the channel margins increases in mid-catchment and lower catchment area on both rivers. Changes in erosive potential of sediments due to processes associated with land clearing (salinity and vegetation degradation) are explored in the next chapter, and patterns of sediment sources and storage of difference channel styles and through the study catchments are investigated in the context of river response to land clearing in Chapter Five.
~ 78 ~ Chapter 3: Land Clearing and Boundary Conditions Summary of Changes in Boundary Conditions Following Land Clearing
This chapter started by posing the question, “has land clearing altered channel boundary conditions”? Data presented in this chapter has established that significant changes to landscape hydrology and sediment transport have occurred in the last few decades in response to land clearing for agriculture. Rivers now carry larger volumes of water, with larger flood peaks, more rapid runoff, and more frequent occurrence of bankfull discharge. Saturated areas are continuing to expand, and will do so until the middle of this century when hydrologic equilibrium is expected to be reached. Sediment transport rates are now significantly higher than the pre-European regime under which the river channels evolved. While the rates have increased between 50 and 500 times, sediment transport rates remain low on a world scale.
These changes have fundamentally altered sediment and water flux through these rivers, and consequently landscape stability. Landscape sensitivity has been altered through an increase in the erosive potential through both magnitude and frequency mechanisms. While sediment volumes reaching the channel and mobilisation through the channel have increased, they are also characterised by a strong downstream variability and consequently how changes in water and sediment flux affect channel response.
This chapter presented evidence of significant changes to boundary conditions. The sensitivity and response of the landscape is often determined or strongly impacted upon by system resistivity. The effects of landscape connectivity and changes in erosive thresholds, on system resistivity, or buffering of systems from perturbation are explored in the following chapter.
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Chapter 4: Effects of Land Management on Landscape Connectivity
Large internal lakes are common in upper catchments across southwestern Australia. The flat landscape means that these features capture runoff. Though some groundwater discharges reaches river channels, the landscape is often highly decoupled. Paperbark surrounds this lake set in the middle of a paddock in the upper Dalyup River
Connectivity is a component of system resistivity, which determines the sensitivity of landscapes to perturbation (Brunsden, 2001). While erosive forces have been altered by land clearing and salinity, the response of systems is determined by the forces both driving change and those resisting it. This chapter investigates the role that system connectivity plays in changes to the stability of river reaches in response to the altered landscape that now controls river evolution in southwestern Australia. Specifically, this chapter investigates the effects that changes in land use and in particular, the effect of current agricultural land management methods on system sensitivity through altering connectivity within these landscapes.
Section 4.1 examines effects of land management on system resistivity, through alteration of hydrologic connectivity. This first part of this chapter investigates hydrologic disconnection of landscape caused by farm dams. This is based on an unsubmitted manuscript by Callow and Smettem entitled: “Decoupling landscapes: the effect of farm dams and constructed banks on hydrologic connectivity in agricultural landscapes”. Section 4.2 examines how landscape connectivity is represented and modelled using digital elevation models (DEMs). The effect of analysis methods used to adjust DEMs to reflect the known hydrologic pathways, and there impact on subsequent terrain analysis is investigated. This work is based on the paper by Callow et al. (2007) “How does modifying a DEM to reflect known hydrology affect subsequent terrain analysis?” (see Appendix 1B).
~ 81 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity 4.1.Decoupling landscapes: the effect of farm dams and constructed banks on hydrologic connectivity in agricultural landscapes. 2 In studies of environmental systems, the term “coupling” was first applied by Brunsden and Thornes (1979) to describe the linkage between components of a landscape, in the context of system sensitivity. Brunsden (1993a) identified three types of coupling: not coupled, coupled and decoupled systems. Non-coupled systems are robust as internal barriers prevent the transmission of change, whereas coupled systems are more sensitive to perturbation as energy and material is freely transferred. Decoupled systems were once coupled, but have become non- coupled due to creation of a transmission barrier (Brunsden, 1993a). Coupling has been strongly linked to the sensitivity of geomorphic systems to changes through the ability of systems to either transmit or buffer downsystem response (e.g. Brunsden and Thornes, 1979; Brunsden, 1993a; Brunsden, 2001; Harvey, 2001; Lang et al., 2003).
Coupling relationships are usually studied between specific landscape components, typically within hillslope, hillslope-channel, tributary junction and reach-to-reach coupling (Harvey, 2002). The importance of in-stream coupling relationships is acknowledged as a control on the transmission of sediment waves and its control of patterns of in-channel sediment storage (see Hooke, 2003). Coupling is not a static quality. Temporal variability in coupling affects both the transfer of water and sediment through geomorphic systems and the sensitivity of the system to perturbation. Obvious examples of anthropogenic change of system coupling are dam and reservoir construction, their impact on downstream connectivity and the potential for geomorphic response (e.g. Johnson, 1990; Guillen and Palanques, 1992; Kearsley et al., 1994; Grams and Schmidt, 2002; Cluett, 2005).
While human impacts on hillslope processes following land clearing for agriculture are widely recognised (Walling, 1999; Hooke, 2000; Rutherfurd, 2000; Wohl, 2000a; Knox, 2001; Wolman, 2002; Lang et al., 2003), human impacts on coupling within-hillslope and between hillslope and channel remain relatively unstudied. The replacement of native vegetation systems by less-efficient water-using agricultural crops and pastures (Zhang et al., 1999) increases runoff, and surface exposure combined with increased overland flow increases sediment transport rates (Storey et al., 1964; Wolman, 1967; Walling, 1999; Brierley and Stankoviansky, 2002; Wolman, 2002; Lang et al., 2003). Land clearing alters thresholds for channel formation in favour of increased drainage density and headward expansion of first order streams (Montgomery and Dietrich, 1988; Montgomery and Dietrich, 1989; Montgomery and Dietrich,
2 This section is based on the manuscript: Callow, J.N., Smettem, K.R.J. in prep. Decoupling landscapes: the effect of farm dams and constructed banks on hydrologic connectivity in agricultural landscapes. ~ 82 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity 1992). These changes move systems toward instability, though coupling mechanisms control the transmission of changes downsystem and therefore control potential for channel morphological response.
Harvey (2002) described a limited range of studies that have investigated the temporal variability of hillslope-channel coupling, citing examples related to changes in the magnitude and frequency of erosive events affecting sediment erosion from badland and gully systems (Wells and Gutierrez, 1982; Faulkner, 1988; Harvey, 1997). Harvey (1992, 2001) also identified landscape evolution processes over long timescales that alter coupling relationships in landscapes. These investigations describe temporal variability in hillslope-channel coupling through environmental (climatic) variability and longer term changes in landscape structure. However, the potential for humans to alter hillslope coupling over shorter timescales has not been explicitly investigated.
Agriculturalists rely on rainfall to grow crops and pasture for grazing livestock under dryland conditions. Water is plentiful during wetter months, but year-round water is required for stock and domestic purposes. The response of land managers has been to construct small earthen dams and associated diversion banks to re-route and store surface runoff on hillslopes to ensure on-farm water security. Beavis & Howden (1996) commented that the impact of farm dams was similar to in-stream reservoirs, which decrease streamflow and sediment loads. Where the frequency of farm dams is sufficiently numerous, their potential to affect system connectivity becomes significant. This section investigates a contemporary example of land management practices that has changed landscape processes, focusing on an example from the upper Kent River catchment (see Figure 4.1).
Potential for dams to affect landscape coupling Farm dams have become an integral part of agricultural landscapes in dryland regions. As pressure on water resources increases due to climatic variation, increased extraction and environmental flow requirements, some research identifies the impact of farm dams on streamflow (e.g. Sinclair Knight Merz, 2000b; EGIS, 2002; Savadamuthu, 2002; Teoh, 2002). Farm dams and earthen banks are constructed for the same reasons as larger in-channel dams, to alter the spatial and temporal distribution of water resources. Where the direct upslope contributing area for dams is insufficient to capture enough runoff, earth banks (locally termed “grade banks”) may be constructed by mounding dirt across hillslopes to collect overland runoff. Deeper drains or “interceptor banks” up to 2 metres deep, designed to intercept both overland runoff and shallow lateral subsurface throughflow in texture contrast soils, are also constructed across slopes. Banks run laterally across hillslopes at gentle gradients to intercept and then divert water into dams for storage, and significantly increase the catchment area of
~ 83 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity dams. In doing so, they decrease the area of hillslope in direct hydrologic connection with river channels. Banks are also used for soil conservation by reducing slope length and erosion rates (Keen, 1998).
Figure 4.1 Location of the twelve basins in the upper Kent River catchment and rainfall isohyets.
In the study region, design guidelines stipulate that banks are constructed to retain all rainfall events less than 1:5 year Annual Recurrence Interval (ARI) event, and a farm dam should have sufficient capacity to store and withstand at least a 1 in 20 year ARI rainfall event (Keen, 1998; Agriculture Western Australia, 2003). Inspection of bank capacities in-field indicates that these features are often constructed well in-excess of this design capacity. As a result, farm dams have the capacity to remove large areas from direct hydrologic connection with the original sub- catchment outlet.
~ 84 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Beavis and Howden (1996) found that the number of farm dams in a sub-catchment of the Murray-Darling Basin (MDB) had increased since the 1960s, resulting in a reduction in streamflow in sub-catchments of between 4 to 62%. For the Hoddles Creek and Diamond Creek catchment in Victoria, Sinclair Knight Merz (2000b) found that each Ml of farm dam capacity resulted in a 2 Ml and 2.4 Ml reduction in annual streamflow (respectively), with median flow
(Q50) 14.7% lower. Schreider et al. (2002) also identified a reduction in annual streamflow for tributaries of the Murray-Darling due to increases in farm dam capacity, amounting to between 0.3 – 3.3% of annual streamflow since the 1970s. Other studies have found reductions in annual streamflow of 1.4% for the Little Yarra River (EGIS, 2002), 18% for the Upper Marne River (Savadamuthu, 2002), and 3-20% for the Onkaparinga River (Teoh, 2002).
Additional to their effect on runoff interception and reduction of streamflow, farm dams trap sediment liberated on hillslopes and therefore decouple sediment transport from hillslopes to channels. Because of the large design capacity of farm dams, outflow only occurs when design capacity is exceeded. Farm dams therefore have a high sediment trap efficiency (Verstraeten and Poesen, 2000; Farmer and Coles, 2003). Because of the high trap efficiency relative to total capacity (compared to in-stream dams and reservoirs), sedimentation rates are high and design lifespan is low for farm dams (Verstraeten and Poesen, 2000; Farmer and Coles, 2003). Dams and banks are therefore important landscape features affecting the downsystem transmission of changes in runoff and erosion processes on hillslopes.
Representing and Modelling Landscapes Digital Elevation Models (DEMs) have become one of the most widely used tools for representing landscapes and modelling processes, and are useful in several disciplines including ecology, hydrology, geomorphology. By representing the landscape using a grid of elevation values, flux between cells in a downslope direction can be used to model catchment hydrologic processes. Accurate representation of landscape processes is dependent on the DEM quality and the algorithms used to process landscape data (Quinn et al., 1991; Hutchinson and Dowling, 1994; Wolock and Price, 1994; Zhang and Montgomery, 1994; Gyasi-Agyei et al., 1995; Gallant and Hutchinson, 1997; Tarboton, 1997; Jones, 1998; Walker and Willgoose, 1999; McMaster, 2002; Tarboton and Ames, 2002; Gallant and Dowling, 2003; Callow et al., 2007).
While the impact of human infrastructure such as roads has been incorporated into DEMs (e.g. Croke et al., 1999; Luce, 2002; Duke et al., 2003), and tools for estimating the impact of farm dams on streamflow (Sinclair Knight Merz, 2000b), the effect of farm dams and associated earthworks (i.e. earthen banks to increase catchment area of dams) have not been explicitly incorporated into DEMs for investigating hydrologic and geomorphic change in basin characteristics and processes.
~ 85 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Changes in Dam Frequency and their Catchments Investigation Methodology Twelve basins were selected (basins (i) – (xii), see Figure 4.1 for locations) for detailed analysis. Sub-catchments directly coupled to the main channel of the Kent River (see Figure 4.1), and ranged in area from 0.5 to 4 km2. Rectified aerial photographs from 1965, 1973 and 1999 were used to identify farm dams and banks. A 10-metre DEM generated as part of the “Land Monitor” project (Allen and Beeston, 1999; Caccetta et al., 2000) was used to model basin characteristics. Figure 4.2 details the process used to incorporate banks and dams into the DEM of the basins. The DEM was modified firstly by pit-filling using the most commonly-used filling algorithm of O'Callaghan and Mark (1984), then clipped into separate basins (basins (i) to (xii)). Under this “natural” catchment scenario, all internal pits were filled runoff was routed to the edge of the DEM (as is the standard practice, e.g. see Doan (2000) or Ackerman (2002) ).
A series of steps were then undertaken simulating changes in the “modified”, agricultural landscape. Banks and dams were digitised and made into grids based on the catchment from the 1999 aerial photography. Banks were accounted for first by raising the elevation of “bank” cells by 1m. The resulting surface was then pit-filled to ensure that flow was forced along banks. Care was taken to ensure that no “leakage” occurred between cells along the bank when digitising banks, or following pit-filling. Dams were then accounted for by lowering the elevation by 10 metres, and the resulting basin was extracted. Areas draining into a dam or where a bank had rerouted flow across the slope and into a neighbouring catchment were removed from the modified DEM.
Figure 4.2 Investigation methodology used to incorporate the hydrologic influence of farm dams and banks into DEMs.
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Figure 4.3 Changes in area of the hillslope in basin (ii) retaining a hydrological connection with the main river channel without (A & B) and with (C & D) the effects of banks and farm dams incorporated into the DEM.
Figure 4.3 shows an example from basin (ii) under natural (Figure 4.3A and B) and modified conditions (Figure 4.3C and D). Two dams and 840m of banks remove 15% of the upper catchment from hydrologic connection with the catchment outlet. Lower in the catchment, a long bank runs from right to left, capturing water that has flowed down the right side of the catchment, and then routes it laterally across the hillslope before it discharges downstream into the Kent River. This illustrates the dual effect of banks in reducing effective catchment area where water is routed into dams, or increasing residence time where water is intercepted and forced across hillslopes before reaching the catchment outlet.
~ 87 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Changes in Effective Catchment Area Increased Dam Frequency as a Human Response to Climate Change Studies in other dryland settings have reported increases in the number of farm dams since the 1970s, citing productivity pressures and a desire to increase on-farm water security as the reasons for increases in farm dams (Beavis and Howden, 1996; Schreider et al., 2002). While these pressures also exist in the Kent River catchment, southwestern Western Australia has also experienced a stepped drop in annual rainfall of 15% since the 1970s (see Figure 3.4, and Smith et al., 2000; Indian Ocean Climate Initiative, 2002). This has compounded pressures on farm water security and is likely to have increased pressures to construct dams. Banks increase individual dam catchment area in light of reduced rainfall and so offer increased storage reliability for existing dams.
Changes in the number of farm dams were investigated for 1965, 1973 and 1999 from aerial photographs. Table 4.1 details the results for the twelve study sub-catchments (See Figure 4.1 for their locations), identifying an increase in the average number of dams from 2.6 to 5.4 dams per basin, and a doubling in the number of dams per hectare to 2.8 (Table 4.1). In Figure 4.4 the number of dams is plotted against basin area. Trend lines for the various years indicate increased number of dams across all basins. The R2 for the various years also becomes higher, suggesting that the construction of dams is approaching the maximum number for each basin. By 1999 all of the twelve basins had dams and there is less variation between basins.
Table 4.1 Total number of dams in each study catchment in 1965, 1973 and 1999. 1965 1973 1999 AREA Catchment (ha) Number Ave Number Ave Number Ave of Dams per ha of Dams per ha of Dams per ha i 2.52 4 1.59 4 1.59 5 1.98 ii 0.59 0 0.00 1 1.69 1 1.69 iii 1.34 1 0.75 2 1.49 3 2.24 iv 3.76 6 1.60 7 1.86 14 3.72 v 1.6 1 0.63 1 0.63 3 1.88 vi 0.97 4 4.12 4 4.12 5 5.15 vii 1.73 2 1.16 3 1.73 3 1.73 viii 3.47 1 0.29 5 1.44 9 2.59 ix 2.11 6 2.84 6 2.84 8 3.79 x 0.9 2 2.22 3 3.33 3 3.33 xi 1.86 2 1.08 2 1.08 4 2.15 xii 1.93 2 1.04 5 2.59 7 3.63 Average 1.9 2.6 1.4 3.6 2.0 5.4 2.8
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Figure 4.4 Relationship between basin area (ha) and number of dams in 1965, 1973 and 1999. Interestingly the R2 values increase through time. This suggests that the number of dams in a basin may be evolving toward some type of maximum value for each basin, dependent on the basin area.
Spatial Extent of Landscape Decoupling Results showing the reduction in effective catchment area following dam and bank construction are presented in Table 4.2. Overall, catchments experienced an average 39.5% reduction in the area maintaining hydrologic connection with the catchment outlet in 1999 compared to the natural state, though the range was 4.3% to 86.7%. Interestingly the total number of dams does not appear to correlate well with the total reduction in catchment area (R2 = 0.44), and there is no correlation between the length of banks either (R2 = 0.01). Rather, less easily quantifiable factors such as the landscape position of individual dams appears an important factor. Dams constructed in concave areas of the lower landscape, locally called “gully dams”, have the greatest impact on catchment area. Of the five basins showing the most extreme reductions in effective catchment area, four result from construction of gully dams lower in the landscape. The fifth basin (basin (vi)), had five dams (the highest dam density at 5.15 dams per ha) and 2.77 km of banks constructed in the upper portion of a 0.97km2 catchment. Results from Basin (iv) further highlight the importance of landscape position on the magnitude of catchment area change. With 14 dams in a 3.76 km2 catchment, it had the most dams of any basin and the third highest density, yet the dams were generally in high landscape positions and not in depressions, which resulted in a 35.1% reduction in catchment area, less than the average across the twelve basins.
Within the group of basins without gully dams, three were selected to investigate the relative influence of dams and banks on discharge and basin characteristics. Basin (i) is close to the average basin size, and had both dams (5) and banks (90m) which caused a 31.7% reduction in catchment area. Basin (ii) has 840 m of banks; some of which channel water into its two dams and others that redistribute flow within the basin but retain hydrologic connection to the outlet.
~ 89 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Basin (ii) allows the effects of banks which reroute water to be assessed. Basin (iv) represents a larger basin with an average (35%) reduction in area due solely to 14 dams.
Table 4.2 Effects of banks and dams on catchment area Natural Modified Catchment Catchment Catchment Area Number Length of Area Change Notes: (km2) of dams banks (km) (km2) (km2) (%) i 2.52 5 0.09 1.72 0.8 -31.7 Dams higher in landscape ii 0.59 1 0.84 0.5 0.09 -15.2 Banks route some flow out of catchment iii 1.34 3 0.83 0.86 0.48 -35.8 Banks route all runoff to dams iv 3.76 14 0 2.44 1.32 -35.1 Large number of banks, one gully dam higher in landscape v 1.6 3 0 1.14 0.46 -28.8 Dams mid-slope, but not in gullies vi 0.97 5 2.77 0.17 0.8 -82.5 Banks route most runoff into dams vii 1.73 3 0 1.66 0.07 -4.3 Dams high in landscape viii 3.47 9 0 1.45 2.02 -58.2 Gully dam on main channel ix 2.11 8 0.52 1.77 0.34 -16.1 All dams high in landscape x 0.9 3 0.65 0.12 0.78 -86.7 Gully dam on main channel xi 1.86 4 3.93 0.95 0.91 -51 Gully dam higher in landscape xii 1.93 7 0 0.45 1.48 -76.5 Gully dam on main channel
Landscape Structure in Agricultural Catchments Identifying Changes in Catchment Function Commonly used geomorphic metrics were used to investigate changes in basin topography, and potential for geomorphic processes to be affected by dams and banks capturing runoff from hillslopes before it reaches the main river channel. Four common geomorphic descriptors; “cumulative area distribution” (CAD), “hypsometric curve” (HC), “simplified width function” (SWF) and the “instantaneous unit hydrograph” (IUH), were selected to investigate basins i, ii and iv. These were applied to each basin under “natural catchment” and “modified catchment” conditions. The geomorphic descriptors selected here have been used extensively to quantitatively investigate or describe a variety of processes (see reviews of the CAD, HC and SWF by: Hancock and Willgoose (2001) and Hancock (2005), and the work of Rodriguez- Iturbe and Valdes (1979), Gupta et al. (1980) and Rodriguez-Iturbe and Rinaldo (2001) on the IUH).
The calculation and application of these geomorphic statistics is detailed briefly. The CAD plots the area of the catchment that has a drainage area greater than or equal to a specific drainage area. The plot has been used as an indicator of the areas of a catchment that saturate and
~ 90 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity generate saturation-excess overland flow (the dominant overland flow mechanism in the Kent River catchment) (Hancock and Willgoose, 2001; Willgoose and Perera, 2001). The HC is a non-dimensionalised plot of elevation against area, and was first used by Strahler (1952; 1964) to describe the age or maturity of landscapes. More recently Willgoose and Hancock (1998) linked its shape to specific erosional processes, catchment geometry and network form. The SWF plots the width of a catchment against the distance from the catchment outlet. Traditionally, “width” has been measured by the number of streams at a particular distance form the outlet, often referred to as the “standard width function” (Boggs et al., 2001). The width function as applied here is the “simplified width function” (SWF), which plots all cells, and is a measure of hydraulic response of a catchment, mirroring the shape of the IUH when routed using constant velocity (Hancock and Willgoose, 2001). Finally the IUH, represents the hydrologic response of the catchment, plotting the time taken for 1 unit (in this paper a figure of 1mm is used) of rainfall falling on every cell to reach the channel outlet, and therefore representing an overland flow response to rainfall under 100% runoff.
Geomorphic statistics were calculated on the natural and modified DEMs using the ArcEvolve extension for ArcView created by Boggs et al. (2001). A cellular time-isochrone model using a modified Clark’s technique (Clark, 1945) as described by Usul & Yilman (2002) (see Equation 4.1 – 4.3), was selected to determine the shape of the IUH. Hydrographs were determined in a GIS using Equations 4.1 – 4.3 evaluated on a 10m DEM. Because of the small basin sizes, no differentiation between channelised and unchannelised flow was factored into the flow routing, and storage attenuation coefficients were ignored due to the ungauged nature of the basins.
0.7 0.8 §1000 · 1.347L f ¨ 9¸ © CN ¹ Tc 0.5 (4.1) 190S p where Tc is the time to concentration in minutes, Lf is the maximum length of the watershed in metres, CN is the Soil Conservation Service (SCS) Curve Number (SCS-CN) for the basin
(determined to be 84 (ASCE, 1996)), and Sp is the average slope of the basin (determined from GIS analysis) in percent (Usul and Yilmaz, 2002).
Tc Tt Lc WeightGrid (4.2) L f where Tt is the travel time to the basin outlet for each cell in minutes, Lc is the flow length of each cell to the basin outlet in metres (Usul and Yilmaz, 2002), and WeightGrid is either nil for constant velocity flow routing, or for variable velocity; 1 WeightGrid (4.3) 1 2 R 3 S n
~ 91 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity where n is the manning roughness coefficient, R is the depth of flow (based on a classification of a cells flow accumulation – see (Usul and Yilmaz, 2002), and S is the slope of the cell in metres per metre.
Have Dams and Banks Affected Basin Geomorphology? All basins showed little change from the natural to modified condition for both the CAD and hypsometric curve statistics (Figure 4.5). The shape of the CAD is important for defining the area which becomes saturated and generates overland flow, and the curve has three distinct regions: the area at the top of the catchment (“S” portion of the curve at the far left) dominated by diffuse flow mechanisms; the log-linear middle section where flow becomes channelised; and the final region marked by a drop off in the curve where minor tributaries join to form a larger, high order stream (Hancock, 2005). For each basin, the shape of the CAD remained unchanged, as did boundaries of the different regions, though the pixel areas decreased due to reduced catchment area. For the HC, the normalised area-elevation plots changed very little for basins (i) and (ii), maintaining a shape similar to a “mature” drainage shape (based on Strahler, 1952). Basin (iv) shows some deviation, with a more “mature” curve under the modified condition due to the removal of more of the steeper parts of the upper catchment. The CAD and HC are descriptors of geomorphic processes and based on results from these statistics (Figure 4.5), and despite reduced catchment area of 15% to 35%, little change in catchment function is predicted.
The SWF and the IUH are both descriptors of the runoff signature of catchments. Under the modified condition, discharge signatures from all three basins were affected by reduced catchment area. The SWF and IUH highlight the effect that bank construction has on increased residence time for basin (ii). Basin (i) lost part of the upper catchment due to dam construction. This is seen in the IUH, where the rising limb of the hydrograph under modified condition, mirrors that of the catchment under natural condition. Drainage is captured in the upper catchment and the hydrograph peaks at 10 m3 sec-1 (rather than 13 m3 sec-1 under the natural scenario, a 22% reduction in peak discharge). A similar effect is evident for basin (iv), however, the dams lower in the landscape capture runoff sooner; the rising limb is affected more quickly and consequently peak discharge is reduced by 29%. The effects of constructed banks that reroute water flow internally, but maintain a hydrologic connection with the catchment outlet is evident for basin (ii), where residence time is increased. Unlike basins (i) and (iv) where all water rerouted by banks is captured by dams, time to peak discharge is delayed by the banks. The effects of the banks in concentrating flow from different portions of the catchment also causes the change in the hydrograph shape from smooth, to one with three distinct peaks.
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Figure 4.5 Effects of bank and farm dam construction of geomorphic statistics for basin i, ii and iv.
Conclusions The results from this investigation confirm those from previous research that farm dams and banks alter the discharge regime from catchments (Henschke and Bessell-Browne, 1983; Davies et al., 1988; Keen, 1998; Sinclair Knight Merz, 2000b; Savadamuthu, 2002; Schreider et al., 2002; Teoh, 2002; McMurray, 2003). By including banks and dams in a DEM to better represent the hydrologic reality of an agricultural catchment, basins were decoupled. The total number of farm dams and length of banks appears less significant than their landscape position, which has an important bearing in the reduction catchment area. The greatest effect came from “gully” dams in low landscape positions which disconnected up to 85% of the catchment area from the outlet. Banks which retain a hydrologic connection with the basin outlet reroute flow, increase residence time and can result in localised increases in discharge and a more complex, multi peaked hydrograph. They also serve to decouple sediment transport potential by capturing
~ 93 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity flow (and entrained sediment) and rerouting it across slopes at low gradients. Catchments dominated by dams had broader, flatter and smooth hydrographs with shorter flow duration, whereas those with numerous banks have delayed peak discharge, multiple peaks and longer flow duration.
Overall, change to geomorphic statistics that represent process characteristics (cumulative area distribution and hypsometric curve) changed little, despite effective removal of over 35% of the catchment (due to banks and dams). The geomorphic statistics that represent discharge response (simplified width function and the instantaneous unit hydrograph) changed significantly, with reduced discharge where dams were present, and increased residence times and reduced flood peaks where banks rerouted flow. These findings have implications for hydrologic prediction in agricultural catchments, as all hydrologic models (empirical, distributed, semi-distributed and lumped) calculate discharge as a function of basin area. Extraction of input parameters for hydrologic modelling from DEMs is becoming increasingly commonplace (Doan, 2000; Ackerman, 2002), though catchment area of a pit-filled DEM may not match the hydrologic reality of basins within agriculturally development catchments.
Modelling runoff in agricultural landscapes affected by banks and dams presented many challenges. In this analysis, simple geomorphic statistics were used to represent the discharge response of catchments (simplified width function and the instantaneous unit hydrograph) as the basins were ungauged. These metrics assume 100% runoff and overland flow in representing discharge response. In this environment, overland runoff is rare in upper catchment areas and runoff models in this landscape need to address issues such as effects of banks and dams at the catchment-scale, variability in dam storage capacity over time due to sedimentation, dam leakage and how development of saturated sources areas and saturation-excess overland flow is impacted. This investigation reveals the significant potential for banks and dams to affect runoff potential in agricultural landscapes at the sub-catchment scale.
Geomorphic response to land clearing has seen many examples of significant increases in water and sediment flux following removal of forests and woodlands. These changes have resulted in morphological response in rivers to accommodate increased water and sediment flux under agricultural land uses. In this catchment, changes to landscape processes as a result of agriculture have been tempered by the changes to the landscape as a result of agricultural activities. The design guidelines for dams means that they are able to retain all but the most extreme rainfall events and therefore apart from leakage or dam failure, portions of a basin upslope of dams and associated banks are removed from hydrologic connection with the main river channel. These banks and dams remove not only potential discharge from reaching the main river channel, but also sediment liberated from upslope areas that is now stored in dams.
~ 94 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Changes to catchment area have important implications for the erosion processes in these catchments. While upslope of dams, eroded material is now stored in dams, downslope, the catchment area of lower order streams and gullies is now significantly less. The relationship between slope length and the generation of channelised flow and position of stream head (Montgomery and Dietrich, 1988; Montgomery and Dietrich, 1989; Tarboton et al., 1991; Montgomery and Dietrich, 1992; Tarboton et al., 1992) and erosion rates (Wischmeir and Smith, 1961; Moore and Burch, 1986; Renard et al., 1991; Moore and Wilson, 1992) is widely acknowledged. Changes in hillslope-channel coupling may accompany land use changes, and have the potential to affect the transmission of hillslope changes through systems.
4.2.Spatially Representing Landscape Connectivity
How Does Modifying a DEM to Reflect Known Hydrology Affect Subsequent Terrain Analysis?3
The previous section highlighted the effects of current land management practices on hydrologic landscape connectivity. Spatially representing landscape connectivity and landscape integrity is important when modelling and representing fluvial processes at the catchment-scale. Hydrologic representativeness is therefore required of DEMs that are used to model landscape processes. Efforts to improve DEM performance in replicating known hydrology have included a variety of soft (i.e. algorithm-based approaches) and hard techniques, such as “stream burning” or “surface reconditioning” (e.g. Agree or ANUDEM). Using a representation of the known stream network, these methods trench or mathematically warp the original DEM to improve how accurately stream position, stream length and catchment boundaries reflect known hydrologic conditions. However, these techniques permanently alter the DEM and may affect further analyses (e.g. slope). This section explores the impact that commonly used hydrologic correction methods (stream burning, Agree.aml and ANUDEM v 4.6.3 and ANUDEM v5.1) have on the overall nature of a DEM, finding that different methods produce non-convergent outcomes for catchment parameters (such as catchment boundaries, stream position and length), and differentially compromise secondary terrain analysis. This section also provides a basis for determining ways of correcting for hydrologic processes using DEMs for use in subsequent analyses in this thesis.
Detecting surface hydrology features in flat landscapes using DEMs is a problem, and new analysis methods (Garbrecht and Martz, 1997; Gallant and Dowling, 2003) and ways to
Callow, J.N., Van Niel, K. and Boggs, G.S., 2007. How does modifying a DEM to reflect known hydrology affect subsequent terrain analysis? Journal of Hydrology, 332(1-2): 30-39. (A copy of this paper is presented in Appendix 1B) ~ 95 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity recondition DEMs for improved performance have been suggested (Hutchinson, 1989; Hellweger, 1996; Soille et al., 2003). Whilst some of these methods are soft solutions (e.g. flow algorithms), others permanently alter the DEM. Soft methods are relatively well understood (Tarboton, 1997; Jones, 2002; Tarboton and Ames, 2002) and while impacts of DEM error on secondary terrain analyses have been studied (Fisher, 1998; Holmes et al., 2000; Van Niel et al., 2004), the impact of permanent alteration of a DEM on any further analysis has not been investigated. This section explores the impact that altering DEM cell values to better represent the known hydrology of a landscape (hydrologic correction) has on the overall nature of a DEM, and how the changes impact on any further terrain analysis. It demonstrates different, non- convergent outcomes for secondary terrain analyses due to the type of method used.
DEMs are widely used for modelling surface hydrology. Analyses include the automatic delineation of catchment areas (O'Callaghan and Mark, 1984; Martz and De Jong, 1998), development of terrain characteristics (Moore et al., 1991) and drainage networks (Fairfield and Leymarie, 1991), the detection of channel heads (Montgomery and Dietrich, 1988; 1992), estimating hydrology and soil moisture (Beven and Kirkby, 1979; O'Loughlin, 1986; McKenzie et al., 2003; English et al., 2004), determination of flow accumulation (Peuker and Douglas, 1975) and flow direction and routing (Tarboton, 1997; 2002), and automated extraction of parameters for hydrologic or hydraulic modelling (Doan, 2000; Ackerman, 2002).
The accuracy with which a DEM is able to replicate the hydrologic reality of a catchment is determined by the scale of capture (i.e. cell size), the precision (i.e. vertical accuracy and relative accuracy between adjacent, upstream and downstream cells) and strength of the landscape gradient (i.e. flatness) (Quinn et al., 1991; Hutchinson and Dowling, 1994; Wolock and Price, 1994; Zhang and Montgomery, 1994; Gyasi-Agyei et al., 1995; Gallant and Hutchinson, 1997; Walker and Willgoose, 1999; McMaster, 2002). Algorithms used to represent and extract real-world processes from a DEM are also significant (Tarboton, 1997; Jones, 1998; Tarboton and Ames, 2002; Gallant and Dowling, 2003).
Thus, a number of methods for DEM improvement have been suggested, such as the removal of spurious sinks (Jenson and Dominque, 1988; Soille et al., 2003), incorporation of vector stream data for stream burning (Maidment, 1996; Mizgallewicz and Maidment, 1996; Saunders, 1999) or surface reconditioning (Hutchinson, 1989; Hellweger, 1996; Hutchinson, 2004). Further problems in delineating surface hydrology are caused by human impacts that have altered catchment hydrologic processes. For example, the incorporation of road data into modelling of surface runoff has been identified as a significant process (Croke et al., 1999; Luce, 2002; Duke et al., 2003).
~ 96 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Flat landscapes, such as the study catchment, which has a mean drop per pixel of 0.32 m, require a vertical accuracy of 32 cm for a 10-m DEM, according to the Gyasi-Agyei criteria (Gyasi-Agyei et al., 1995). A DEM of the study region was purchased from the Department of Land Information (DLI). Generated as part of the Land Monitor Project (Allen and Beeston, 1999; Caccetta et al., 2000), the DEM has a grid size of 10 metres and a vertical accuracy of ±1m (Caccetta et al., 2000). This DEM is at the higher end of available, catchment-scale DEMs in terms of the grid size and vertical accuracy, compared to products by professional agencies such as the United States Geological Survey (USGS 30m DEMs), Shuttle Radar Topography Mission (SRTM 30m or 90m products) and British Ordnance Survey (Land Form Profile 10m DEMs). While DEMs created from Light Detecting And Ranging (LIDAR) methods offer better resolution and precision, they are not typically available at the catchment scale and are usually produced for a specific project, rather than being widely available. While this DEM can delineate hydrology with good accuracy and precision in many areas, flatter regions which have a mean drop per pixel up to two orders of magnitude lower than the catchment mean, require a DEM with centimetre to sub-centimetre accuracy. DEMs of this accuracy are not available for this study area, or for many others at the catchment or sub-catchment scale.
The minimum resolution and precision of a DEM are important when analysing surface hydrology (Quinn et al., 1991; Hutchinson and Dowling, 1994; Wolock and Price, 1994; Zhang and Montgomery, 1994; Gyasi-Agyei et al., 1995; Gallant and Hutchinson, 1997; Walker and Willgoose, 1999; McMaster, 2002). However, the required level of resolution and precision is often not available, particularly for vast, flat landscapes. In these circumstances, methods of DEM hydrologic correction must be used. However, many further analyses are often conducted. There is little understanding of the effect that the introduction of additional error by hydrologic correcting a DEM has on subsequent analysis.
While pit filling and flow direction algorithms are well known to affect hydrologic analysis (e.g. Tarboton, 1997; Tarboton and Ames, 2002; Soille et al., 2003), this section focuses on effects of hydrologic correction algorithms. All DEMs used were prepared using a pit filling algorithm (Jenson and Dominque, 1988), and flow direction calculated using a D-8 algorithm (O'Callaghan and Mark, 1984). This is the most commonly used approach when preparing and processing DEMs for hydrologic analysis (Saunders, 1999; Jones, 2002). Hydrologic modelling with a pit-filled version of the raw Land Monitor DEM revealed disagreement with the expected water catchment boundaries and stream network data in some (flatter) sub-catchments. The raw DEM routes flow through a series of lakes in the northwest of the catchment, and into the neighbouring Frankland River catchment (Figure 4.6). Field work and consultation with local farmers and land managers identified how flow should be routed through the lakes and channels and into the Kent River catchment, as depicted in Figure 4.7. Thus, alteration of the DEM to
~ 97 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity replicate the new hydrologic reality of the study catchment is required. This section discusses surface reconditioning methods and explores their impact on the nature of the DEM and subsequent terrain analyses (slope) conducted using the hydrologically corrected DEMs.
Figure 4.6 The upper Kent River study catchment, showing how the raw DEM incorrectly routes flow out of the Kent River catchment and into the neighbouring Frankland River catchment to the west.
This study investigates three algorithms for modifying a DEM to reflect known hydrology; ‘stream burning’, ‘Agree’ and ‘ANUDEM’. While a number of algorithms have been developed for this purpose, these algorithms were selected as they are more widely used (e.g. Saunders, 1999; Renssen and Knoop, 2000; Turcotte et al., 2001; Doll and Lehner, 2002; Soille et al., 2003), and represent three scales of DEM modification that algorithms are based on. That is, stream burning only modifies the elevation value of stream cells within a DEM, while the Agree algorithm modifies the value of streams cells and the surrounding area within a user defined distance. The ANUDEM algorithm can potentially modify a whole landscape. These algorithms,
~ 98 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity therefore, provide three contrasting approaches that are likely to have variable effects on secondary terrain analysis.
Figure 4.7 Expected flow directions and catchment boundary from field work and people with expert knowledge.
Stream burning was developed to improve the replication of stream positions by using a raster representation of a vector stream network to trench known hydrologic patterns into a DEM at a user specified depth (Maidment, 1996; Mizgallewicz and Maidment, 1996; Saunders, 1999). Depths of 1m, 2m, 5m, 10m and 100m deep were used to assess the trenching depth required to correct hydrology. Stream burning has been used successfully in many other projects (Saunders, 1999), but with some limitations, such as distorted watershed boundaries (Saunders, 1999) and the creation of parallel streams (Hellweger, 1996). Stream burning also creates a discrepancy between the original DEM and the trenched “stream” cells, leading to a dramatic jump in elevation, which is likely to affect derived properties such as slope, particularly when a deep trench is required to correct hydrology. The advantages of this method are its simplicity, computational efficiency and the restriction of changes to fewer cells in the landscape.
Agree (Hellweger, 1996) uses a raster representation of the known stream network to lower the landscape across a user-specified horizontal buffer distance and depth as well as burning a stream at a user-selected depth. This method addresses the parallel drainage problems through creation of an even sloped surface across the buffer distance, but in so doing, smoothes the landscape, creating an even gradient and uniform aspect perpendicular to the stream channel. A greater number of cells are changed and a drop between flat areas and stream cells still occurs where a channel is trenched. After unsuccessful trial settings of 250m, 500m and 750m with drops of 1m, 2m and 5m across the plain with 1m and 2m drop at the channel, a setting of
~ 99 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity 1000m buffer distance, vertical drop of 5m across the buffer plain and a channelised depth of 2m was selected, which replicated the true site hydrology well.
ANUDEM (Hutchinson, 1988; Hutchinson, 1989; Hutchinson, 2003; Hutchinson, 2004) creates a smooth surface without sinks by imposing a global drainage condition via an iterative drainage enforcement algorithm, which is based on input data that can include irregularly spaced elevation data points (spot heights), contour lines, streamlines, sink points (lakes) and cliff lines. Elevation across the entire DEM can be altered when creating a new surface that enforces drainage and eliminates abrupt jumps between the stream and non stream cells. Although ANUDEM allows the user a larger range of input variables to correct hydrology, for the sake of simplicity and consistency across the three different methods, only the stream network data was used to enforce drainage. ArcGIS 9 was used to create a two metre contour dataset (with the contour interval representing half the vertical accuracy of the original DEM) and the UNGENERATE command used in ARC/INFO to prepare the contour and stream network data for processing in ANUDEM 5.1 using default values for roughness penalties and standard errors (Hutchinson, 2004). ANUDEM 4.6.3 (Hutchinson, 2003) was also used through the TOPOGRID command in ArcGIS 9, with the data prepared as above and processes using suggested default values. Both results were output as 10m grids.
The modified DEMs created using these four different methods were then pit filled (Jenson and Dominque, 1988), before a stream network was extracted. This was created using a critical contributing area value of 1.5km2, as determined from application of the area/slope method (Tarboton et al., 1991) and checked against stream head position on aerial photography. The catchment boundary was delineated from an outlet point at the junction of this sub-catchment with the main channel of the Kent River. Catchment area and stream networks derived for each DEM were compared visually against the expected result and values for catchment area, stream length, mean and maximum slope used to quantify the success of different methods.
The successful and least severe (i.e. the shallowest trench) Stream burning, Agree and ANUDEM v4 and ANUDEM v5 DEMs were then further analysed to determine the spatial pattern and magnitude of changes caused by correcting hydrology. An assumption was made that while the raw DEM contained error (causing the incorrect routing of flow), it represented the best representation of real catchment topography. A residual grid was created by subtracting the various hydrologically corrected grids from the original DEM (pit filled version of the raw DEM). Three-dimensional plots with histograms and values for mean, standard deviation, total Root Mean Squared (RMS) error, minimum and maximum change were used as a means of assessing the magnitude and spatial patterns of changes caused by hydrologic correction. Slope (Horn, 1981) (in percent) was calculated from the raw and corrected DEMs, with residuals
~ 100 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity calculated as above to determine the effects that hydrologic correction on a subsequent terrain analysis.
Comparison of Algorithm Performance Stream burning required a 10m deep trench in this flat landscape to make DEM-derived hydrology match the known reality of the catchment. Figure 4.8b-c identifies that trenching up to 5m has little effect on stream position and catchment area. Trenching depths of 1m, 2m and 5m marginally increased stream length, while catchment area remained the same as the original DEM. Stream burning with 10m and 100m trench replicated stream network length (-1%) and position (Figure 4.8D) best out of all methods, with catchment area close to the expected value (-7%). While trenching 10m or 100m produced identical catchment areas and similar stream lengths, the deeper trench caused mean slope (2.4% to 5.2%) and maximum slope (59.7% to 562.8%) to increase significantly compared to the original DEM. Also of note are the truncated catchment boundaries in the lower catchment (Figure 4.8D & 3E), a characteristic downfall of Stream burning as identified by Saunders (1999).
Agree, ANUDEM v4 and ANUDEM v5 all improved the delineation of catchment boundary and stream locations over the original DEM. Figure 4.8F shows that the catchment delineated by Agree is close to the expected result (+3%) and has corrected the problem with the truncated catchment boundary in the lower catchment (see Table 4.3). Stream length was slightly shorter than expected (-3%), and stream position replication was poorer than other successful methods, as an area of overlapping buffer distance caused drainage to meander tortuously. Smoothing topography across the buffer distance modulated the effects of Stream burning, resulting in the smallest increase in mean slope (+3%), though maximum slope increased significantly (+172%).
ANUDEM v4 and ANUDEM v5 performed similarly, producing catchment areas close to the expected result (-3% and -7% respectively), with ANUDEM v4 and Agree the best at replicating catchment area (ANUDEM v4 -3%, Agree +3%). Replication of stream length was acceptable for both ANUDEM methods (+5%, +6% respectively), as was stream position though some differences in the upper catchment are evident (compare Figure 4.8G and 3F to 3A). Results for increased mean and maximum slope did, however, show some difference between the two methods. ANUDEM v4 performed better than ANUDEM v5 for mean slope (+6% to +9%), but ANUDEM v5 performed best at minimising increases in maximum slope (+37% to +57%). Both ANUDEM v4 and ANUDEM v5 performed well in replicating stream length, position and catchment area and were consistent with the known hydrology of the catchment.
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Figure 4.8 The catchment area and stream position expected from expert knowledge (a), against the results from; the original DEM data (b), unsuccessful Stream burning to 5m (c), successful Stream burning at 10m and 100m (d & e (note: similar results with different depth of trench), Agree (f), ANUDEM version 4.6.3 (g) and ANUDEM version 5.1 (h).
~ 102 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Table 4.3 Results achieved by the original DEM and different hydrologic correction methods in replicating known hydrologic conditions and catchment parameters. The most successful method for each parameter is identified in bold. Hydro-logically Catchment Area Stream Length Mean Slope Max Slope Correct (km2) (km) (%) (%) Expected Result 69.5 34.7 2.4 16.9 Original DEM NO 11.9 (-83%) 5.2 (-85%) 3.5 (+46%) 12.5 (-26%) Trench 1 m NO 11.9 (-83%) 5.2 (-85%) 3.6 (+50%) 12.6 (-25%) Trench 2m NO 11.9 (-83%) 5.7 (-84%) 3.6 (+50%) 15.2 (-10%) Trench 5m NO 11.9 (-83%) 5.7 (-84%) 3.8 (+58%) 31.8 (+88%) Trench 10 m YES 64.5 (-7%) 34.2 (-1%) 2.7 (+14%) 59.7 (+253%) Trench 100 m YES 64.5 (-7%) 34.3 (-1%) 5.2 (+117%)562.8 (+3230%) Agree YES 71.6 (+3%) 33.7 (-3%) 2.5 (+3%) 46 (+172%) ANUDEM 4 YES 67.6 (-3%) 36.3 (+5%) 2.6 (+6%) 26.6 (+57%) ANUDEM 5 YES 64.5 (-7%) 36.9 (+6%) 2.6 (+9%) 23.15 (+37%)
Residual grids of elevation (Figure 4.9) show the spatial pattern and magnitude of changes caused by the different hydrologic correction methods, compared to the original (pit-filled) DEM. Stream burning with a 10m trench changed the least number of cells, with the lowest mean difference from the original DEM (-0.01m lower), lowest range and RMS (0.265) (Figure 4.9A). Agree altered cells across a larger area, and caused significant lowering where topography was flattened across the buffer zone. The resulting surface is on average 0.70m lower, with more cells altered than any other method and the highest RMS error (2.277) and range. ANUDEM v4 and v5 produced a slightly lower surface (-0.43m and -0.42m respectively) (Figure 4.9C-D). ANUDEM v5 performed slightly better than ANUDEM v4, with a lower residual, standard deviation, range and RMS error, though results were very similar. For the ANUDEM results (Figure 4.9C-D), slight undulation in the error surface away from the stream network results from conversion of grid elevation data to contour data for processing by the ANUDEM algorithm and back to a DEM. Stream burning produced the least change in elevation, with ANUDEM v5 and v4 performing equally second best. Agree caused the most significant change in elevation compared to the original surface
The slope residuals plots in Figure 4.10 identify the spatial patterns of changes caused by the different hydrologic correction methods on a secondary terrain variable. The ways that different hydrologic correction methods have altered elevation to improve hydrology (Figure 4.9) are reflected in the spatial patterns and magnitudes of change to a secondary terrain variable (slope). Stream burning increased mean slope by 0.05% and altered the least number of cells, but it also caused the largest increase in individual cell slope of all methods.
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Figure 4.9 Residual surface showing difference in elevation between the original (pit- filled) DEM and hydrologic corrected (pit-filled) DEM created using Stream burning (a), Agree (b), ANUDEM v4.6.3 (c) and ANUDEM v5.1 (d). Note that vertical axis has been reversed for the purposes of display.
Agree had the greatest effect on elevation, flattening the lower portion of the catchment (see Figure 4.9B). This smoothing of the landscape across the buffer distance reduced overall slope, and counteracted the effects of the 2m trench which increased slope adjacent to the stream cells. The resulting error surface has a lower mean slope (-0.02%) than the original (pit-filled) DEM, and low RMS (0.623) and range. If Agree was used in a flatter landscape, without the dissected
~ 104 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity terrain in the lower part of the sub-catchment, smoothing of the landscape across the buffer distance would be less significant and mean slope would increase.
Figure 4.10 Residual surface showing difference in slope between the original (pit- filled) DEM and hydrologic corrected (pit-filled) DEM created using Stream burning (a), Agree (b), ANUDEM v4.6.3 (c) and ANUDEM v5.1 (d).
ANUDEM v4 and v5 performed relatively well, with minimal increases in slope (0.15% and 0.21% steeper respectively). RMS was lowest for ANUDEM v5 (1.347), with ANUDEM v4 very close (1.349). ANUDEM v5 has a tighter range than ANUDEM v4, with both close to results for ~ 105 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity Agree (ANUDEM v5 has a tighter range than Agree). The results for the ANUDEM residuals are also affected by the minor changes away from the stream network used to warp elevation. Based on the raw statistics for the error surfaces, Agree appears to create the least change, however, visual inspection of error surfaces reveals that both ANUDEM methods (Figure 4.9C-D) create smooth error surfaces in comparison to both Agree and particularly Stream burning.
Impact of the Hydrologic Correction Algorithms on DEMs DEMs are an important tool for spatially modelling hydrology, but surface reconditioning of DEMs is often required to improve their representation of true hydrology. However, correction of the surface hydrology has been shown in this study to further impact terrain analyses, and selection of the method chosen will lead to very different results. Stream burning was the simplest method, altering the least number of cells and performing well for replicating stream length and position, but performed poorly for calculating catchment area and significantly affected subsequent terrain analysis of slope proximal to the channel network. Agree performed well for determining catchment area and reasonably for stream length. While mean slope was close to the expected result, hydrologically correcting using Agree fundamentally changed the way that the landscape was represented in the DEM. This was evident from the change in elevation (See Figure 4.9B). ANUDEM v4 and ANUDEM v5 produced very similar results to each other, performed consistently across most of the criteria, and maintained landscape integrity. ANUDEM v4 was equal best for determining catchment area (with Agree), while ANUDEM v5 caused the least increase in maximum slope, though the other methods were superior for determining stream length and mean catchment slope (refer to Table 4.3).
The dataset used in this study is at the better end of available catchment scale DEMs, compared with those used in other studies, in both its grid resolution and vertical accuracy. However, it is unable to replicate hydrology with the accuracy required for modelling hydrologic processes across the study catchment. While the performance of the raw DEM for delineating stream position and catchment area in this portion of the study area is poor, in other equally flat parts of the catchment its performance was acceptable. Elsewhere, the raw DEM replicated “natural” hydrology accurately, but the actions of land managers in diverting flow means that hydrologic correction was required to replicate the new hydrologic reality of the catchment.
The difficulty of delineating drainage direction in this landscape is further highlighted because delineation of the correct flow paths required consultation with people who have decades of expert knowledge of the area and the fact that there was even some discrepancy between these experts (that was finally reconciled). Using only the stream position predicted by experts with knowledge of the area, four different methods were able to hydrologically correct the DEM so that the catchment area and stream network matched or approximated the predicted result. If the
~ 106 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity purpose of a study is to create a DEM that is able to replicate the stream position, length and catchment boundaries, then all methods were relatively successful.
Many projects involving the use of DEMs require accurate development of secondary parameters such as aspect, slope, wetness index or predicted rates of water movement through the landscape. Surface reconditioning compromises these outcomes. In this study, the precision and minimisation of additional error was different between the methods and for different parameters. The variable performance of these models indicates the importance of assessing their suitability for deriving specific secondary attributes on a case-by-case basis.
Hydrologically correcting DEMs is necessary for many hydrologic modelling projects. All of the methods provided a significant improvement for the delineation of the catchment boundary and stream position on the original dataset, by warping the dataset. However, the DEM may ultimately be altered in such a way that further hydrologic calculation cannot be performed with the certainty and accuracy required to model further hydrologic processes in the catchment.
Summary: Impacts of Land Clearing and Land Management on Connectivity
Agricultural development of catchments has affected hydrologic pathways. In the upper Kent River catchment, farm dam construction has reduced the hillslope area in hydrological connection with the river. The location of the basins investigated in this research in relatively steeply-sloped sub-catchments has additional implications for the connectivity of sediment sourced from hillslope erosion. While this study focuses purely on hydrologic pathways, disconnecting water flux will also disconnect sediment flux. Other research and reports highlights the volume of sediment collected in farm dams (Neil and Galloway, 1989; Verstraeten and Poesen, 2000; Farmer and Coles, 2003; Verstraeten and Prosser, 2005). The results of the analyses presented in this landscape are applicable to other agricultural landscapes, and highlight the potential magnitude and spatial pattern of landscape disconnectivity. It also highlights the importance of considering the impact of farm dams in spatial hydrologic modelling in agricultural landscapes, particularly when they are constructed in lower landscape positions and intercept channelised flow.
The second section also highlights difficulty in accounting for the hydrological reality of landscape connectivity caused by both DEM error and by human actions. Current approaches for modelling hydrological and sediment flux rely strongly on terrain derivatives such as catchment areas, slope, and hillslope lengths. Standard practice is to process landscapes to force all flow to exit from a catchment, and then to warp the dataset to replicate the known
~ 107 ~ Chapter 4: Chapter 4: Effects of Land Management on Landscape Connectivity hydrological patterns. This process can ultimately compromise the dataset and its ability to represent other terrain aspects aside from hydrologic pathways.
The investigation presented in this chapter also emphasises how standard practices for preparing DEMs do not necessarily reflect the reality in flat landscapes and those disturbed by human actions. DEMs used for research in this thesis were altered using ANUDEM to correct the catch-scale hydrologic patterns. Large lakes and known stream locations were included in the algorithm so that the hydrological reality of the catchment was accurately represented for the catchment in its current condition. While consideration was given to the incorporation of dams and banks into DEMs of the entire catchment, there were numerous factors that prevented this: x In the Section 4.1, a single cell editor was required to increase and decrease cell heights for banks and dams (respectively) to ensure that flow was forced along banks and did not break through banks. Using single-cell editing at the catchment-scale was not feasible for either catchment. x Efforts to incorporate banks and dams using vector representations digitised from aerial photographs, required large changes in cell values to prevent flow from breaking through banks when a pit-filling algorithm was used to force flow along banks. Such large changes in cell values have the same effects as trenching had on the suitability of the DEM for subsequent terrain analysis. x Finally, the focus on subsequent analysis was modelling river channel processes, such as stream power. Preliminary work identified that stream slope rather than catchment area, was the most sensitive parameter in the subsequent analysis.
While this chapter presents methods for incorporating dams and banks into a DEM, they also highlight the limitation of these methods and how they compromise application of hydrologically correct DEMs for terrain analysis. Few studies have considered the impacts of humans on altering hydrological pathways, though a growing body of research highlights the potential importance of these factors. An extension of the work presented in this chapter would be to more widely consider the sensitivity of hydrologic models to these changes in hydrologic pathways and whether incorporating these features into DEMs can improve hydrologic prediction. There is a need to increase awareness of the ways that DEMs are processed and subsequently used, and to develop new and more appropriate analysis methods. This is however, is beyond the scope of this thesis.
~ 108 ~ Chapter 5: Land Clearing, Salinity and Thresholds
Chapter 5: Land Clearing, Salinity and Thresholds
The effects of increased stream salinity and waterlogging on riparian vegetation are clear along many rivers in southwestern Australia. At Wattersons Farm Gauging Station, upper Kent River (Station Number: 604003) most Paperbarks on the valley floor have died.
Landscape salinisation is one of the greatest environmental problems facing southwestern Australia (National Land and Water Resources Audit, 2001; Mayer et al., 2005; Environmental Protection Authority, 2006). It has been known for most of the last century that land clearing has caused stream salinity to increase in southwestern Australia (Mann, 1907; Wood, 1924; Wood and Wilsmore, 1928; Beresford et al., 2001). Low rainfall, flat gradients and deep regolith have accumulated salt that originated as an oceanic aerosol in the landscape (Hingston and Gailitis, 1976). This resulted in many rivers along the south coast being naturally brackish or saline (Hatton and Salama, 1999; Hatton et al., 2003). Land clearing has altered water balance, increasing recharge and brought saline groundwater to the surface. As a consequence, stream salinity has increased in all cleared catchments, and continues to rise in many (Maybeck et al., 2003; Lothian and Conacher, 2005). Mayer et al. (2005) investigated changes in stream salinity, finding increased stream salinity in two thirds of the rivers in Western Australia over the last decade, with continuing rising trends in cleared catchments where remedial actions (e.g. reforestation) have not been taken. Some river reaches now have mean salinity greater than seawater (Mayer et al., 2005).
Increased stream salinity has resulted in severe vegetation degradation across riparian zones in southwestern Australia (Froend et al., 1987; Cramer and Hobbs, 2002; Briggs and Taws, 2003; Halse et al., 2003; Hart et al., 2003; James et al., 2003; Lymbery et al., 2003; Nielsen et al., 2003; Peck and Hatton, 2003; Pannell and Ewing, 2004; Lothian and Conacher, 2005). The role of vegetation in controlling channel form and stabilising reaches, means that there is potential for rivers in salinising landscapes to become highly unstable (Kouwen and Unny, 1973; Kao and Barfield, 1978; Kouwen et al., 1981; Trimble, 1990; Gippel, 1995; Shields Jr and Gippel, 1995; Hupp and Osterkamp, 1996; Piegay and Gurnell, 1997; Bendix and Hupp, 2000; Tabacchi et al., 2000). Section 5.1 investigates increases in stream salinity, its effects on erosive thresholds and erosive potential. The human response to landscape salinisation is investigated in
~ 109 ~ Chapter 5: Land Clearing, Salinity and Thresholds Section 5.2. The different responses are summarised, and the potential impact of these actions on landscape stability is investigated.
Section 5.1 of this chapter is based on an expanded version of the paper by Callow and Smettem (2007)4.
5.1. Increased Stream Salinity Since Land Clearing Historic documents from early explorers and surveyors who traversed the southern coast before land clearing, record that many streams and pools were undrinkable by humans and stock prior to land clearing (see Eyre, 1845; Wilson, 1968; Roe, unknown). The earliest recorded European account of the Kent River was from T.B Wilson (Wilson, 1968), who travelled through the catchment in 1828, reporting that the water in the upper catchment was drinkable in some sections of the river, but many pools and lakes were undrinkable because of the salt. E.J. Eyre (Eyre, 1845) passed the Dalyup River in 1841 and made the first recorded observations of water quality for many rivers in the south coastal rivers region. Based on his map and the journal entries, it is known that he was around the Dalyup River between 29th and 31st May 1841, but the specific journal entry relating to the Dalyup River cannot be determined. Many of the rivers Eyre found were brackish, and saline, though some fresher lakes and streams were also found: May 29th “Came to a small salt water stream running seawards ... I hoped more (water) might be procured and perhaps fresh water, by tracing higher up... but I could find no fresh water” May 30 “...continued to a salt water river, broad and apparently deep near the sea. Four miles beyond this river we came to another channel of salt water. Travelling two miles further we came to a very pretty fresh water lake of moderate size. May 31 ... we came to a salt water river ... another salt water stream ... water in both branches brackish
Other oral histories have reported brackish and saline condition of many river in southwestern Australia prior to settlement (Hassell, 1975; Bignell, 1977; Bennett and Macpherson, 2002). While primary salinity affected water quality, riparian vegetation and channel morphology, secondary salinity caused by land clearing has significantly increased stream salinity and affected vegetation. These changes are more pronounced in upper catchment, lower rainfall areas. Flushing rates are lower, greater quantities of salt are stored and the landscape is typically flat and groundwater salinity is very high (de Broekert and Coles, 2004). Consequently, rising watertables have the potential to affect large areas of land. The first section of this chapter
Callow, J.N. and Smettem, K.R.J., 2007. Channel response to a new hydrological regime in southwestern Australia. Geomorphology, 84(3-4): 254-276. (A copy of this paper is presented in Appendix 1A) ~ 110 ~ Chapter 5: Land Clearing, Salinity and Thresholds investigates changes in stream salinity in the Kent River and Dalyup River. The effect that these changes have had on riparian vegetation and the effect of this on erosive thresholds are investigated.
5.1.1. Salinity Trends in the Kent River Continuous salinity data have been collected from Styx Junction (Station: 604053) and Rocky Glen (Station: 604001) since 1978. Since this time, mean annual flow-weighted salinity at Styx Junction has increased from 1175 to 1687 mg l-1 (Figure 5.1), and from 3140 to 3785 mg l-1 at Rocky Glen (Department of Environment, 2005e). Figure 5.1 shows the trend since 1956, combining discrete salinity measurements collected at Styx Junction prior to 1978, and continuous data. This shows that water at the bottom of the catchment was of potable quality (< 500 mg l-1) prior to land clearing in the upper catchment. Recently, two additional gauging stations have been installed in the upper catchment; South Perillup (Station: 604002) and Watterson’s Farm (Station: 604003) (refer to Figure 3.9 for location). At these gauges, mean annual flow-weighted salinity is 6100 mg l-1 (Station: 604002) and 6550 mg l-1 (Station: 604003). At these stations, readings as high as 75,000 mg l-1 have been recorded during periods of flushing and baseflow. The higher salinity from these reaches is diluted by the lower salinity and higher runoff from the lower catchment, resulting in a salinity gradient that decreases downstream. This decreasing downstream trend in salinity under higher rainfall and fresher discharge is highlighted in Figure 5.2.
Figure 5.1 Salinity trend for the Styx Junction station at the bottom of the Kent River catchment based on a box plot of discrete sampling (1956 – 1978) and a line plot of mean annual flow-weighted salinity from continuously sampled data (1979-2004). Data from: Department of Environment (2005e). Box plot of discrete sampling data from 1956-1978 show a rising trend from 500 mgl-1 (limit of potable water) at the start of the record, to an average of 1600 mgl-1 at present.
~ 111 ~ Chapter 5: Land Clearing, Salinity and Thresholds
Figure 5.2 Downstream salinity trends in the Kent River before and after clearing. Higher rainfall in the lower catchment results in increased discharge (A). Fresher discharge and a larger volume results in the marked downstream gradient in salinity (B). The change in salinity from the pre-clearing regime to the current rates is also highlighted, this transformation has occurred in only the last 50 years.
The gauging data from Rocky Glen and Styx Junction, allows the contribution of salinity from the lower catchment to be calculated by using the total salt load through each of these stations and the annual discharge from the lower catchment (based on data presented in Chapter 3.1.3). Figure 5.3 shows salinity for the lower catchment rose from 300 mg l-1 at the start of the record and appears to have stabilised at a new average of 490 mg l-1 (1986-2001 average). Adopting 300 mg l-1 as a mean pre-clearing salinity for the lower catchment and averaging the increase to the present rate, mean annual salinity for the upper catchment was estimated from 1956-1978 (Figure 5.3B). This estimate compares well to actual gauging data for 1979-2003 (Figure 5.3B).
~ 112 ~ Chapter 5: Land Clearing, Salinity and Thresholds Based on this analysis, salinity is estimated to have been around 700-750 mg l-1 at Rocky Glen prior to land clearing and has risen to 3750 - 3800 mg l-1 (1986-2001 average).
Using the limited data (1999-2003) from the two new stations in the upper catchment, changes in salinity were also estimated. South Perillup contributed 42% of the flow and 43% of the salt load received at the Rocky Glen Station. Using these figures, pre-clearing mean annual salinity at the South Perillup Station was estimated at approximately 750 mg l-1, and has increased to 4,420 mg l-1 since clearing (Table 4.1). Wattersons Farm Station contributed 25% of the salt load but only 17% of the flow at Rocky Glen, suggesting that preclearing salinity was 2,250 – 2,500 mg l-1 (now 7,500 – 8,000 mg l-1). Estimating preclearing salinity using mass balance suggests that salinity in the upper catchment has increased at least four fold, from around 1,000 - 2,500 mg l-1 to 4,500 – 8,000 mg l-1.
Figure 5.3 Predicted salinity trends for the upper Kent River Catchment at Rocky Glen Gauging Station since land clearing. Salinity in the upper catchment shows relative stability for the years 1957 to 1964, representing the pre-clearing levels (average 734 mg l-1), which rose until 1986, and has stabilised at 3,785 mg l-1 since then.
Table 5.1 Pre clearing and current salinity in the Kent River. Mean salinity has increased by 150% in the lower catchment and between two and four times in the upper catchment. Station Pre-clearing salinity mg l-1 Present salinity 1986-2001 mg l-1 Styx Junction 490* 1690* Lower Catchment 300* 490* Rocky Glen 725 - 750** 3800* South Perillup Road 750** 4250 - 4500* Wattersons Farm 2250 - 2500** 7500 – 8000* *Based on gauging data **Estimated value, based on modelling described above
5.1.2. Salinity Trends in the Dalyup River The Dalyup River has no gauging station, nor has extensive monitoring of salinity been undertaken. Minor discrete sampling in Lake Gore by the Department of Conservation and Land Management (CALM), and at Esperance Downs Research Station by the Department of Agriculture gives some indication of salinity levels (see Appendix 5A). Discrete sampling for Lake Gore (Appendix 5A) shows that salinity is lowest at the end of winter, averaging 20,000
~ 113 ~ Chapter 5: Land Clearing, Salinity and Thresholds mg l-1, rising as a result of evaporation over summer and spiking upwards during the flushing flows in April and May, when salinity is around 50,000 - 60,000 mg l-1. Average salinity across the record was 39,600 mg l-1. Data collected by the Department of Agriculture from a tributary of the Dalyup River that flows through the Esperance Downs Research Station, has brackish flow, with a mean salinity of 2,675 mg l-1, with salinity peaking at 10,000 -15,000 mg l-1 at times of flushing following moderate rainfall (see Appendix 5A).
Snapshot salinity sampling was undertaken in the Dalyup River on 22nd June 2006 as part of field work to understand the magnitude and spatial distribution of salinity. Salinity was sampled at 24 locations and varied from 92,785 mg l-1 where the Dalyup River crosses Griffith Road, to 19,910 mg l-1 at the site 3 km above Lake Gore at the bottom of the catchment (see Appendix 5B for data). The downstream trend in salinity is identified in Figure 5.4.
Figure 5.4 Snapshot of stream salinity in the Dalyup River catchment (sampled on 22nd June 2005)
Stream salinity data from gauging stations in the Lort River and Young River were used to analyse the changes in salinity following land clearing in adjacent catchments on the Esperance Sandplain. The Munglinup catchment was cleared in 1982/83, while the Melaleuka catchment has remained uncleared. Figure 5.5 shows that salinity in the Melaleuka catchment has remained
~ 114 ~ Chapter 5: Land Clearing, Salinity and Thresholds unchanged since the late 1970s, averaging 125 mg l-1. The Munglinup catchment had salinities that mirrored those in the Melaleuka catchment when it was uncleared, ranging between 100- 200 mg l-1. Following clearing, salinity increased for around a decade and appears to have reached equilibrium, averaging 30,000 mg l-1 since 1990.
Figure 5.5 Salinity from the Munglinup and Melaluka gauging stations on the Young River, showing the effect of land clearing on salinity. Data from: Department of Environment (2005c; 2005b)
Changes in Stream Salinity Following Land Clearing
Table 5.2 highlights the salt tolerance of different flora and fauna against measured salinity data from rivers and groundwater in southwestern Australia. While the tolerance of plants to salinity varies with other factors such as waterlogging and drought tolerance (Munns, 2002; Barrett- Lennard et al., 2003; Loch et al., 2003), the potential for increased in stream salinity to affect riparian vegetation and channel hydraulics is highlighted. Data collected and analysed from both the Kent River and the Dalyup River highlights the magnitude of the increased salinity following land clearing. At present, approximately 4.3 million hectares is affected by salinity and shallow saline watertables. An estimated 8.8 million hectares is at risk by 2050, when hydrologic equilibrium is predicted to be reached (see Appendix 5C). An estimated 25 – 35% of these landscapes are likely to be affected by shallow (less than 2 m from the surface) saline watertables (Kelly, 1995; Ferdowsian et al., 1996; Salama et al., 1997; Agriculture Western Australia, 2000; Short et al., 2000; National Land and Water Resources Audit, 2001). Even with immediate remedial action, larger areas of these landscapes are already affected by secondary salinity and this area will continue expanding (Ferdowsian et al., 1996; Hatton and Salama, 1999; Commander et al., 2001; George et al., 2001; Hatton and Ruprecht, 2001; Hatton et al.,
~ 115 ~ Chapter 5: Land Clearing, Salinity and Thresholds 2003). Stream salinity trends are likely to continue rising until hydrologic equilibrium is reached, and will remain high for many hundreds of years (Ferdowsian et al., 1996; Hatton and Salama, 1999; George et al., 2001; Hatton and Ruprecht, 2001; Jolly et al., 2001; Hatton et al., 2003).
Table 5.2 Some measured water quality data from rivers in southwestern Australia compared to various ecological tolerance values. Salinity (mg l-1) Use of water, tolerance of salinity and Source 500 World Health Organisation (WHO) standard for (Conacher and Conacher, human consumption (potable water) 1995) 825 Threshold value for damage to environmental values (ANZECC & ARMCANZ, 2000; of rivers Goss, 2003) 1000 Aquatic biota adversely affected (Nielsen et al., 2003) 1500 WHO maximum permissible limit for human (Conacher and Conacher, consumption 1995) 2500 Maximum tolerance of pasture (Conacher and Conacher, 1995) 6700 Mean annual flow-weighted salinity for Avon River (Department of Environment, (Stn: 615062) 2005a) 7000 Drinking water for horses (Conacher and Conacher, 1995) 7300 Drinking water for dairy cattle (Conacher and Conacher, 1995) 8200 Mean annual flow-weighted salinity for upper Kent (Water and Rivers River (Stn: 604003) Commission, 2003) 9000 Maximum tolerance of wheat (Conacher and Conacher, 1995) 9000 - 10,000 LD50 for Swamp Yate (E. occidentalis) seedlings (Blake, 1981; van der Moezel and Bell, 1987) 10,000 Maximum drinking water tolerance of many (Halse et al., 1993) waterbirds – Ibis, heron, pelican, black swan, coot 10,200 Drinking water for beef cattle, weaners, lambs and (Conacher and Conacher, ewes in milk 1995) 11,600 Maximum tolerance of tall wheat grass (Conacher and Conacher, 1995) 13,200 Maximum drinking water tolerance of adult sheep (Conacher and Conacher, 1995) 21,000 Swampy Paperbark (M. ericifolia) stops growing (Ladiges et al., 1981) 23,400 LD50*1 for River Red Gum (E. camaldulensis) (Blake, 1981) seedlings 27,000 Mature River Red Gum (E. camaldulensis) can (Mensforth et al., 1994) withstand for short periods 30,000 Swampy Paperbark (M. ericifolia) can withstand for (Bird, 1978) short periods 35,000 Mean salinity of seawater (Conacher and Conacher, 1995) 35,000 - 40,000 Mean annual salinity of groundwater discharge from (de Broekert and Coles, 2004) deep drain in eastern wheatbelt 44,000 Samphire (Halosarcia Sp.) can withstand and grown (English et al., 1999; English et when waterlogged al., 2001) >50,000 Baseflow salinity for upper Kent River (Stn: 604003) (Water and Rivers over 4 ½, from January 2001 Commission, 2003) 55,000 Upper limit for camels to drink (Conacher and Conacher, 1995) 80,000 - Typical peak salinity during flushing flows and (Department of Environment, 100,000 baseflow periods, Young River. 2005a) 92,000 Salinity measured at Dalyup Site 20, June 2005 Field Measurement 110,000 Salinity of groundwater in one eastern wheatbelt (de Broekert and Coles, 2004) catchment 110,000 - Maximum tolerance of Samphire (not waterlogged) (English et al., 1999; English et 165,000 al., 2001) *1 N.B. LD50 refers to the Lethal Dose sufficient to kill 50% of plant specimens.
~ 116 ~ Chapter 5: Land Clearing, Salinity and Thresholds 5.1.3. Effects of Salinity on Channel Roughness and Sediment Transport Potential Aerial photography from 1946, 1965, 1972 and 1999 over the upper Kent River enables the change in riparian vegetation condition to be mapped as salinity increased. Photography from 1946, combined with forensic field evidence of pre-clearing conditions, shows that the floodplain at Wattersons Farm Gauging Station site was covered by a paperbark (Melaleuca sp.) woodland prior to clearing. Open areas with occasional shrubs and grasses were typically associated with lower lying areas and numerous channels that flow across the wide braiding floodplain, but very few bare patches existed prior to clearing. Change from 1946 to 1972 shows that as salinity increased, the woodland became more open as some salt-sensitive understorey species died and paperbarks thinned, with bare areas expanding gradually. The change from 1972 to the present is dramatic, with only a few remnant paperbarks (typically found on locally higher ground on the floodplain), with most of the floodplain now devoid of vegetation (see image at start of this chapter).
Figure 5.6 Change in vegetation across the floodplain along the reach at Wattersons Farm Gauging Station from 1946, 1965, 1972 and 1999.
Similar changes are observed across the other sites, though the severity of salinity is less and consequently the reduction in vegetation cover is not as high. Spatially, the extent of vegetation degradation is variable between sites such as Wattersons Farm where the entire valley floor is affected, to downstream sections where water quality improves and salinity originates from upstream and consequently only vegetation surrounding the wetted channel has degraded. In these reaches the floodplain remains relatively unchanged. Aerial photography and field evidence such as the lack of dead stumps littered across the floodplain suggests that some sites appear to have had very little vegetation before land clearing and increased salinity.
~ 117 ~ Chapter 5: Land Clearing, Salinity and Thresholds By combining the aerial photography with field evidence, downstream changes in vegetation condition before land clearing and the present condition were estimated (see Table 5.3). Based on this data and cross sections surveyed at study sites, the effects of vegetation degradation on channel velocity and sediment transport potential were modelled (see Table 5.3 and Figure 5.7). Vegetation roughness before clearing and in the present salt-affected state was determined from aerial photography and field evidence (based on Chow, 1959; Arcement and Schneider, 2003; Ladson et al., 2003). Specific stream power (Ȧ) has been used in investigations of channel morphology, channel change potential, and sediment transport capacity (e.g. Magilligan, 1992; Nanson and Croke, 1992; Hooke, 2003; Reinfelds et al., 2004). It was used in this study to relate changes in vegetation roughness to changed erosive potential. Specific stream power (Equation 5.1) was calculated from total stream power (Equation 5.2), with discharge for the bankfull channel calculated from hydraulic geometry obtained from field surveying and estimation of roughness using the slope-area method (or Manning’s Equation - Equation 5.3): Ȧ = ȍ/W (5.1) ȍ = ȖQS (5.2) 1 2 1 Q AR 3 S 2 (5.3) n where Ȧ is specific stream power (W m-2), ȍ is the total stream power per unit length of channel (W m-3), W is the bankfull channel width (m), Ȗ is the specific weight of water (9807 N m-2), Q is the bankfull discharge (m3 sec-1), S is the energy slope, A is the cross sectional area of the bankfull channel (m2), and R is the hydraulic radius (m). Grab samples of bedload were dry- sieved, at 0.5I intervals and the D50 used for estimating transport competence based on Hjulström curves (Hjulström, 1935).
Decreased vegetative roughness due to salinity has increased channel velocity, and stream power across all sites, although the magnitude of these changes is variable (Figure 5.7). Plotting channel velocity changes against D50, shows that the sediment transport potential has increased slightly at most sites (Figure 5.7). Only the high-gradient Site 6 has experienced significant change in stream velocity, though the site shows the least change in vegetation roughness. This result highlights the overwhelming control of channel slope on velocity and stream power, with minor change in severely degraded but low-gradient reaches. Aside from site 7, channel velocities in the low-gradient reaches are barely in excess of the threshold for transport (see Figure 5.7 and Table 5.3).
~ 118 ~ Chapter 5: Land Clearing, Salinity and Thresholds
Figure 5.7 Changes in sediment transport potential following vegetation degradation from salinity (modified after Hjulström, 1935). Refer to Fig. 2 in Callow and Smettem (2007) for a map locating study sites (see Appendix 1A).
Data from Figure 5.7, Figure 5.8 and Table 5.3 identify a relatively minor change in landscape stability following land clearing and salinisation in the upper Kent River. This data suggests that while there has been severe vegetation degradation, increases in specific stream power and velocity are greatest in areas of steep slopes and minor degradation, rather than the areas with the most severe degradation. To further investigate these downstream trends, stream power was modelled for Dalyup River catchment (Figure 5.8). Using field survey data at 21 locations, downstream trends in channel morphology were identified for similar reaches (using the method presented in Chapter 6). Using a DEM, specific stream power was modelled downstream for the Dalyup River on a cellular basis, using Equations 5.1 – 5.3. Figure 5.8 identifies that the greatest increase in stream power is in middle to lower reaches of the catchment (see Figure 5.8B-E), where channel gradients are highest and the channel becomes more incised and confined. In these reaches, minor changes in vegetation condition causes large changes in specific stream power due to the high slope. Peak change occurs 18km from the channel outlet, rather than the areas of maximum vegetation degradation (see Figure 5.8A-B). Data presented in Figure 5.8 suggest that absolute specific stream power and the changes in total specific stream power in the upper catchment is minor, and despite the severe vegetation degradation. As such the potential for channel adjustment as a result of increased erosive power in a saline river channel is more limited in these reaches.
~ 119 ~ Chapter 5: Land Clearing, Salinity and Thresholds
Table 5.3 Estimated changes in channel roughness as a result of salinity, with corresponding changes in bankfull velocity, discharge and stream power.
~ 120 ~ Chapter 5: Land Clearing, Salinity and Thresholds
A
B
C
D
E
Figure 5.8 Downstream trends in specific stream power for the West Dalyup River, showing the modelled change in specific stream power due to reduced vegetation roughness. ~ 121 ~ Chapter 5: Land Clearing, Salinity and Thresholds Across both river catchments, landscape stability has changed as a result of vegetation degradation caused by increased stream salinity. The image at the start of this chapter highlights the severity of salinity and its impact on vegetation. In low-gradient reaches, despite the severity of vegetation degradation, there have been relatively minor changes in erosive potential. Data presented in this chapter highlight how vegetation degradation, stream morphology and stream gradient combine to determining the spatial patterns of changes in system stability following stream salinity. Change in erosive potential is greatest in steeper-sloped areas. Overall, the stream power and channel velocity values are low. Despite the severity of salinity in many upper catchment areas, the landscape has not become highly unstable. By comparison, modelling suggests that steeply-sloped area undergo significant changes in stability under minor vegetation degradation. Where steeper-sloped reaches are cleared of vegetation, or degraded by salinity, there is a large potential for significant channel adjustment. By contrast many upper catchment areas are unlikely to undergo catastrophic changes due to the low channel gradient resulting in only minimal changes in erosive potential.
5.2. Human Responses to Salinity: impacts on system stability. The previous section highlighted the severity and extent of changes in stream salinity since land clearing. The effects and impacts have not been limited to remnant vegetation on floodplains. Areas of productive agricultural land in southwestern Australia in lower lying landscape positions have also been affected. Farmers are faced by the potential that up to 35% of the landscape will be affected by shallow saline watertables, with associated productivity losses (Kelly, 1995; Ferdowsian et al., 1996; Salama et al., 1997; Agriculture Western Australia, 2000; Short et al., 2000; National Land and Water Resources Audit, 2001). Farmers have adapted land management practices, aimed at improving productivity of these areas.
Management strategies have included include; surface drains, argiforestry, pumping and groundwater drainage. Surface drains are constructed across the gradient, designed to intercept surface flow before it can infiltrate as a means of reducing groundwater recharge (such as the banks and dams described in Chapter 4) (Keen, 1998). Vegetative options aimed at increasing transpiration involve many different tree species planted at various densities and producing different agricultural products. Perennial fodder shrubs have been advocated as a higher water- using grazing system (Hatton and Nulsen, 1999; Hatton et al., 2003; Barrett-Lennard et al., 2005). Other farmers have used a mixture of tree belts and traditional pasture/cropping systems (locally known as alley-farming) to increase the amount of rainfall transpired and hence reduce groundwater recharge. These systems typically offer only marginal returns and cannot compete economically with returns from broadacre farming (Pannell, 2001; Barrett-Lennard et al., 2005). Hatton and Nulsen (1999) also identify that returning groundwater tables to pre- European levels requires extensive areas of catchments to be revegetated such that the leaf-area
~ 122 ~ Chapter 5: Land Clearing, Salinity and Thresholds index approximates naturally-vegetated ecosystems. Thus silvicultural industries based on the Tasmanian bluegum (Eucalyptus globulus) and various Pinus species, offer potential for addressing groundwater recharge problems, but have large social costs related to rural downsizing (Curry et al., 2001). These solutions are also more suitable to moderate and higher rainfall zones, where yields are higher, and salinity issues are not as severe. Within the 800 - 600 mm rainfall zone in the Kent River, extensive conversion to agroforestry has occurred in the past decade (Kington and Pannell, 1999; Kington and Smettem, 2000). While agroforestry in the Kent River catchment has resulted in falling groundwater tables (Ryder, 2004b), planting is driven by tax-effective investment companies buying arable land, rather than strategic natural resource management (Tonts and Selwood, 2003). Other land managers have accepted that a portion of their property will be affected by saline groundwater. Some have planted palatable saltbush and other halo-tolerant species on saline land to allow some economic return, while others have abandoned these areas (Barrett-Lennard et al., 2003; Barrett-Lennard et al., 2005).
One of the most contentious proposed solutions has been the use of open deep drains (Jensen, 9/6/2005; Hewitt, 14/7/2004; Zekulich, 14/7/2004; Agriculture Western Australia, 1999; Ali et al., 2004). In low-lying, waterlogged and salt-affected land, land managers have sought to lower watertables by draining saline land (see Figure 5.9 for a typical deep drain in the upper Dalyup River catchment). This “engineering” solution has been implemented over large areas, often without regulation or with knowledge of the downstream effects (Deep Drainage Taskforce, 2000). The removal of saline groundwater requires a disposal location. At present there are two options, the first involves the retention of saline discharge on-farm in receiving basins or where water quality permits, in dams where aquiculture is possible, though this option is often very costly (Deep Drainage Taskforce, 2000; Dogramaci and Degens, 2003; Ali et al., 2004). The remaining option is to dispose of the effluent to the lowest landscape position, either river channels or salt lakes; this is currently the most commonly used method (Deep Drainage Taskforce, 2000). The risks that the disposal of effluent from open deep drains has serious implications for river channel processes and is poorly understood (Dogramaci and Degens, 2003; Ali et al., 2004).
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Figure 5.9 Deep drains in the Dalyup River catchment. Deep drains have been dug in this paddock between 1999 and 2004, and a network of drains in the low-lying parts of the paddock. The photograph of the deep drain at this site (A) identifies the dimensions of the channel, 2m deep and 1.2m wide at the base, with groundwater discharging in the bottom of the channel.
Aside from the environmental implications for effluent disposal, the ability of deep drains to lower groundwater tables varies across various landscapes (Speed and Simons, 1992; Coles et al., 1999; Agriculture Western Australia, 2000; Deep Drainage Taskforce, 2000; Ali et al., 2004). In permeable soils, drains can lower groundwater levels for hundreds of metres either
~ 124 ~ Chapter 5: Land Clearing, Salinity and Thresholds side, though their performance in heavy soils is limited to only a few metres either side (Speed and Simons, 1992; Coles et al., 1999; Ali et al., 2004). A report by Agriculture Western Australia conducted an agronomic and environmental evaluation on the use of deep drains on the Esperance Downs Research Station (EDRS) farm in the Dalyup River catchment, finding that: “In 1990 a study on salinity and waterlogging on the EDRS concluded that saline groundwater cannot be safely or economically drained from the station. Following this the use of deep drainage was eventually abandoned” (Agriculture Western Australia, 2000 p.46).
Less well understood and lacking from agronomic analyses has been the environmental cost of discharging hypersaline water into salt lakes and natural wetlands. At present, the installation of deep drains requires a land manager to inform the Office of the Commission for Soil and Land Conservation within 90 days of commencing construction with a Notice of Intent (NOI) to drain sub-surface water and dispose of it over land, whether on or off their property (Deep Drainage Taskforce, 2000). The NOI document must include information about the type and size of the drainage scheme, but also the approval of downstream land owners, local governments and statutory authorities to the disposal of drainage effluent between the site of the drainage and the receiving point (Deep Drainage Taskforce, 2000). “Landholders have a duty of care to ensure that their management actions such as drainage/pumping do not lead to land degradation or other damage such as through severe erosion, flooding or environmental damage to waterways and wetlands” (Deep Drainage Taskforce, 2000)
“Drainage proposals…will be ‘environmentally friendly’ and not contribute to land degradation such as downstream salinisation, erosion or siltation in the long-term.” (Deep Drainage Taskforce, 2000 p.40)
Despite the legislation, regulations and guideline, extensive areas of deep drainage have been constructed in the south coastal rivers region of Western Australia, many without lodgement of NOI documentation (Agriculture Western Australia, 2000; Deep Drainage Taskforce, 2000; Ali et al., 2004). As yet, no research has considered the downstream impacts of deep drains on receiving watercourses. Observations of the effects of deep drainage on channel processes and morphology was made during field work, and the following section summarise qualitative observation of deep drains in the Dalyup River catchment, where they now cover an extensive area (Agriculture Western Australia, 2000; Water and Rivers Commission, 2002a).
~ 125 ~ Chapter 5: Land Clearing, Salinity and Thresholds Changes in downstream water quality, particularly changes in salinity are the most obvious effect of deep drainage on downstream watercourses. Work by Agriculture Western Australia at the Esperance Downs Research Station (EDRS) found that where 18 hectares were drained, open deep drains removed 9.9 ML (55 mm of groundwater) and 170 t of salt (average salinity 17,170 mg l-1) in one year. Discrete salinity sampling of the Dalyup River in this location (as part of this research) shows that salinity of the river was higher than the average salinity of discharge from this drain, though elsewhere salinity of effluent from deep drains is significantly higher. de Broekert and Coles (2004) measured salinity of deep drain discharge up to 40,000 mg l-1 and groundwater salinity values up to 110,000 mg l-1.
Deep drainage is also associated with causing acid sulphate soils (ASS) to develop and drainage of acidic groundwater. Elevated saline groundwater which contains moderate to high concentration of sulphate anions (linked to terrestrial carbon concentrations), can transform soils into potential ASS over periods of years to decades (Fitzpatrick et al., 2000; Dogramaci and Degens, 2003). Drainage lowers watertables and oxidation creates ASS problems and low groundwater pH from groundwater effluent (Deep Drainage Taskforce, 2000; Fitzpatrick et al., 2000; Dogramaci and Degens, 2003; Ali et al., 2004). Groundwater discharge as low as pH 2.8 are measured in groundwater and drainage effluent (de Broekert and Coles, 2004; Simons and Alderman, 2004). Discharge of hypersaline and moderately acidic effluent has serious implications for riparian vegetation and system ecology.
A B Figure 5.10 The indurated ferricrete evaprolite layer formed above sediment found in deep drains in the upper Dalyup River catchment. It is interpreted that the ferricrete is an evaprolite that forms in summer when flow recedes, causing a hard layer and fortifying sediment below. In some locations several layers, all topped with a ferricrete horizon were found overlying each other, interpreted as annual accumulation layers filling deep drains.
Additional to the eco-hydrological and biological implications of effluent quality, is the effect of constructing an artificial stream network on catchment discharge and flood peaks. Agriculture Western Australia mapped 340km of drains in the upper Dalyup River catchment in 2000 (Agriculture Western Australia, 2000), and their extent has increased in subsequent years, though the exact extent is unknown (Water and Rivers Commission, 2002a). Deep drains typically drain wide, flat valleys that had no previously defined channel. Groundwater that ~ 126 ~ Chapter 5: Land Clearing, Salinity and Thresholds otherwise percolates slowly through the landscape, is now rapidly removed downstream. In addition, many drains intercept surface discharge and so are likely to increase flood peaks and reduce time to peak discharge. Drains are deep, steep sided and have high velocities compared to unchannelised surface and subsurface discharge. Field observations at numerous drains in the upper Dalyup River found transport of (medium) sandy bedload under baseflow conditions. The high sediment discharge from deep drains is noted in many management reports and highlight drain sedimentation and the high maintenance cost of dredging drains (AGWA, 2000; WRC, 2002). Field observations as part of this study also found sandy bedload accumulated in the drain that became cemented by a ferricrete layer. In one drain, several layers were evident, interpreted as representing annual layers cemented during summer that reduced the channel capacity by 0.1 to 0.15m per year (see Figure 5.10).
A
B
Figure 5.11 Photo of the West Dalyup River above (A) and below (B) Boydells Road. The unchannelised reach above the road (A) is devoid of vegetation, whereas the channel in the downstream reach (B) has lowered the saline watertable and allowed vegetation condition to improve.
Despite the negative impacts discussed above, field work in this study found that drains also have an important, positive effect. Deep drains dug into the valley floor, lowered groundwater tables and allowed vegetation conditions to significantly improve across valley floors. Figure 5.11 compares two adjacent reaches, and shows the effects of deep drains in lowering groundwater. Mounding of excavated dirt at the margins that allowed vegetation to grow in a landscape where it would not otherwise be possible. Where a deep drain was located, Samphire was found across the entire floodplain, with Paperbark and other halo-tolerant species able to grow in mounded areas, compared to the bare, unchannelised reach upstream (Figure 5.11A). The research presented here highlights the potential that deep drains offer, but also the significant adverse effects that they pose. The current use of deep drains has been ad-hoc, and there is the need to improve the understanding of their effectiveness, where they can best be used and where they should not be used because of the risk of creating an “environmental
~ 127 ~ Chapter 5: Land Clearing, Salinity and Thresholds legacy”. They may, however, offer a management solution for severely degraded reaches and offer a way forward for improving biodiversity and geomorphic stability through revegetating the floodplain.
Summary of Land Clearing, Salinity and Threshold Changes While the link between land clearing and increased stream salinity, and the effects in degrading riparian vegetation condition are widely acknowledged (Froend et al., 1987; Cramer and Hobbs, 2002; Briggs and Taws, 2003; Halse et al., 2003; Hart et al., 2003; James et al., 2003; Lymbery et al., 2003; Nielsen et al., 2003; Peck and Hatton, 2003; Pannell and Ewing, 2004; Lothian and Conacher, 2005), prior to this research the impact of reduced vegetative roughness on channel stability and erosive potential was unknown. This chapter identifies that while vegetation has been severely degraded, changes in erosive potential in many low-gradient upper catchment reaches have been minor. Modelling changes in channel velocity and stream power due to vegetation degradation from salinity identified the significant role played by stream gradient in determining potential for changes in erosive potential. Consequently low-gradient reaches have undergone little change in erosive potential despite severe vegetation degradation. By contrast steeper-sloped mid catchment areas where limited vegetation degradation has occurred have undergone larger increases in erosive potential due to degradation of vegetation surrounding the wetted channel perimeter.
Across both river catchments, changes in erosive potential are highly variable. Despite the severity of salinity in many upper catchment areas, the landscape has not become highly unstable. Erosive potential has been increased and erosive thresholds lowered by vegetation degradation. Stripping of sediments in low-gradient reaches has occurred where changes in landscape stability caused reaches to exceed transport thresholds, though response to land clearing and salinity has not been catastrophic because of the low landscape gradient. In comparison high-gradient reaches in the mid and lower catchments, particularly those in the lower Dalyup River where vegetation has been cleared from the channel margins and floodplain, are more unstable under the current salinity and hydrologic regime.
Additional to existing impacts of diffuse land clearing are more point-source impacts due to the way that humans have reacted to increased stream salinity through the construction of deep drains. Deep drains have further affected landscape sensitivity in the catchment, though they also show some potential for improving vegetation condition. This chapter highlights the complex response to land clearing and changed land use in this environment, and underlies the need to better understand the impacts of these changes on river sensitivity, river response and system trajectory. These are factors that are considered in the following chapter, which investigates the response of the reaches in the study catchment to altered landscape stability.
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Chapter 6: River Response to Land Clearing and Salinisation
This crossing on the Dalyup River was washed out by the March 2000 flood. While this reach was severely eroded, other reaches in the upper catchment were relatively unaffected by the flood event. Photographer: Steve Janicke, Department of Environment, May 2000.
Introduction
The previous chapters have identified the changes that have occurred in the landscape over the past five decades in southwestern Australian catchments (see Table 6.1). Land clearing, salinisation and changes in land management practices have altered landscape stability and the sensitivity to change. Chapters Two, Three and Four have established the high spatial variability of independent channel controls (rainfall, geology, soils, vegetation), in changes in boundary conditions (sediment and water flux), in perturbation of erosive thresholds (due to salinity and associated vegetation degradation) and in transmission of impacts through the system (connectivity) (see Table 6.1). The spatial variability of these factors both within and between the study catchments underlies the complexity of these geomorphic systems.
~ 129 ~ Chapter 6: River Response Given this spatio-temporal variability and the multiple factors that have been perturbed, there is a need to identify components of the overall system that have similar characteristics and behave in similar ways to understand the response of the river morphology. A founding component of the landscape sensitivity concept is that perturbation brings about a response that is a “sensible, recognisable, sustained but complex response” (Brunsden, 2001 p.99). At the catchment-scale, changes may seem to be chaotic, but studying morphologically similar reaches may yield understandable, predictable and indeed “sensible” responses to perturbation. This research therefore requires a methodology which groups components of each study catchment, in a way that allows comparison between similar groups, in different settings (i.e. under different environmental perturbation gradients). By applying a reach-based channel classification, this research sought to understand the response of particular reach types common to both the Kent River and Dalyup River, under different degrees of perturbation.
Table 6.1 Summary of the changes in landscape stability following land clearing in the study catchments Landscape Stability Kent River Dalyup River Factor Chapter 2 - Land Use Land Clearing 39% cleared 96% cleared Land Clearing Pattern Only upper catchment cleared, Entire catchment cleared, most plantation forestry expanding, floodplains cleared to the bankfull lower catchment forested channel margin Chapter 3 - Hydrology Annual Rainfall -10.5% -2% Extreme Rainfall -20% for <70 mm day-1, +10% to +15% for < 60 mm per day +80 to +240% for > 90 mm day-1 +40% for >90 mm per day Annual Discharge Double to triple pre-clearing rate Greater than one order of magnitude Event/Flood Discharge Peak discharge ~ two to five Bankfull discharge reached annually, times larger for given rainfall was ~ 1:10 – 1:20 years event Chapter 3 - Sediment Sources Stripping of mid-channel islands, Stripping of mid-channel islands, minor hillslope and bank erosion channel avulsion in mid to lower catchment Loads 500 times pre-clearing rate 50 times pre-clearing rate Chapter 4 - Connectivity Hillslope-channel Up to 40% of hillslope decoupled, Not investigated – based on aerial limited to upper catchment photography less catchment area decoupled than in Kent River Chapter 5 - Vegetation / Thresholds Mean Salinity ~ four times greater pre-clearing > 1 order of magnitude increase, from rate, from 2,000 – 8,000 mg l-1 ~20,000 mg l-1 - ~90,000 mg l-1 Peak Salinity Up to 60,000 mg l-1 Up to 90,000 mg l-1 Erosive thresholds Upper catchment minimal Upper catchment minimal change, change, mid catchment up to mid catchment up to 30-40% increase 10% increase in stream power in stream power mid to lower catchment
6.1. Reach-Based Assessment of River Response The variability in fluvial forms through systems, and the scales of spatio-temporal variation in process-response behaviour have led geomorphologists to appreciate the time and space scales over which processes occur, and the need to view reach-scale phenomena within a holistic catchment context (Schumm, 1977; Schumm, 1988; Downs and Gregory, 1993; Gilvear, 1999;
~ 130 ~ Chapter 6: River Response Kondolf and Piegay, 2003b; Downs and Gregory, 2004; Brierley and Fryirs, 2005). Working at the reach-scale, has required using methods to group or classify reaches, such that response to perturbation can be understood and potentially used to predict behaviours in reaches that have similar characteristics.
The earliest classification schemes were based on stream order, discussed and developed by Horton (1945), Strahler (1957) and Shreve (1967), and the relationship between channel width, discharge and catchment area (Lacey, 1930; Clark, 1945; Cao and Knight, 1996.; Savenije, 2003). Relating channel morphology to certain boundary conditions (typically slope, discharge, particle size) can be traced to the work of Lane (1955; 1957) on stable channel design, and the pioneering work by Leopold and Wolman (1957) on the relationship of meandering or braided river planform to slope thresholds. Much of this work was expanded by Schumm (1977; 1985) into the channel continuum concept which characterises river type by variations in slope and sediment character, and refined by others to include factors such as stream power (Church, 1992). The past two decades have seen numerous classification schemes based on form (morphological) (e.g. Whiting and Bradley, 1993; Woolfe and Balzary, 1996; Hand, 1997; Bengeyfield, 1999; Goodwin, 1999; Kondolf et al., 2003) and process-based classifications schemes (e.g. Nanson and Croke, 1992; Rosgen, 1994; Downs, 1995; Thorne, 1997; Montgomery and Buffington, 1998; Brierley and Fryirs, 2000; Parsons et al., 2004), that have increasingly become the basis for river management (Kondolf et al., 2003; Downs and Gregory, 2004; Brierley and Fryirs, 2005).
A significant focus of geomorphological research through the 1980s and 1990s sought universally applicable classification schemes. The classification scheme of Rosgen (1985; 1994; 1996) is the most recognisable classification scheme, and has been widely applied to river management. After at least two decades of work, neither the Rosgen classification scheme (e.g. see Miller and Ritter, 1996; Brierley and Fryirs, 2000; Simon et al., 2005), or any subsequent classification methodology has provided a universally applicable classification and management tool. Reasons for this can be summarised by the comments of Montgomery (2001) (quoted in Brierley and Fryirs (2005)) that “No simple cookbooks or manuals can capture the inherent regional complexity or interactions between geomorphic processes, riverine habitat and ecological systems”. Many classification schemes are developed for specific purposes, such as: floodplain evolution (Nanson and Croke, 1992); predicting erosion and sedimentation patterns (Woolfe and Balzary, 1996); determining recovery potential (e.g. Brierley and Fryirs, 2000; Brierley et al., 2002; Brierley and Fryirs, 2005); or geo-ecological habitat assessment (e.g. Parsons et al., 2004). These classification techniques, therefore, need to be carefully applied to appropriate regional settings and research purposes (Goodwin, 1999).
~ 131 ~ Chapter 6: River Response In applying a reach-based investigation methodology, the geography, geomorphology, climate, channel planform characteristics, and the desired river management application all affect the selection of a suitable classification scheme. The goal of this study is to investigate the response of river reaches to changes in landscape stability caused by changes in boundary conditions linked to land clearing and reduced erosive thresholds associated with vegetation degradation from increased salinity. The aim is to understand how rivers are changing in response to these perturbations, and how river management strategies can influence the trajectory of the system.
The RiverStyles® Framework, is a reach-based method developed by Brierley, Fryirs and others (Brierley and Fryirs, 2000; Brierley et al., 2002; Brierley and Fryirs, 2005) to analyse river reach character, behaviour and patterns, framed within a catchment context. While it was developed in catchments located in New South Wales (Australia) (see critical review by Parsons et al. (2004)), the aim was to develop a flexible framework allowing the identification of reach types (or River styles) specific to particular regions, and therefore more widespread application. It also shares the goals of this research, i.e. understanding system trajectory, rehabilitation potential and how river management can be framed within a geomorphic context. The RiverStyles® Framework was selected for application to this project for three key reasons.
Firstly, it has been applied successfully to a range of other Australian rivers (see Brierley et al., 1999; Ferguson and Brierley, 1999; Brierley and Fryirs, 2000; Brooks and Brierley, 2000; Fryirs and Brierley, 2001; Thomson et al., 2001; Brierley et al., 2002; Fryirs, 2003; Brooks and Brierley, 2004; Thomson et al., 2004; Brierley and Fryirs, 2005). This is significant as many other river classification schemes have been developed for temperate-climate rivers in North America and Europe, and may not necessarily be suited to the highly unique Australian rivers (Finlayson and McMahon, 1988; Warner, 1988; Tooth and Nanson, 1995; Brierley and Fryirs, 2000; Brizga and Finlayson, 2000b; Nanson et al., 2002). While most of the development of this framework has been in the coastal rivers of New South Wales (see Brierley et al., 1999; Ferguson and Brierley, 1999; Brierley and Fryirs, 2000; Brooks and Brierley, 2000; Fryirs and Brierley, 2001; Thomson et al., 2001; Brierley et al., 2002; Fryirs, 2003; Brooks and Brierley, 2004; Thomson et al., 2004; Brierley and Fryirs, 2005), there are no other suitable classification schemes that have been applied in climates and settings similar to those in southwestern Western Australia. While there are other classification schemes for rivers that have been developed and applied in Australia (see Nanson and Croke, 1992; Nanson and Knighton, 1996; Water and Rivers Commission, 2002b; Parsons et al., 2004), the goals of these classification schemes are significantly different to those of this project, whereas the RiverStyles® Framework was developed on rivers that have undergone similar changes (i.e. recent land clearing) to those in southwestern Australia and with similar goals.
~ 132 ~ Chapter 6: River Response Secondly, the RiverStyles® Framework has been developed in a way that allows addition of new river styles that are found in a location and that have not been previously identified (Brierley and Fryirs, 2000; Brierley et al., 2002; Brierley and Fryirs, 2005). This flexibility is an important attribute for southwestern Australia, which has several unique features such as the ancient landscape and “backwards” catchments characterised by low rainfall and gradient upper catchments, a rainfall gradient that increases downstream and steeply-sloped mid-catchment (Pen, 1999; Commander et al., 2001; Callow and Smettem, 2004; Twidale, 2004). The potential for unique river styles and arrangement of styles within a catchment is particularly high in this setting.
The third factor in favour of selecting the RiverStyles® Framework is that it has been applied elsewhere in investigating reach response in a catchment, and in identifying the recovery potential of reaches. The RiverStyles® Framework is the most suitable existing classification scheme for application to this research in terms of where and why it was developed.
6.2. Identifying Morphologically Similar Reaches 6.2.1. Classification Methodology The RiverStyles® Framework is a hierarchical classification scheme, implemented in a top- down procedure that considers scales of river processes, patterns, behaviour and system trajectory: from the catchment, to landscape units, near-uniform reaches and finally geomorphic units within these reaches. The process of classification begins (Stage One), with a desktop analysis using a range of datasets, descending through the scales of investigation, with the goal of producing a river styles map that identifies the distribution of styles throughout the catchment (see Brierley and Fryirs, 2005). This forms the basis for Stage Two, whereby the geomorphic conditions of the different identified reach river styles are investigated. This field-based assessment of geomorphic condition then forms the basis for Stage Three, where the evolution and trajectory of each particular reach style or morphotype, and determined from this catchment-wide investigation. The final stage (four) considers the potential for management strategies to be effective in achieving meaningful ecological outcomes, by viewing the potential evolution of individual river styles, taking into account their unique organisation within a catchment and how upstream processes affect their individual recovery potential and overall success of river management strategies (Brierley and Fryirs, 2000; Brierley et al., 2002; Brierley and Fryirs, 2005).
This chapter presents data based on geomorphological investigation of river styles found in the Kent River and Dalyup River in southwestern Australia, using the procedure summarised above, and in more detail in the numerous publications on the RiverStyles® Framework (see Brierley and Fryirs, 2000; Brierley et al., 2002; Brierley and Fryirs, 2005). Section 6.2.2. outlines the
~ 133 ~ Chapter 6: River Response results of identifying and investigating river styles within the two study catchments. This presents data from Stages One and Two of the RiverStyles® Framework. Stage One of the framework was undertaken using a desktop analysis of river character and behaviour using a variety of spatial data sources, coordinated using a GIS: x DEMs that represent the hydrological reality of each catchment (see Chapter 4), x channel elevation (from the DEM), x catchment area (determined using a D-8 flow accumulation algorithm), x channel slope (based on a vertical slice method applied to the elevation data (see Reinfelds et al., 2004)), x catchment slope (derived from the DEM), x valley width (mapped from valley slope), x soil, geology and vegetation mapped at 1:250,000 scale (largest scale available), x recent and historical aerial photography: o Kent River: 2000 @ 1:25,000 scale 1972 @ 1:40,000 scale 1972 @ 1:86,400 scale 1946 @ 1:40,000 scale o Dalyup River 2004 @ 1:25,000 scale 1999 @ 1:25,000 scale 1969 @ 1:40,000 scale Near-identical (i.e. those with similar geomorphic characteristics based on the above data) river reaches were mapped, though the detail of mapping was limited by the scale of the available resources (particularly the aerial photography which was at best 1:25,000 scale (1.4m pixels)). The pioneering nature of this research, and no prior field experience of the study area, limited the ability for this initial stage to accurately identify and map the three-dimensional morphology of river reaches, based on plan-view data, to classify reaches into particular river styles. The lack of previous field-based knowledge of the area is noted as a particular limitation of the RiverStyles® Framework.
Indeed, some river reaches were incorrectly mapped, and using this desktop study as the basis to determine the field program may have serious limitation in catchments that are not well known to those using this framework. This may partially relate to limitations from the scale of the aerial photography Brierley and Fryirs (2000) used 1:12,500; and others suggest using data at scales less than 1:10,000 for mapping geomorphic units (see Gilvear and Bryant, 2003). Larger- scale photography for mapping areas with dense tree cover, and narrow river channels (e.g. all of the mid and lower Kent River catchment) is unlikely to resolve this for some reaches.
~ 134 ~ Chapter 6: River Response Rapid reconnaissance trips of the catchments were conducted following this initial work, visiting easily accessible areas, to make brief observations of river character. These areas were selected, based on initial mapping of river styles as part of the Stage One analysis. River reaches that had been initially mapped were compared to on-ground patterns of river geomorphology. This provided the basis to refine and better interpret the river styles and to determine where more detailed, in-field analysis of representative reaches should be undertaken. The field sites were selected to permit investigation of each style under stages of degradation such that the evolution of each river style could be understood. The extent of this analysis was ultimately determined by the frequency of each particular style, and range of degraded states that existed.
The hierarchical classification methodology constructed by Brierley et al. (2002, see Figure 6.1), was applied to the Kent River and Dalyup River. This primarily classifies reach type on the degree of valley confinement (confined, partially confined, alluvial valley setting) and then by different physical characteristics, dependent on the initial class. Brierley et al. (2002) identified a total of 21 river styles in coastal rivers in New South Wales (see Brierley et al., 2002 p.97). In applying this classification methodology to the study catchments, (expanding initial work presented in Callow and Smettem (2004) on the upper Kent River catchment), eleven different river styles were identified, seven identical to styles identified by Brierley et al. (2002). In total 52 representative reaches were investigated across the two rivers (31 in the Kent River and 21 in the Dalyup River).
Field work involved collection of quantitative and qualitative geomorphic data that was initially based on a detailed channel and floodplain cross section and longitudinal survey of the low-flow water surface slope using a Geodimeter 400 Total Station, or dumpy level in less accessible locations. Along the cross section profiles, a forensic or retroductive geomorphic approach was applied to determine recent responses of reaches (erosional/depositional) to changes in boundary conditions and thresholds to increased water and sediment discharge and degraded vegetation condition. Present vegetation condition was noted, and estimations of past vegetation condition estimated based on the density of tree stumps and sites where they have been eroded (this was later combined with historical aerial photography). Vegetation roughness (Manning’s “n”) for present and pre-clearing condition was estimated based on published sources (Chow, 1959; Hicks and Mason, 1991; Arcement and Schneider, 2003; Ladson et al., 2003; Lang et al., 2004). The pattern of vegetation response was noted on field sketches, which were later transferred onto the surveyed cross sections (see Figure 6.5 to Figure 6.15). Floodplain stratigraphy was also investigated along some reaches to characterise the underlying material, using a combination of data collected by the Department of Agriculture as part of drilling programs. These were used to investigate channel substrate composition and depth to bedrock.
~ 135 ~ Chapter 6: River Response
Figure 6.1 Classification hierarchy of the RiverStyles® Framework, from Brierley et al. (2002). River styles are primarily differentiated based on the degree of valley confinement (confined, partially confined, alluvial), and then different criteria within these classes.
The following section (Section 6.2.2.) presents the data from Stages One and Two of the investigation of river character, identifying unique reaches throughout the two study catchments. Section 6.3 investigates the evolution of each unique river styles, using the range of different examples of river of that particular River Style, under different degrees and types of perturbation. This chapter provides the bases for an investigation of the potential for management of these rivers, and to applying this understanding at a regional scale, themes that are explored in Chapter 7.
6.2.2. Reach Styles in Two South Coastal Western Australian Rivers The landscape setting of the two catchments is strongly related to the trends in rainfall, geological, soils and vegetation discussed in Chapter 2. The regions are characterised by geomorphic zones that differ significantly from the tri-zonal fluvial systems observed in most settings with a steep upland catchment sediment source area, middle transport zone and low gradient lowland accumulation zone (e.g. see Schumm, 1977; Schumm, 1988; Sear and Newson, 1993; Kondolf, 1994; Kondolf et al., 2003). Rather, southwestern Australia catchments begin with sporadic channels that drain an ancient and flat plateau surface (Yilgarn Craton) where stream gradients are low (Salama et al., 1993b; Pen, 1999; Commander et al., 2001), and surface soils are sandy (Ruprecht and Stoneman, 1993; Agriculture Western Australia, 2002a). The fluvial geomorphology is often complex due to the age, and varied palaeoclimates. This has
~ 136 ~ Chapter 6: River Response resulted in complex evolution of drainage (Mulcahy, 1967; Cope, 1974; Ollier, 1988; Ferdowsian and Greenham, 1992; Ferdowsian and Ryder, 1997; Beard, 1999; Pen, 1999; Beard, 2000; Goodreid, 2000; Commander et al., 2001; Beard, 2003; Harper and Gilkes, 2004; Twidale, 2004).
Downstream, rivers have rejuvenated, and mid catchments are dominated by increasingly confined and steep valleys, often with river channels surrounded and controlled by exposed bedrock. These valleys become open over short distances as outcropping bedrock disappears into the coastal plain. Sea level fluctuations over the Holocene have seen phases of river incision during the last glacial maximum (20,000 YBP), followed by subsequent valley flooding, formation of contemporary estuaries from 3,500 YBP, and their subsequent progressive extinction due to sedimentation and wave action (Hodgkin and Clark, 1988; Olsen and Skitmore, 1991; Hodgkin, 1998; Hodgkin and Hesp, 1998; Radke et al., 2004). Within this geomorphic setting, five landscape settings were identified: Flat Uplands, Ancient Drainage, Dissected Terrain, Steep Confined Valley, and Coastal Plain (see Table 6.2 and Figure 6.2).
Despite the relatively strong east-to-west gradient in the landscape (refer to Chapter 2), the study catchments share similar downstream patterns in their landscape setting, drainage from flat uplands with portions of ancient drainage, through a more confined mid catchment where the channel is underlain by bedrock, before emerging and flowing for a short distance across the coastal plain and terminating in an inland basin. This similarity in landscape setting suggests that these rivers may share similar types and organisation of river styles. Table 6.2 details the physiographic characteristics of the different landscape units and the geological, soil-landscape, vegetation assemblages associated with them, while Figure 6.2 maps the spatial distribution of the units at the catchment-scale.
The next step was to use this knowledge in combination with the river styles classification hierarchy (refer to Figure 6.1) to identify the styles found within each river (see Figure 6.3 and Figure 6.4). The methodology detailed in Brierley and Fryirs (2005), was applied through the Kent River and Dalyup River. Figure 6.3 identifies the eleven different river styles that were found across the two catchments.
~ 137 ~ Chapter 6: River Response Table 6.2 Landform Units of the Kent River and Dalyup River Landscape Catchment Setting Geology and Soil Vegetation Channel Valley Unit position Landscape Associations slope Width Flat Upper Gently Yilgarn Plateau, Open Jarrah- Low to <100m Uplands catchment, rounded sandy-duplex and Marri woodland, moderate typically hills with sandy-gravel soils paperbark with (<0.0005 lower-order even slopes mixed sedge m/m) streams to wide species on valley floor floodplains.
Ancient Wide valley Wide, flat Yilgarn Plateau, Open Jarrah- Very Low Up to Drainage floors in the valley alluvial and Marri woodland, (< 0.0001 1500m, upper bottoms colluvial valley-fill paperbark with m/m) mean catchment developed over tertiary mixed sedge, 100- over a sediments, over naturally saline 300m clayey country rock and valley fill waterlogged areas with no vegetation
Dissected Mid Dissected Albany-Fraser Open Marri- Low to 50-200m Terrain catchment gravelly Complex, sandy Jarrah moderate ridges, and gravelly woodlands that (<0.0005 undulating duplex soils, over becomes closed m/m) country, granite and gneiss, in better soils some exposed that is and higher exposed exposed lower in rainfall areas bedrock the landscape
Steep Mid Dissected Albany-Fraser Open and Steep (up < 80m Confined catchment, with Complex,granitic closed Marri- to 0.004 Valley steeper- exposed remnants and Jarrah m/m) sloped country gneissic bedrock woodlands, areas rock on frequently some Karri located in ridges, outcropping stands dissected valley sides landscape and valley floor
Coastal Lower Flat lowland Sandplain overlies Closed Jarrah- Low Narrow Plain catchment, plain country rock, Marri woodland, (<0.0002 river adjacent to comprising which is exposed occasional Karri m/m) channel coast aeolian and at the channel bed pockets, (<100m) alluvial at many locations grading to a incised deposits coastal into woodland and coastal shrubland with plain heathlands in poorer soils
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Figure 6.2 Landscape Units of the Kent and Dalyup Rivers
Figure 6.3 River styles of the Kent River and Dalyup River
Figure 6.4 shows the classification tree, with four new river styles added to the work presented in Brierley et al. (2002) and Brierley and Fryirs (2005): “low sinuosity planform controlled fine grained”, “low sinuosity bedrock”, “fine grained multi channel”, “multi channel sand bed”. The “low sinuosity planform controlled fine grained” and “fine grained multi channel” are new additions to the river styles classification tree, and fit easily within the structure, presented in Brierley et al. (2002) and Brierley and Fryirs (2005). The “low sinuosity bedrock” is a variant of the “low sinuosity boulder bed” from the work of Brierley et al. (2002) and Brierley and Fryirs (2005). The term “bedrock” rather than “boulder bed” is more applicable in this setting given that a continuous bedrock sheet underlies the channel, with limited boulders found in the ~ 139 ~ Chapter 6: River Response channel. The “multi channel sand bed” appears related to the “multi channel sand belt” of Brierley et al. (2002) and Brierley and Fryirs (2005) who have chosen to classify this reach type within the single channel reaches in their hierarchy. In the classification applied to the Kent River and Dalyup River, this river style has been located within the multiple channel branch, and this seems a more appropriate position given that Brierley et al. (2002) and Brierley and Fryirs (2005) have used the number of channels as the secondary factor (after degree of confinement) for distinguishing river styles in an alluvial setting, yet chosen to classify a multi- channel river style under the single channel branch of the classification tree.
The following eleven River styles were identified in the Kent River and Dalyup River: x Gorge x Occasional floodplain pockets x Bedrock controlled discontinuous floodplain x Low sinuosity planform controlled discontinuous floodplain x Low sinuosity planform controlled fine grained x Valley fill x Channelised fill x Low sinuosity fine grained x Low sinuosity bedrock x Fine grained multi channel x Multi channel sand bed
For the 52 representative reaches that were investigated, cross section data with current vegetation condition is presented in Figure 6.5 to Figure 6.15. Overall, reaches have been organised from the most confined and bedrock controlled reaches to those in an alluvial setting. Within each river style, they have been ordered from the most pristine example, through to the most degraded, (irrespective of which river or their location in the river). Table 6.3 summarises the characteristics of each particular river style, in terms of the geomorphic units and controls within each style.
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Figure 6.4 River styles tree for reaches found across the Kent River and Dalyup River. The greyed box identifies new “River Styles” identified by this research.
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Figure 6.5 “Gorge” and “occasional floodplain pockets” river styles.
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Figure 6.6 “Occasional floodplain pockets” and “bedrock controlled discontinuous floodplain” river styles.
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Figure 6.7 “Low sinuosity planform controlled discontinuous floodplain” river style.
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Figure 6.8 “Planform controlled low sinuosity fine grained” and “valley fill” river styles.
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Figure 6.9 “Valley fill” and “channelised fill” river styles.
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Figure 6.10 “Channelised fill” river style.
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Figure 6.11 “Low sinuosity fine grained” river style.
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Figure 6.12 “Low sinuosity fine grained” and “low sinuosity bedrock” river styles.
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Figure 6.13 “Low sinuosity bedrock” and “fine grained multi channel” river styles.
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Figure 6.14 “Fine grained multi channel” river style.
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Figure 6.15 “Fine grained multi channel” and “multi channel sand bed” river styles.
~ 152 ~ Chapter 6: River Response Table 6.3 Characteristics of the different river styles found in the Kent and Dalyup Rivers
~ 153 ~ Chapter 6: River Response Table 6.3 continued.
~ 154 ~ Chapter 6: River Response Table 6.3 continued.
~ 155 ~ Chapter 6: River Response Table 6.3 continued.
~ 156 ~ Chapter 6: River Response 6.3. Changes in Morphologically Similar River Reaches Most of the eleven different channel reach styles were found across a range of degraded states during the field work. Six styles were found in both catchments. Across all reaches within a particular river style, there was a strong similarity in the types of responses and characteristics. All eleven morphotypes are described herein, representing Stage Three of the RiverStyles® Framework. For each style, results of the geomorphic investigation across all reaches of that style are summarised in terms of their contribution toward understanding the behaviour and evolution of the different river styles. The styles are ordered as in the previous section, beginning with the confined styles which are strongly influenced by bedrock and narrow valleys, to the more open and alluvial settings.
Gorge:
Location of the “Gorge” river style in the Kent River (left), highlighted by the red circle. Gorge sections are only found on the Kent River, located in the area just downstream from Break Road and continue for 3 kilometres downstream. Here the channel is incised into the country rock and the channel composed of steeper chute sections and often boulder pools in a stepped/cascading arrangement. The chute sections are conduits for sediment and water. Pools have not accumulated coarse sediment, indicating that low coarse sediment loads are carried through these reaches and accumulated sediments are flushed during high flows. Channel gradients and stream power are greatest in these reaches, though there is no potential for channel adjustment given the channel is laterally and vertically confined by country rock.
The gorge section marks the area where Maley (1976), began his survey of the canoeing recreation potential of the Kent River (between Break Road and Owingup Swamp). As part of this work, Maley took 51 photographs along the lower Kent River and made some detailed notes about the condition of the river. Unfortunately, only six photos remain from this set. His detailed notes allowed the sites to be identified and repeat photographs to be taken. These results are presented below in Figure 6.16.
~ 157 ~ Chapter 6: River Response Figure 6.16 Repeat photography of gorge sections in the lower Kent River, originally photographed by Ian Maley in 1976. 1976 Photography reproduced with permission of the Environmental Protection Agency, Western Australia. 1976 April 2004
Photograph 4049
Photograph 4052
Photograph 4061
Photograph 4062
~ 158 ~ Chapter 6: River Response Maley also noted along this reach and reaches further downstream, that: x “water was not suitable for drinking, but suitable for cooking etc.”, (due to the salinity) x “Except for the occasional and out of character open pool this river is extremely small in width – often only a few metres” x “With the start of the Karri at point 42, the river is, except for the pools referred to above, choked continuously with logs.” x “Again only an almost negligible amount of erosion resulting from man’s activities was noted: o Where cattle drink on a cleared bank at point 52, and; o Where cattle water at point 110”
Of the six photo sites, four are located in the gorge section, and were re-photographed to examine change over the 28 year period (see Figure 6.16). Unsurprisingly, gross channel morphology has not changed. Vegetation at the channel margins (where present) appears to have changed little. Water quality gauging data from Styx Junction (just downstream of the gorge sections) shows that in the late 1970s mean salinity was 1200 mg l-1, but had increased to 2300 mg l-1 at the time of the repeat photography. Despite a doubling of stream salinity in the 28 years between the photo pairs, the geomorphic effectiveness of fringing vegetation has not degraded.
Occasional floodplain pockets:
Location of the “occasional floodplain pockets” river style in the Kent River (left), highlighted by the red circle. This style is associated with the steeper areas of the Kent River catchment, either side of the gorge sections described above. This style is distinguished by the steeply-sloped valley that surrounds the incised channel, with narrow discontinuous floodplain pockets, and a channel continuously underlain by bedrock. It is less common than the “partially confined valley with bedrock-controlled discontinuous floodplain” that is found in both catchments, but they share some similarities in the reach geomorphic characteristics and processes of change that are occurring. Because of its position in the mid to lower Kent River catchment, salinity increases in this style have not been as extreme as in upstream areas (e.g. see Figure 5.2), or on the Dalyup River. Consequently no detectable change in the density or geomorphic effectiveness of riparian vegetation was detected from analysis of historic aerial photography or geomorphic field evidence.
~ 159 ~ Chapter 6: River Response Repeat photography carried out on the two remaining sites (corresponding to this style) show no vegetation degradation (Figure 6.5). Site 4075 shows regeneration of riparian vegetation on the left bank, where dairy grazing has been abandoned, despite a doubling of stream salinity. While there has been only limited clearing of bank vegetation along the lower Kent River, and the period over which vegetation has taken to regrow is unknown (aside from postdating 1976), Figure 6.5 identifies the potential for river revegetation through stock exclusion in “mildly” salinising rivers.
The position of reaches of this style in a lower salinity region of the river and the strong revegetation of Site 4075 indicate that vegetation has not been degraded in response to increasing stream salinity. No erosion was observed in any of these reaches. There was however, evidence of sediment accumulation and minor incision of secondary channels consistent with dynamic equilibrium within these reaches. The storage of sediment within these reaches appears linked to role of large woody debris (LWD) evident in photography from numerous sites (refer to Kent River Sites 22, 23, 27 in Figure 6.5), and dense riparian vegetation. Significant increases in stream salinity are likely to result in reduced density of riparian vegetation and eventually the supply of LWD is likely to be degraded. Determining the source of LWD was not extensively investigated, but based on the size of debris in relation to surrounding sources it would appear to be local. Vegetation degradation may be turning these reaches from their current role as chutes or sediment neutral reaches (i.e. where deposition balances erosion), to a source of sediment for downstream reaches. These reaches are highly susceptible to any erosive response resulting from vegetation degradation due to relatively high slope and confined nature of the channel in these reaches. Based on field investigation and aerial photography, changes in salinity gradient have not been sufficient to degrade vegetation significantly as yet. Similar styles with examples of vegetation clearing and degradation, such as the bedrock controlled discontinuous floodplain reaches, suggest a significant potential for shift towards erosion in these reaches following vegetation degradation.
~ 160 ~ Chapter 6: River Response Figure 6.17 Repeat photography of occasional floodplain pocket sections in the lower Kent River, originally photographed by Ian Maley in 1976. 1976 Photography reproduced with permission of the Environmental Protection Agency, Western Australia.
1976 April 2004
Photograph 4075
Photograph 4080
Bedrock controlled discontinuous floodplain:
Location of the “bedrock controlled discontinuous floodplain” river style in the Kent River (left) and Dalyup River (right), highlighted by the red circle. This style is associated with the dissected terrain and steep confined valley landscapes, in the mid part of both study catchments. These reaches are located downstream from Brownings Road to the South Coastal Highway on the Dalyup River, and upstream of the gorge and occasional floodplain pocket styles (Break Road area) on the Kent River. The channel is less steeply sloped and wider than the two previous styles with strong valley confinement (compare cross sections from Figure 6.6 to Figure 6.5), though bedrock crops out continuously along the channel floor. Bedrock is often exposed laterally, or is overlain by thin layers of sediment. On the Dalyup River this river style is found in areas where vegetation has been extensively cleared for agriculture. At Dalyup Site 8, vegetation has been degraded by grazing and recent fire and is a good example of a reach of this type in a degraded state. The other sites have a wide riparian buffer (Dalyup Site 11), or in the case of Kent Site 24 are located in an uncleared part of the catchment.
~ 161 ~ Chapter 6: River Response In pristine condition, sediment is stored at the channel margins and on the discontinuous floodplain benches. The underlying bedrock means that the channel bed has limited potential for sediment storage in-channel, with some sediment retained on lateral bars and more significant deposits on the floodplain benches. In the Kent River the dense vegetation and low sediment loads (particularly coarse sediment in comparison to the Dalyup) means that the floodplains are constructed from predominantly fine-grained material (mud and fine sand), and potential for erosive response is limited. Moderate stream gradients and the bedrock basement mean that reaches often act as chutes, with no sediment storage potential. However, once flow reaches bankfull, sediment is stored on lateral bars and benches. In a degraded state, and under higher flows, this lateral sediment storage becomes available for downstream removal, with the channel at risk of scour and transformation towards a bedrock chute. Dalyup Site 8 highlights management problems associated with degraded reaches of this style. Large quantities of sediment are stored on floodplains, becoming available for downstream transport during large floods from erosion or avulsion. At present, upstream processes play an important role in determining the direction of change in this reach. Coarse sediment input from upstream erosion (see Figure 6.20) has resulted in accumulation of sediment at the channel margins in this reach (shown in Figure 6.6). These various responses are schematised in Figure 6.18, which demonstrates the potential direction of channel response under variations in sediment load, discharge and stream salinity, based on the changes identified from the three reaches that represent this river style.
Figure 6.18 Example of potential directions of change for the bedrock controlled discontinuous floodplain river style under changes in sediment load, discharge and stream salinity (vegetation roughness).
~ 162 ~ Chapter 6: River Response Low sinuosity planform controlled discontinuous floodplain:
Location of the “low sinuosity planform controlled discontinuous floodplain” river style in the Kent River (left) and Dalyup River (right), highlighted by the red circle. This river style is found extensively in the mid to upper catchments of both the Kent River and Dalyup River, typically located in the dissected landscape, in less confined setting either side of the more confined and steeply sloped areas. Valley width increases and the floodplain becomes more continuous, although more confined and bedrock-controlled sections result in a discontinuous floodplain. The bedrock is typically located close to the channel bed, outcropping sporadically and does not exert any lateral control on the channel. In the Kent River (Sites 17 and 18), lateral benches adjoin the channel and store a mixture of mud and fine to medium sand. These sites are located in the uncleared mid Kent River catchment, but near the upper catchment where salinity is higher (see Figure 5.2, which shows the salinity gradient through the catchment). Consequently, vegetation surrounding the wetted perimeter has degraded, but vegetation across the rest of the floodplain has remained in good condition. In the Dalyup River, vegetation has degraded across the floodplain due to land clearing, grazing and under the influence of higher salinity in the reach (as opposed to the upstream influence on the Kent River, see Dalyup Sites 6, 9, and 10).
In the mid to upper Kent River catchment, these reaches have responded to increased sediment loads, sourced from upstream reaches, by depositing sediment on lateral floodplain benches (Figure 6.19A and B). Within the wetted channel perimeter, vegetation degradation has led to scour of loose sediment, though the dense paperbark vegetation that surrounds the channel in these reaches has a dense root mat that is fortifying the sediment and preventing further channel incision (Figure 6.19 C). In the Dalyup River, these reaches also store large volumes of sediment in the channel margins, on lateral benches and on floodplain terraces. Stratigraphic field evidence shows that recent deposition of fine to medium sand on these benches and on the floodplain of up 5-10cm per event and with 20-30cm of deposition over the past 10-20 years (based on the age of buried vegetation), particularly with deposition associated with dense vegetation.
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Figure 6.19 Processes of and limitation on channel adjustment at Kent Sites 17 and 18. Sediment is accumulated at the channel margins at Kent Site 18 (Photo A and B), while at Kent Site 17, the root mat associated with the paperbark at the channel margins is fortifying underlying (older) sediments from erosion. This dense (relatively continuous) root mat was found at many locations across the Kent River catchment, where paperbark clumps occupied the riparian zone.
In most reaches, sediment is being stored at the channel margins. There is the potential that under flood conditions and influenced by vegetation degradation, that much of this sediment could be moved downstream. An example is the Dalyup Site 10 during the 2000 flood. Vegetation has been left surrounding the channel, but cleared across the remainder of the floodplain. Flow exceeded the channel capacity and flowed across the point bar. Higher velocity across the uncleared floodplain underlain by easily erodible medium-fine sand resulted in significant adjustment of the channel. Figure 6.20 shows data from aerial photography before (1999) and after (2000) the flood event, and the new channel that was carved on the floodplain. The channel has eroded sediment to a depth of between 2-4 metres, creating a new channel 50 metres wide and removing around 60,000m3 of sediment, which filled downstream pools.
The effect of channel avulsion is evident from the photos in Figure 6.21, 300m downstream of the end of the channel avulsion and 3 km further downstream. This shows a pool that has been filled with sediment from the event. Based on the aerial photo and field observations, 900 – 1000m downstream of the avulsion is now choked with sediment. Interestingly, the observable downstream impact of the event is limited to around 1km. This indicates the poor downstream sediment transport competence, and also how effective pools are in trapping sediment. Over time it is likely that some material filling these pools will be removed downstream, though the sediment slugs appears to be only locally mobile (i.e. only hundreds of metres per large event). While the medium-grained sand is able to be transported under most flow conditions, the sediment slug has not moved far downstream since the flood. The avulsive event illustrated in Figure 6.20 was observed at a number of reaches of this river style in the Dalyup River during field work, and from aerial photo analysis. Channel avulsion appears to be the major source of
~ 164 ~ Chapter 6: River Response coarse sediment for the lower Dalyup River. Based on aerial photography and field data of average depths of avulsion channels between 300,000 – 400,000 m3 of sediment has been removed from channel avulsions during this one event (equivalent to 30 to 40 times the annual load reaching Lake Gore – see data presented in Chapter 3).These reaches store large volumes of relatively coarse sediment and pose significant risks to downstream reaches, and in particular to the Ramsar-listed Lake Gore on the Dalyup River. Maintaining dense vegetation cover at the channel margins (on the lateral benches) and across the entire floodplain is important in controlling the potential for these reaches to switch from sediment storage to sediment source areas, as occurred during the 2000 floods. Avulsions were associated with areas of pasture. No incidence of avulsions found for areas with continuous native vegetation cover across the floodplain. The role of vegetation in modulating flood impacts is further explored in Chapter 7.
Figure 6.20 Avulsion channel at Site 10, illustrating the significant risk posed to agricultural land adjacent to rivers and the impact such events can have on the river geomorphology.
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A B Figure 6.21 Pools, located 500m downstream (A) and 3km downstream (b) of the avulsion at Dalyup Site 10 (see Figure 6.20). These images shows the effects of sedimentation from avulsion on pool morphology and waterbird habit.
Planform controlled low sinuosity fine grained:
Location of the “planform controlled low sinuosity fine grained” river style in the Kent River (left), highlighted by the red circle. This style is found in the mid to upper Kent River catchment, associated with more open valleys than the similar “low sinuosity planform controlled discontinuous floodplain” reaches. Here the floodplain is continuous, with a small and narrow channel that is typically straight, though high sinuosity stretches do occur within relatively straight valley sides. The floodplain vegetation is variable across the different reaches of this style, from a relatively pristine reach (Kent Site 16) to reaches degraded by salinity (Kent Sites 14 and 15, see Figure 6.9). These degraded reaches have responded in a similar way to Kent Sites 17 and 18 (due to a similar location and salinity levels and source), with severe degradation of the in-channel vegetation but little change on the floodplain.
These reaches share many similarities with the “low sinuosity planform controlled discontinuous floodplain” reaches, the key differences being the wider valley and less confined setting, floodplains that are constructed from mud rather than mixed fine sand and mud, and continuous floodplain. Paperbark vegetation thickets on the channel margins and floodplain play a similar role as in the previous river style, with the dense root mat fortifying the channel and floodplain from vertical and lateral incision in reaches that are being eroded (see Figure 6.19 C). The lack of large volumes of coarse sediment at the channel margins and on the floodplain, and a wider valley makes this style more stable and less prone to avulsion. No significant changes were observed at Kent Site 14 after the April 2005 flood.
~ 166 ~ Chapter 6: River Response Kent Site 16, the most downstream site of the three examples of this style, is relatively unaffected by vegetation degradation, and fine to medium sediment sources from upstream are being retained at the channel margins and in small lateral bars. At the other sites, degradation of vegetation surrounding the wetted channel perimeter has resulted in erosion of sediment exposing the dense root mat at Kent Site 14 and 15. Across the floodplain, vegetation remains in a similar condition to the pre-clearing state (based on field study and historic aerial photography). Some evidence of minor scour of secondary channels on the floodplain at Kent Site 14 was evident; it is unknown whether this is in response to increased discharge and more frequent overbank flow or to natural variability. Though stream gradients are moderate to steep in these reaches (0.001 – 0.0015 m m-1), degradation of vegetation and reduction in erosive thresholds is restricted to the wetted primary channel perimeter. The vegetated, fine-grained floodplain and dense root mat underlying the channel has resulted in little response in these reaches to landscape perturbation.
Valley fill:
Location of the “valley fill” river style in the Kent River (left) and Dalyup River (right), highlighted by the red circle. This style is typical of the ancient drainage valley reaches in the far upper Kent River and Dalyup River catchment. A wide floodplain that covers the entire valley floor has no defined channel. Channel gradients are exceptionally low, with gradient below 0.0001 m m-1 (falling less than 10 cm per kilometre). Both the Dalyup and West Dalyup Rivers around Griffiths Road at the top of the Dalyup River catchment have this style, though the West Dalyup has been transformed from the “valley fill” to a modified version of the “channelised fill” river style by the open deep drain that has been dug (refer to Dalyup Site 21, Figure 6.9). In some reaches, Samphire is the only groundcover, though stunted open mallee woodland (Dalyup River) or open Jarrah-Marri woodland (Kent River) are found on locally elevated areas at the margins where salinity and water logging do not limit growth. Other areas (e.g. Kent Site 5) are inundated for long periods of the year and are devoid of vegetation. Historic aerial photography and field evidence (i.e. lack of dead trees as with other areas) suggest that these reaches may never have had vegetation covering even in the pre-cleared landscape (see Figure 6.22). This observation is supported by oral history (R. Webb, pers. comm.).
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A
B
D
C Figure 6.22 Lake Carrabundup vegetation and stratigraphy. Historic and recent aerial photography from 1946 to 1999 (Figure 6.22 A), and the recent oblique images looking upstream (Figure 6.22 B) and downstream (Figure 6.22 C) shows that there was little vegetation before land clearing, evidenced by the lack of trees in both aerial images, or dead trees in the oblique photography. Figure 6.22 D shows a stratigraphic log, taken to the north and east of Lake Carrabundup (approximately 500 north of the top of this image), that shows the younger colluvial hillslope materials that overlie the valley fill sediments that were found across this and other reaches in the ancient drainage valleys.
Low channel slopes in these reaches (fall of less than 10cm km-1) means that velocities and potential for erosion are low. The clay valley fill material (see Figure 6.22 D) that underlies this reach and many others in the upper portion of both catchments is highly resistive to erosion. In the field, the unit was made of cohesive massive clay, with only minor dispersion at the surface, with a thin layer of flocculated clay particles in inundated reaches. In dry areas the clay was extremely hard and did not show evidence of cracking. A channel hydraulics calculation tool (WINXSPRO – see latter discussion in Section 7.4) was used for data from Dalyup Site 20 and found that maximum water velocity for a completely unvegetated floodplain under bankfull flow was 0.21 m sec-1, far below the critical flow velocity required to initiate channel incision of the clay valley fill (which is greater than 5 m sec-1, based on presented in Callow & Smettem, 2007). Given the location of these reaches in areas of both high pre- and post-clearing salinity and low sediment loads carried naturally (evidenced by the lack of islands in the middle of the floodplain valley or sandy bars and banks at the channel margins) or under contemporary conditions (evidenced by the lack of accumulation of sediment surrounding the channel or on
~ 168 ~ Chapter 6: River Response the floodplain) these channels have changed very little. While vegetation has severely degraded, channel materials and low channel gradients (and consequently stream power) has not resulted in significant adjustment of channel morphology. While there has been minor erosion of sandy material from mid-channel islands, there has been an only minor change in the volume (despite a more significant increase as a percentage) of sediment contributed from hillslopes and tributaries since land clearing and salinisation.
Channelised fill:
Location of the “channelised fill” river style in the Kent River (left) and Dalyup River (right), highlighted by the red circle. Channelised valley fill reaches are found in a number of locations in both catchments. In the upper catchments, reaches of this style are found where a sandy layer overlies the clay valley fill (described above). Analysis of longitudinal channel profiles from DEMs, suggests that the location of these reaches in upper catchment settings (particularly on the Kent River) is associated with locally flatter reaches, that have acted as sediment sumps, accumulating sediment sourced from the valley fill and other reaches, possibly over long time periods. In lowland reaches, they have incised into the sandy marine and aeolian sediments on the coastal plain. Consequentially, these reaches are surrounded by material that is often easily erodible and the reach morphology is potentially more dynamic. Erosion can be significant within this style, and can severely impact on downstream reaches. Across the nine examples of this river style, there are a range of reaches in good condition, with dense vegetation that is stabilising the reaches, to reaches that have little vegetation and show the potential for these reaches to act as sediment sources.
The nine examples of this river style demonstrate the role that vegetation condition plays in determining the stability of these reaches. In the lower Kent River, reaches are well vegetated with dense stands of paperbark, wattle and various Eucalypts. Even within these reaches, sediment is stored temporarily at the channel margins in lateral and point bars, and is mobile downstream (see Figure 6.9, Kent Site 30 and Kent Site 13 (upper-mid catchment)). Numerous secondary channels are found on the floodplain, due to the lower erosive thresholds of the fine- medium sand material that make up the floodplain. This is particularly evident at Kent Site 30 and 31 (see Figure 6.9), where the numerous secondary channels suggest anabranching behaviour of these reaches, though based on field and aerial photographic evidence, there does not appear to be dynamic switching of flow between channels across the floodplain, and flow
~ 169 ~ Chapter 6: River Response remains in a stable, single primary channel with a series of large but essentially stable secondary channels.
In higher salinity reaches, and those in cleared areas which are actively grazed (see Figure 6.10, Dalyup Sites 5 and 7), the channel is less stable. Sediment from upstream sources is transported through these reaches, with erosion of stored material evident by contemporary channel enlargement and reworking of floodplain sediments. The examples of this river style on the Dalyup River carry large sandy bedloads, and high stream salinity prevents growth of vegetation within the wetted perimeter of the channel. These sediments are highly mobile. There is potential for vegetation establishment on the floodplain away from the saline river flow, however, current land management practices are preventing this from happening.
Figure 6.23 Changes at Dalyup Site 7 from the March 2000 flood. The lack of geomorphically-effective vegetation surrounding the channel and floodplain resulted in channel widening and large volumes of sandy bedload mobile through this reach.
In comparison to the valley fill reaches, channel gradients are higher (0.0002 – 0.00015 m m-1), and the coarse sediment has a significantly lower erosive threshold. The potential for channel adjustment is demonstrated by the changes in some reaches in the Dalyup River from the 2000 floods (e.g. see Figure 6.23, above). By comparison a large flood in the Kent River in April 2005, saw little change to Kent Site 2, with minor bank erosion (5-10 cm of bank retreat), and scour of a minor secondary channel on the floodplain despite flow reaching a level of 0.5 above the floodplain (based on debris evidence). Here the low to moderate channel gradient (0.00036 m m-1) resulted in relatively slow channel velocities (estimated from modelling to be 0.3 – 0.5 m
~ 170 ~ Chapter 6: River Response sec-1 at bankfull) and pasture was able to retain the sediment and prevent avulsion on the floodplain. Where flow becomes locally channelised by vegetation, slope becomes steeper and where vegetation has degraded, erosive thresholds are exceeded and these reaches become unstable and potentially act as sediment sources for downstream reaches.
Low sinuosity fine grained:
Location of the “low sinuosity fine grained” river style in the Kent River (left) and Dalyup River (right), highlighted by the red circle. This river style is typically found downstream of the “valley fill” reaches, and are characterised by their wide floodplain and lack of a confined channel. These reaches are similar to the valley fill reaches in appearance, but are distinguished by a defined channel and the presence of mid- channel islands. This style is found in open valley settings with low channel gradients (typically 0.00025 – 0.0001 m m-1) on both the Dalyup and West Dalyup Rivers around Speddingup Road, and in the upper and mid Kent River catchment.
Similar to the valley-fill channels, channel gradients are low (0.00025 – 0.0001 m m-1). Greater discharge due to a larger catchment area, and (slightly) steeper stream gradients leads to greater channel velocities, though because of the similar underlying clayey channel material, the boundary material is highly resistant to erosion (see Figure 6.22). Also unlike the valley fill reaches, most of these reaches were naturally vegetated before land clearing, but, except for the reaches in the mid Kent River catchment, (Figure 6.11, Kent Sites 20, 19 and 12), most have experienced severe vegetation degradation due to high salinity.
Associated with the vegetation degradation has been erosion of sediment that had accumulated behind vegetation at the margins of the channel and on small islands elevated above the remainder of the floodplain, though these volumes are minor, with only a few centimetres of coarse sediment having been removed. Evidence of this is exposed tree roots elevated above clayey material. Along the flow channels sediment is temporarily stored at the margins of the flow. Lateral to the primary channel, much of the vegetation has also degraded. Sediment is sometimes stored behind vegetation and dead tree stumps, though storage is temporary and moderate flood conditions remove this material downstream (refer to Figure 6.24).
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Figure 6.24 An example of changes that have occurred in low-sinuosity fine grained river styles at Dalyup Site 18, where vegetation surrounding the wetted channel perimeter has degraded and sediment is now freely transported downstream.
Vegetation has degraded across the areas adjacent to the primary channel, with the floodplain surface now exposed to erosion. Clumps of dead Melaleuca stumps are clearly evident, and at some locations, exposed roots indicate that up to 10-15 cm of sediment has been eroded. No sediment is stored across the bare areas, and rainsplash and sheetflow across these regions carries material to the main channel, where sediment is accumulated. During low flow conditions sediment is transported downstream via the migration of dunes, forming the ripple patterns. During floods, large quantities of this bedload sediment are carried downstream and deposited in pools or downstream reaches.
Kent Site 1 Kent Site 3 Kent Site 9 Coarse sediment 2cm, 8cm 4cm +1cm, NR Underlying clay valley fill NR, 0.5cm, 0.5cm NR, NR, 0.3cm, 0cm 0cm, 0cm, NR N.B. NR means not recovered, all values are negative (erosion) unless otherwise indicated
Figure 6.25 An example of the erosion pin networks (at Kent Site 1), used to measure erosion rates of the clay valley fill material within and between different study reaches. Plots were established on 14th March 2003, and resurveyed (and removed) on 11th April 2005, immediately after the large flood in the Kent River. Data presented above shows the measurements taken from left to right (looking upstream) for Kent Sites 1, 3 and 9.
The resistance of the underlying clay valley fill material to erosion was investigated. Based on field observations, rates of erosion are slow, even in steeped reaches where the massive clay material is found. Erosion pins (consisting of 30 cm lengths of 5 mm metal rod pushed into the ~ 172 ~ Chapter 6: River Response channel bed, to expose exactly 100 mm above the surface) were established at five sites, two of which were low sinuosity fine-grained river style (Kent Sites 1 and 3), one in the fine-grained multi channel (Kent Site 9). Erosion pins and scour chains (scour chains consisted of metal chain links driven into sand bodies such that the top link was level with the sediment surface) were also established at Kent Site 31 (channelised fill) and Kent Site 14 (planform controlled low sinuosity fine grained), but these were underwater and could not be found during subsequent field visits.
Figure 6.25 shows one erosion monitoring site, just upstream of Kent Site 1. The figure presents data from the monitoring period (2 years), which included the large flood in April 2005, though rates of erosion of the underlying clay valley fill material were very low (average 0.1cm yr-1). The behaviour of the clay material was also investigated to determine whether the sediment was dispersive or slaked. An Emerson (1967) slaking test was performed, and found the air-dried material to be non-dispersive, but with the aggregates disintegrating rapidly and flocculating upon wetting (refer to Appendix 6A). When the test was repeated on the block sample at field saturation conditions (i.e. the condition that it was extracted in), the aggregates were highly stable, and no disaggregation was observed. These laboratory results match the field observations that suggest that this material is highly stable and non-dispersive. While minor disaggregation of surficial material does occur, it is limited and leads to only small amounts of surface stripping in areas where flow is concentrated. Minor slaking and flocculation of the surface material appear the only mechanisms whereby erosion of these reaches can occur. These are therefore strongly time-dependent, rather than energy-depended processes, and have resulted in very slow rates of response of these reaches.
Low sinuosity bedrock:
Location of the “low sinuosity bedrock” river style in the Dalyup River (right), highlighted by the red circle. Found in the lower portions of the Dalyup River, this style is similar to the channelised fill styles, though differentiated by the bedrock confinement at the channel basement (rather than the clay basement). This limits vertical incision of the channel and therefore lateral erosion is potentially greater and the channel is laterally more unstable. In the lower Dalyup River (predominantly the West Dalyup River and downstream of its confluence with the Dalyup River) the channel is incised into the coastal plain to within a few kilometres of Lake Gore,
~ 173 ~ Chapter 6: River Response exposing the granitic country rock. There is also an example of this channel style higher in the landscape (Dalyup Site 12), on a tributary of the Dalyup River.
The single channel stream is typically well vegetated along the banks and floodplain, though the surrounding land is cleared and the width of the riparian zone varies, and grazing of stock in the channel affect both vegetation type and density. The river has numerous discontinuous lateral benches, and lateral channels that have been eroded and filled with coarse sediment (refer to Figure 6.12 and Figure 6.13). The channel has large quantities of woody debris (refer to Dalyup Sites 1, 2 and 3 in Figure 6.12 and Figure 6.13), and these trap mobile bedload at the channel margins and on small, discontinuous lateral benches during low to moderate flows, with the vegetated channel margins able to store large quantities of sediment. Erosion scars related to the March 2000 flood are present at many locations along the West Dalyup and Dalyup River, and lower Dalyup River below the confluence. These show the evidence of erosion of sediment stored close to the channel on small lateral bars and benches, and on elevated, discontinuous terrace features that have been eroded to the underlying clay material (see Figure 6.26).
Figure 6.26 Erosion of lateral benches and terraces from the March 2000 flood. Profile for the post-200 channel was surveyed, and the pre-flood profile constructed from aerial photography and field evidence. Photo A and C (see location on 2000 aerial photo image) shows the erosion of secondary channels into the terrace (photo C is located about -45m on the profile). Composition of the floodplains is shown in photo B.
~ 174 ~ Chapter 6: River Response The position of these reaches in the lower catchment and the large volume of available sediment make them important sources of sediment, which can potentially reach Lake Gore. They are also important for their potential to store sediment sourced form upstream, with the vegetation condition critical in determining whether they become sediment source or store. The power of the March 2000 flood demonstrates the sensitivity of these reaches and the increase in discharge frequency and reduced relaxation time is likely to affect this river style (particularly in lowland areas where the effects of discharge increases are at their greatest). Even with complete vegetation cover, there is the potential for these reaches to respond erosively to high magnitude flows, as evidenced by the previous excavation of lateral bars and benches, uncovered by erosion associated with the March 2000 flood.
Fine grained multi channel:
Location of the “fine grained multi channel” river style in the Kent River (left) and Dalyup River (right), highlighted by the red circle. Downstream of the fine grained single channel rivers are sections where the floodplain becomes wider, catchment area and discharge increases and the single channel rivers become multi- channelled. These reaches are typical of the area on the West Dalyup and Dalyup, around Boydell Road, and the Kent River between Lake Carrabundup and Porrarecup Road. These reaches are underlain by a fine-grained clayey valley infill, material similar to the fine-grained single channel reaches. Cross sections show the main difference between these reaches and the single channel fine grained reaches, is often a wider valley setting, multiple channels that are active under low flow conditions, and numerous small islands across the floodplain separating the channels (Figure 6.13, Figure 6.14 and Figure 6.15). Vegetation cover is variable on these islands, though on reaches with more local relief across the floodplain and those with lower salinity, vegetation is more dense and better able to trap sandy material. The majority of these reaches are low to moderate gradient, with drops of 0.0005 – 0.00015 m m -1.
On the mid-channel islands, greater quantities of sediment are stored than in other fine-grained reaches. On these islands up to 40 cm of coarse sand has accumulated, though in many areas the depth is significantly less. Evidence of erosion in reaches where vegetation has degraded suggests that deposition and sediment retention is finely balanced and controlled by vegetation density, with the most unconsolidated sediment on mid-channel islands having been eroded in reaches once vegetation is degraded.
~ 175 ~ Chapter 6: River Response Figure 6.27 illustrates the processes leading to the formation of mid-channel islands in these environments. The striated appearance arises from deposition of alternating layers of coarse sand (white colour) and fines (dark material), most likely deposited as flood couplets. In Figure 6.27, this sits on top of the clayey valley fill material (indicated by the knife), that underlays the entire floodplain. The laminated material that makes up the island is typically stable when vegetated, though once vegetation degrades, this material can be eroded. The underlies clay valley-fill material is very resistant to erosion (see discussion above and Figure 6.25). The island material with lower erosive thresholds is preferentially eroded, though over time some incision into the underlying clay valley fill material will occur, where vegetation is unable to re- establish itself and to fortify sediment.
The laminated appearance of the fine sediment that has been deposited to create these features suggest that they have been created due to vertical accretion over long time periods of time due to the transport of fine sandy material during floods, followed by organic clay material accumulating under lower flow condition. Dating of these materials would provide further information on the rates of accumulation and the climate and land-use conditions under which they were created. There is no evidence of vegetation burial in more vegetated reaches where these features persist. This suggests that they are not forming under contemporary conditions and widespread erosion of the laminated layer is observed under the conditions of increased discharge and vegetation degradation.
Figure 6.27 Stratigraphy of a mid-channel island, typical of those found in the mid and upper reaches of the Dalyup and West Dalyup Rivers. Photo taken on the West Dalyup River, 500m upstream from Spedingup Road.
~ 176 ~ Chapter 6: River Response Multi channel sand bed:
Location of the “multi channel sand bed” river style in the Dalyup River (right), highlighted by the red circle. This style is restricted to a small area along the Dalyup River, where a large volume of sand has filled the floodplain, overlying a deeper clay valley infill. This is essentially a fine grained multi channel reach that has filled with coarse sediment. The result is a primary channel that braids across a wide primary flow channel and transports a large quantity of highly-mobile bedload. There are numerous secondary channels that are active in low to moderate flows, separated by sandy, mid-channel islands, some with samphire and occasional paperbark shrubs that partially fortify the sediment, limited by the high salinity and the high mobility of these sediments. Sediment supply from upstream appears to be balancing or in-excess of sediment denudation, though it is important to note that this is a dynamic reach that is likely to contribute a large volume of sediment to downstream reaches.
Aerial photography identifies a large deep drain that has been used to channelise the river for 3km upstream of this reach, may be responsible for increased sediment input. Aerial photography and stratigraphic evidence indicates that this reach has always had a sand bed, and is not a contemporary variation of a fine grained multi channel reach that has filled with coarse sediment (see Figure 6.28). The longevity of this reach style is linked to the balance of sediment input and output. At present this reach contributes large volumes of sediment downstream and retention mechanisms (vegetation) are limited by high stream salinity. If upstream sediment is being sourced from depletion of in-channel stores, there is potential for run-down of these stores and that diminished upstream sediment supply. Continued vegetation degradation and diminished upstream supply would cause this reach to evolve toward a channel style that reflects the underlying material, as the coarse sand fill is transported through downstream reaches to fill downstream pools along the Dalyup River.
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Figure 6.28 Evolution of the multi channel sand bed reach at Dalyup Site 13, from 1969 to after the 2000 flood. The exposed sand sheets have expanded since 1969, due to vegetation degradation. While sediment has been eroded downstream, supply from upstream reaches has resulted in no net degradation.
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Summary
The RiverStyles® Framework (2002) was originally developed in coastal catchments of New South Wales, but with the aim of providing a geomorphological and physically process-based method for use in many landscapes for assessing the evolution of river reaches placed within a catchment context. In applying this method for this project, the RiverStyles® Framework has provided a successful method for identifying similar river styles within the Kent River and Dalyup River catchment, and comparing the evolution of these channels under different strengths of perturbation. The variability in channel evolution of particular reaches is evident, and highlights the importance of considering the direction of channel evolution in the context of the landscape and the catchment position and the processes occurring upstream.
The flexibility of this method to allow the addition of new river styles identified in this study was a key factor in applying this method to a landscape setting that differs from the landscapes where the method was developed. While this method was successful, a number of observations and cautionary observations were made that may have implications for the adoption and application of this method. Reaches identified by previous researchers from the Bega River, of that the same river style found in both their rivers the river investigated in this study are evolving in different directions in response to similar perturbations (land clearing). These responses in different settings underlie the importance of recognising regional differences when assessing river channel evolution. On this basis, the RiverStyles® Framework provides a valuable regional tool for assessing river character, behaviour and response, rather than a guidebook for managing particular river styles as recognised by Brierley and Fryirs (2005).
Application of the classification methodology was at times difficult. Some of the reaches appear misclassified (such as the multi channel style classified within a single channel division), though the biggest difficulty was in identifying boundaries between different styles. The classification scheme is largely subjective and relies on qualitative, supervised interpretation and, therefore, there is the potential for different classifications of the same reach.
Overcoming the issues associated with geomorphic classification that is dependent on qualitative analysis is limited by the inherent diversity within and between river systems, and difficulty in applying simple quantitative spatial models for classification. Classification using more quantitative methods applied within in a GIS framework was tried: based on catchment parameters such as catchment area, geology, soils, valley width. However, the diversity and variability of reaches through a system and the relatively coarse resolution of available spatial data cannot account for the variability of channel form and uniqueness of individual rivers. While it would be possible to develop these types of models, they are likely to be highly
~ 179 ~ Chapter 6: River Response regionally specific (particularly in this environment). A mixture of quantitative and qualitative analyses with extensive field examination is required to understand river channel evolution in this, and most other settings. The RiverStyles® Framework therefore provides what appears to be a universally applicable and flexible tool that can be applied at the regional-scale to evaluate channel evolution and to determine how management strategies can influence reach trajectory.
Evolution and the response of the eleven river styles identified in the Kent River and Dalyup River is summarised in Table 6.4. The variable response of different river styles is highlighted, additional to the variability within individual styles under the influence of vegetation degradation, increased sediment loads from upstream, and from increased discharge and large flood events. The evolution of these channels and the potential for management strategies to influence the trajectory of system evolution is examined in Chapter 7.
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Table 6.4 Geomorphic evolution characteristics of the different river styles of the Kent River and Dalyup River, southwestern Australia, highlighting the changing function of river reaches from the pre-European (uncleared), to the current degraded and salt-affected condition.
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Chapter 7: Recovery and Management of Rivers in Saline Landscapes
Photograph: May 2000 Photograph: June 2005 Vegetation recovery following the March 2000 flood in the Dalyup River. These images are taken at the same site, five years apart and show vegetation recovery in a river reach where salinity levels were similar to seawater (30,000 mg l-1) when the most recent photograph was taken in June 2005.
7.1. Introduction While the nature and magnitude of morphologic responses in these cleared, saline dryland catchments is now appreciated for particular river styles, the potential for river management to offer ways to stabilise these reaches and to modulate downstream impacts remains unknown. While secondary salinity is associated with land clearing in dryland environments across the world (Ghassemi et al., 1995; Conacher and Sala, 1998), there are no settings directly comparable to southwestern Australia that offer examples of river management strategies (i.e. with similar stream salinity, hydrology, geomorphology and land use changes). Because of the high-cost of river management and the relatively low-cost of land, in most other agricultural settings, vegetation has been the primary tool used to achieve management goals (Hey, 1996; Rutherfurd et al., 2000; Downs and Gregory, 2004; Brierley and Fryirs, 2005). The potential for river management strategies to offer meaningful outcomes for saline landscapes appears limited, given data presented in Chapter 5. The paper by Hatton and Salama (1999) entitled “Is it feasible to restore the salinity-affected rivers of the Western Australian wheatbelt?” concludes by stating (p316):
“We may have to accept that some changes in the hydraulic and hydrochemical characteristics of the system may be irreversible…[and while]…there is an ethical compulsion…there may be little we can realistically do to control or reverse this process” ~ 183 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes This chapter examines and quantifies the potential for river management to stabilise degrading reaches in a saline, dryland river. Patterns of vegetation growth in the two study catchments form the basis for developing a model for the use of vegetation as a management tool in saline environments. The potential effectiveness of vegetation in stabilising rivers is then assessed using a numerical channel hydraulics model. This chapter focuses on two specific questions: x Where can vegetation be used for river management in saline landscapes?; and x Can vegetation be effective in stabilising reaches and in influencing channel evolution for particular river styles in a saline landscape?
7.2. Potential for River Management in a Saline Landscape 7.2.1. Applying River Management in a Saline Landscape River management has traditionally focused on controlling flow for utilitarian exploitation, or for modulating impacts on infrastructure and property adjacent to rivers (Revenga et al., 2000; Vorosmarty and Sahagian, 2000; Downs and Gregory, 2004). Intervention has involved engineering solutions to specific problems, such as fortifying eroding banks and redesigning channels for the optimal movement of sediment and water (Hey, 1996; Downs and Gregory, 2004). Following the failure of many of the river engineering projects of the mid to later twentieth century, awareness of rivers as dynamic geomorphic and ecological systems has grown. River management has also evolved to consider management strategies in the context of an integrated and holistic catchment-scale approaches based on sustainability (e.g. Brooks, 1995; Hey, 1996; Gilvear, 1999; Brierley and Fryirs, 2000; Douglas, 2000; Newson and Newson, 2000; Gordon et al., 2001; Brierley et al., 2002; Newson, 2002; Fryirs, 2003; Downs and Gregory, 2004; Brierley and Fryirs, 2005; Costelloe et al., 2005).
River management research originated in North America and Western Europe, but is now applied across the world to a range of river types, environmental settings, and types of perturbation that include; river regulation, urbanisation, climate change, base-level change and land clearing (Décamps 1993; Ward et al., 2001; Bernauer, 2002; Downs and Gregory, 2004). Consequently, river management tools and strategies are applied, adjusted, customised and developed across a myriad of settings and for a range of perturbations. Despite the range of environments over which river management is practiced, there are three management philosophies that are applied, originating from the field of restoration ecology: restoration, rehabilitation, and remediation (Palmer et al., 1997; Rutherfurd et al., 2000; Young, 2000; Lovett and Edgar, 2002).
River restoration aims to return all components of the river system (e.g. water quality, sediment and flow regime, channel geometry, aquatic and riparian floras and fauna) to the pre-modified condition. This requires all aspects of system sensitivity driving change to be reversed, to
~ 184 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes promote recovery of degraded components of the river ecosystem. Rehabilitation aims to improve the condition of degraded streams, to return rivers towards a condition that most closely resembles the pre-modified state. This considers the limitations of river management, within the altered controls on river processes, to create a river ecosystem in balance with the new regime. Where the magnitude of changes to boundary conditions are irreversible and have altered in such as way that the existing channel is unstable under the new regime, river remediation aims to maximise ecological diversity to create the most functional and geomorphically stable ecosystem within the new constraints on the system (Hobbs and Norton, 1996; Lovett and Edgar, 2002).
Much of the research and work on river management in Australia has involved adapting river management tools to work within local constraints such as large climatic and hydrological variability, low continental elevation, and the recent nature of human disturbance (Warner, 1988; Tooth and Nanson, 1995; Brizga and Finlayson, 2000b; Douglas, 2000; Rutherfurd and Gippel, 2001). The focus of river management and research in Australia, has been on coastal rivers of eastern Australia and the Murray Darling Basin (MDB) (Tooth and Nanson, 1995; Rutherfurd and Gippel, 2001). Southwestern Australia has been specifically identified for the dearth of research on river processes and management (Tooth and Nanson, 1995). In southwestern Australia the majority of river management work is undertaken by local catchment groups, funded through programs such as Landcare and the Natural Heritage Trust; with advice from Land Care Conservation District (LCDC) offices (Department of Agriculture), Department of Water (DoW), Department of Environment (DoE), and the Department of Conservation (DoC) (Conacher and Conacher, 1995; Pen, 1999; Conacher, 2002). There are a limited number of targeted programs such as the Water Resource Recovery Catchments projects (DoE) and Natural Diversity Recovery Catchments (DoC) (Lothian and Conacher, 2005).
Management plans for rivers in southwestern Australian typically identify river restoration as the primary goal; for example the Dalyup River Action Plan states that the primary aim is to “to protect and restore the Dalyup and West Dalyup Rivers” (Water and Rivers Commission, 2002a p.1:1). While management goals for river rehabilitation are publicly and politically “palatable” (Possingham, 2001; Hobbs et al., 2003), they are unrealistic in this landscape given the severity of changes in boundary conditions, erosive thresholds and stream salinity. While the potential for meaningful geomorphic and ecological river management outcomes through rehabilitation or remediation remains poorly understood, river restoration is an unrealistic strategy given the fundamental changes in these landscapes as outlined in this research. Research on ecological consequences of landscape salinisation suggests that rising saline watertables and stream salinity will cause changes in the structure of riparian ecotones (Froend et al., 1987; Cramer and Hobbs, 2002; Briggs and Taws, 2003; Halse et al., 2003; Hart et al., 2003; Hobbs et al., 2003;
~ 185 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes James et al., 2003; Lymbery et al., 2003; Nielsen et al., 2003). River rehabilitation schemes will need to work within the new limits on plant growth. Cramer and Hobbs (2002) identified the need to recognise zones of the river channel where management can succeed under the new regime, and that the effects of salinity, waterlogging, soil type, and topography will determine the potential for rehabilitation- or remediation-based management strategies to succeed.
7.2.2. Potential Tools for River Management in a Saline Landscape Numerous river “rehabilitation” manuals, books, research articles and other publications extensively review and discuss the range of river management techniques that are available and applied across a range of settings (e.g. see Thorne et al., 1997; Rutherfurd et al., 2000; Water and Rivers Commission, 2001; Downs and Gregory, 2004; Brierley and Fryirs, 2005; Cottingham et al., 2005). Despite the wide range of available tools, the high costs of intensive intervention, such as; re-engineering channels, building hard structures and surfaces, replanting and reintroducing woody debris to rivers, limits their application for many projects (e.g. see data in Rutherfurd et al., 2000 p253-255). In agricultural catchments, the land value in comparison to restoration costs has meant that river management strategies have concentrated on vegetation- based initiatives. These aim at improving vegetation condition across the riparian corridor through fencing to exclude stock, weed and fire management, and limited revegetation. By improving vegetation condition and increasing the roughness surrounding the channel, velocity and stream power are reduced and the channel can be stabilised. This chapter examines the potential for these tools to work in saline landscapes.
Salinity tolerance varies significantly between species, and is affected by drought tolerance and adaptive mechanisms such as storing salts in vacuoles to prevent plant cell death (Munns, 2002; Barrett-Lennard et al., 2003; Loch et al., 2003). Tolerance to absolute salinity varies within species, affected by waterlogging, soil texture and pH (Barrett-Lennard et al., 2003; Loch et al., 2003), and life-stage (i.e. seedlings and saplings are more sensitive than mature individuals) (Bailey et al., 2002; Nielsen et al., 2003; Agriculture Western Australia, 2004b). Based on controlled field trials and greenhouse experiments, there are extensive data on salinity tolerance for many species (see Bailey et al., 2002; Agriculture Western Australia, 2004b). However, the complexity of river channels means there is a limited understanding of how shallow saline watertables will change riparian vegetation assemblages (Cramer and Hobbs, 2002; Hobbs et al., 2003). In addition, there are few quantitative models or tools to determine where, and which species, can be used for managing disturbed riparian corridors in saline landscapes (Hobbs and Norton, 1996; Cramer and Hobbs, 2002; Hobbs et al., 2003).
Research on salinity trends presented in Chapter 5 stress the severity of current stream salinity in comparison to selected tolerance values of ecological systems (see Table 5.2). Research from
~ 186 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes dryland rivers in the MDB, predict that salinity will peak around 100 years after clearing and freshen over the next 400 years until a new equilibrium is reached. By comparison, in southwestern Australia, the freshening cycle is predicted to last between hundreds of years for higher rainfall catchments, to potentially one hundred thousand years for catchments with low rainfall, flat landscape and high salt storages (Hookey, 1987; Salama et al., 1993a; George et al., 2001; Hatton et al., 2003). While changed farming systems offer some potential to improve water quality, most of the valley floors across southwestern Australia are already saline and waterlogged, or will become so in the following decades (Ferdowsian et al., 1996; Hatton and Salama, 1999; Hatton and Nulsen, 1999; George et al., 2001; Hatton and Ruprecht, 2001; National Land and Water Resources Audit, 2001; Cramer and Hobbs, 2002; Hatton et al., 2003). Engineering works offer some potential to alleviate salinity levels (Thomas and Williamson, 2001; Coyne et al., 2002). As with more interventionist river management strategies, they are significantly more costly and so are restricted to the highest value assets (Froend et al., 1997; Hatton and Salama, 1999). River management strategies in these rivers therefore need to work within the limitations which are likely to persist over management timescales. Species suitable to the new ecological and geomorphic niches that exist in these river systems need to be identified, so that their potential for vegetation-based river management can be assessed. This is the basis of the following section.
7.3. Vegetation Regeneration and Growth in Saline Landscapes 7.3.1. Response of Vegetation in Salinising Landscapes: Downstream Trends in Salinity, Plant Growth and Regeneration. The potential for vegetation to be used in river management was investigated in both the Kent River and Dalyup River catchments. Botanically dominant and the most geomorphically important species were identified during field work, and are outlined below. More detailed information on their structure, habitat, location and salinity tolerance is provided in Appendix 7A. Trees and Large Shrubs x Marri (Corymbia calophylla formally Eucalyptus calophylla) x Flat-topped Yate (Eucalyptus occidentalis) x Golden Wreath Wattle (Acacia saligna) x Paperbark (Melaleuca sp), typically: Swampy Paperbark (M. rhaphiophylla), Salt Water Paperbark (M. cuticularis) Native Shrubs, Sedges and Rushes x Juncus sp., typically: Spiny Rush (J. acutus), Pale Rush (J. pallidus), Shore Rush (J. kraussii) x Sword sedge (Lepidosperma sp): Spreading Sword Sedge (L. effusum) Coastal Sword Sedge (L. gladiatum) ~ 187 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes x Gahnia Sp., typically: G. Decomposita or G. trifida Weeds x Samphire (Halosarcia sp) x Rye Grass (Lolium sp.) x African Love Grass (Eragrostic curvula) x Kikuyu (Pennisetum clandestimum)
Salinity data was combined with patterns of vegetation occurrence. Identification of thresholds for tolerance, growth and regeneration, were used to indicate where particular species could be used for river revegetation and management, using the criteria below: x tolerance (where mature individuals survived but did not actively grow), or x growth (where mature individuals survived and grew), or for x regeneration (where juvenile plants were able to grow).
This work did not consider the potential for introducing exotic species, though the research methods used here could be applied to find suitable species from other landscapes. The influence of soil substrate (clay valley fill, laminated clay/fine sand, sand), mid-channel islands, waterlogging and local micro topography was accounted for by analysis of vegetation patterns across river cross sections and under different salinity gradients, for particular river styles. The initial focus of work was on the Kent River, with conclusions tested on the Dalyup River.
Salinity and Vegetation Patterns in the Kent River
Figure 7.1 shows the variation in daily salinity, along with values for mean daily and flow- weighted mean salinity from April 2000 – April 2002 at Watterson’s Farm Gauging Station (Station: 604003), and tolerance levels of Paperbark and Samphire based on published data (see Appendix 7A). Given the tolerance of mature Paperbark to salinity is approximately 40,000 – 50,000 mg l-1, values for mean flow-weighted salinity (8,200 mg l-1) and mean daily salinity (28,000 mg l-1) do not explain the death of vegetation observed across the floodplain and the lack of regeneration. Analysis of data from the Australian Salt Sensitivity Database (Bailey et al., 2002) and from other references (Bird, 1978; van der Moezel and Bell, 1987; Marcar et al., 2002; Barrett-Lennard et al., 2003), shows that trials of salinity tolerance were typically conducted over periods ranging from several weeks to a few months (typically 6-8 weeks). The hypothesis that the maximum salinity over a period of similar duration (i.e. 60 days) rather than measures of mean salinity, was the factor limiting plant growth, was tested. The salinity value that was exceeded by 60 continuous days was determined from the gauging record (see Figure 7.1). At this site, peak salinity exceeding 47,000 mg l-1 for 60 days, a value that corresponded
~ 188 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes with published data of vegetation tolerance, and explained vegetation response at this reach (see image for Kent Site 3 in Figure 7.6 A).
Figure 7.1 Salinity variation over a two year period at Watterson’s Farm Gauging Station (604003), April 2000 – April 2002, in comparison to regeneration and tolerance limits of Paperbark and Samphire.
The hypothesis that maximum 60 day salinity determines vegetation growth potential was further tested by plotting the downstream salinity variability based on data presented in Figure 7.2, from the four gauging stations on the Kent River. Three river sections that represented boundaries of vegetation response to salinity were selected for detailed analysis (Figure 7.2). Section one is representative of the upper catchment area, characterised by valley fill, low sinuosity fine grained and fine grained multi channel river styles. Limited survival of Paperbark with no regeneration was observed across the waterlogged floodplain, with only Samphire growing. Section two is the transitional zone between the flat and saline shallow watertables of the upper catchment and less saline downstream areas, where valley width narrowed and the landscape changed to the more dissected terrain. There was a marked transition in vegetation with changes in river styles in this section, from fine grained multi channel styles to channelised fill and planform controlled low sinuosity fine grained reaches. Section three is located in the uncleared portion of the catchment, with salinity originating from upstream (see Figure 7.2 B). River styles were dominated by planform controlled low sinuosity fine grained and low sinuosity planform controlled discontinuous floodplain reaches, set in narrow and steep valleys.
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Figure 7.2 Downstream trend in maximum 60 day salinity, based on gauging data, with zones of response identified. Section 1 represents the severely degraded site. Section 2 is at the boundary between sites where groundwater has reached the surface resulting in high salinity and the downstream sites where water quality freshens. (Section 3) shows where degradation is purely of the wetted perimeter in relation to high salinity off-site. Downstream of the Lunch Pool location (Kent Site 21), degradation of vegetation surrounding the wetted perimeter ceases and there is no evidence of vegetation decline in the lower catchment.
Historic (1946) and recent (1999) aerial photography was used in combination with field observations and Digital Multi Spectral Video (DMSV) imagery to determine changes in vegetation patterns since land clearing and compared to salinity data from Figure 7.2. In May 1999, DMSV imagery was collected over the upper Kent River by SpecTerra Systems Pty Ltd
~ 190 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes (reproduced here with permission from the Department of Environment), and processed by SpecTerra to produce a Plant Cell Density (PCD) index, using wavelengths that correspond to actively growing plant cells as a measurement of leaf chlorophyll production. The PCD index showed where vegetation was dead, surviving or actively growing (transparent area indicated no growth, blue indicated very little growth (i.e. tolerance) and pink indicated vigorous growth; see Figure 7.3, Figure 7.4 and Figure 7.5).
Figure 7.3 shows little growth at Section 1, with high salinity levels and waterlogged floodplain resulting in only Samphire being able to grow (yellow colour). Some Paperbark shrubs were able to tolerate these saline conditions, and were found on locally elevated areas such as lateral bars and mid-channel islands, indicating the importance of localised topography for vegetation growth in saline environments (see image for this reach in Figure 7.6A). A saline groundwater table and waterlogged valley floor has transformed this site from an open Paperbark woodland prior to land clearing, to a floodplain valley where only halophytic groundcovers regenerate and few Paperbarks survive.
Section 2 covers an area significant for the transition in salinity, valley width, river style and vegetation growth patterns. Kent Site 8 is similar to Kent Site 3, with the river valley dominated by Samphire and limited Paperbark. From Kent Site 9, there is evidence of Paperbark regeneration and growth of salt-tolerant weeds such as Rye Grass. Data on salinity tolerance of Paperbark seedlings (Bird, 1978; van der Moezel and Bell, 1987; Marcar et al., 2002) suggest a limit of around 25,000 mg l-1; a figure that matched 60-day salinity data for this reach (see Figure 7.2). Downstream at Kent Site 10, water quality improved and vegetation changed rapidly to a Yate flat, with more active growth evident from the DMSV imagery (Figure 7.6D). Paperbarks regenerate more readily and began to form thickets at the top of the channel banks. Further downstream at Kent Site 11, vegetation was less affected by salinity, waterlogging, and grazing (unlike Kent Site 10), and consequently vegetation is more dense and vigorous (see Figure 7.6 E). Paperbarks form non-continuous but relatively dense thickets. Other species such as various sedge and rush species (including Ghania, Ledidospermin) and Flat-topped Yate were close to the channel. Marri grow on the locally elevated areas of the floodplain away from the channel. DMSV imagery shows active growth across the floodplain in an area where maximum salinity is approximately 10,000mg l-1. The transition in river style to a channelised valley fill, results in better drained soils with the greater variation in micro-topography likely to assist plant growth.
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Figure 7.3 Section 1: Watterson’s Farm Gauging Station (Kent Site 3). Severe vegetation degradation has occurred across the floodplain, with only limited growth of Samphire on locally elevated terrain. Prior to land clearing the area was an open Paperbark swamp. (DMSV imagery reproduced with permission of Department of Environment, Albany).
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Figure 7.4 Section 2: Poorrarecup Road to Perillup Gauging Station (Kent Sites 8 – 11). At the upstream end of this section, the Poorrarecup East and West (Kent Sites 8 and 9) have limited vegetation growth; though Paperbarks are able to higher salinity in locally elevated areas (particular around Kent Site 9). There was a rapid transition in vegetation condition downstream towards Kent Sites 10 and 11, where Yates and Paperbarks grow in association with mixed sedge and rush species. These trends are highlighted by the images from these reaches in Figure 7.6 B-E. (DMSV imagery reproduced with permission of Department of Environment, Albany).
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Figure 7.5 Section 1: South Perillup Road (Kent Site 14). At Kent Site 14 (South Perillup Road), vegetation degradation was restricted to the wetted channel perimeter, but was severe. This was evident from changes between 1946 and 1999, and highlighted in the DMSV imagery which showed healthy growth across the floodplain, but a bare area where the channel was located. This pattern of off- side salinity degradation was further highlighted by the image for this reach in Figure 7.6 F. (DMSV imagery reproduced with permission of Department of Environment, Albany).
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Kent Site 3
SITE 1 A
Kent Site 8
B
Kent Site 9
C
Kent Site10
D
Kent Site11
E DOWNSTREAM SITE 2
Kent Site 14 SITE 3 F
Figure 7.6 Trends in downstream vegetation condition under shallow saline groundwater at Kent Sites 3, 8, 9, 10, 11, and 14.
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Figure 7.7 Conceptual model of vegetation recovery potential in a saline landscape. The potential for key species to grow across a range of stream salinity values is identified, along with zones where revegetation is possible (with species appropriate to the salinity gradient), and where catchment-scale processes control the potential for vegetation-based river management.
Section 3 represents the vegetation response found throughout the dissected and confined valleys of the mid-catchment. At South Perillup Road (Kent Site 14), only vegetation ~ 196 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes surrounding the wetted channel perimeter had degraded (Figure 7.6 F). The death of in-channel vegetation is evident from aerial photography, DMSV imagery and photography of the field site. Vegetation across the remainder of the floodplain is unaffected by salinity. This pattern of disturbance continued for approximately 25 km downstream, and can be seen in images from Kent Site 14 through to Kent Site 21 presented in Chapter 6. Reaches where vegetation was unaffected by rising salinity, corresponding to a maximum 60-day salinity threshold of around 7,500 mg l-1. Figure 6.17 shows a good example of this response, with local land management, rather than salinity, controlling vegetation. Where cattle were excluded from grazing a stream bank, riparian vegetation has regenerated under an increasing salinity gradient.
Figure 7.7 summarises, as a conceptual model, the salinity tolerance and growth potential observed for the key species, based on data presented above. The changing zones of vegetation degradation due to salinity across the floodplain and downstream highlight the areas where vegetation-based river management has remedial potential in these saline landscapes. Figure 7.7 demonstrates that vegetation-based management strategies in high salinity reaches and low- lying landscape positions are limited by the current salinity gradient (controlled by catchment- scale vegetation patterns (Hatton et al., 2003). In less saline reaches, local land management practices (such as grazing) determine vegetation growth potential. It is in these locations where river management can be applied. Variability in specific salinity tolerance thresholds was observed, due to influences in waterlogging and soil texture, though these changes are strongly associated with particular geomorphic features (lateral bars and benches, and mid-channel islands) and river styles. This conceptual model (Figure 7.7) provides a basis from which to assess its applicability for describing the recovery of vegetation in the Dalyup River, for the five year period since the March 2000 floods.
Salinity and Vegetation Patterns in the Dalyup River
Water quality data presented in Chapter 5 identifies the magnitude and spatial patterns of stream salinity in the Dalyup River. Readings from gauging stations on the Lort River and Young River taken at the same time as the snapshot sampling correspond to peak 60-day salinity values. It is reasonable to assume that the snapshot sampling at this time provides a good estimation of the salinity values that limit vegetation growth on the Dalyup River. 259 sites along the lower Dalyup and West Dalyup Rivers were photographed after the 2000 flood (May 2000), with 99 relocated and re-photographed in June 2005. Repeat photography has been used extensively for analysis of landscape change, particularly for analysing change in vegetation communities (e.g. Hart and Laycock, 1996; Lewis, 2001; Kull, 2005). Figure 7.8 shows photo pairs that demonstrate the type of responses in vegetation observed across the entire survey area (refer to Appendix 7B for all images). Species were identified from the photos and field investigation,
~ 197 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes and correlated with snapshot salinity data. This was then tested against the conceptual model developed from data in the Kent River, as presented in Figure 7.7.
The dominant (or co-dominant) species re-growth five years after the March 2000 flood were tabulated for different geomorphic surfaces (Table 7.1). Species dominance was considered for around the wetted channel margins (areas inundated several times per year), and across the floodplain (areas inundated only during the largest flood events). At every photo site the dominant (or co-dominant) species growing or recolonising areas was noted, along with geomorphic surfaces. Surfaces were classified as “no disturbance” (no erosion or sand deposition), “minor erosion”, “eroded to clay basement”, “minor sand deposition”, and “thick sand sheet” (minor and major deposition were differentiated by whether pre-flood groundcover vegetation emerged above the deposited sand sheet).
Table 7.1 Number of repeat photo sites where a particular species dominated (or was co-dominant) revegetation for a range of geomorphic surfaces, located next to the wetted channel and on the floodplain in the lower and mid Dalyup River. Other African Sedge/ Eucalypt/ Pasture/ Position/Surface Samphire Paperbark Acacia Love Rush Casuarina Kikuyu Grass Shrub Channel Margin No disturbance 2 2 2 1 - 2 1 Minor erosion 4 6 5 1 - - 1 Eroded to clay 3 3 3 - - - 1 basement Minor sand 4 9 11 3 - - 3 deposition Thick sand sheet - 2 5 1 - - 2 Floodplain No disturbance - 1 2 3 1 5 5 Minor erosion 1 2 2 2 - 1 4 Eroded to clay - 4 5 2 1 2 2 basement Minor sand 1 1 4 4 6 1 13 deposition Thick sand sheet - - - 4 - - 16
Table 7.1 shows the dominance of particular species in colonising different geomorphic surfaces. Across the surveyed sites, the salinity gradient varied from just below 30,000 mg l-1 near Brownings Road on the West Dalyup River, to 20,000 mg l-1 several kilometres above Lake Gore. The conceptual model presented in Figure 7.7 suggests that regeneration in upstream areas would be limited to Paperbark and Samphire, with water tolerant eucalypts and native sedges able to grown within several kilometres of the upper extent of the repeat photo survey. Sites 245 and 250 (see Figure 7.8) are at the upstream limit of sites surveyed, and had the highest salinity, 26,000 - 29,000 mg l-1. At both sites vegetation grows around the wetted channel perimeter, and in the case of Site 245, there was good colonisation of mid-channel ~ 198 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes islands which now have dense sedge and rush covering. At Site 250, sand has filled the pool due to an upstream avulsion (see Figure 5.20 for details of avulsion). No vegetation has been able to colonise the highly mobile and waterlogged sediment. The difference in vegetation response between these two sites further highlights the importance of local topography, in combination with salinity gradient, in controlling vegetation recovery potential.
Overall, the conceptual model matches observed in-channel regeneration across the 30km covered by the repeat photography survey. Some variability was noted in relation to different river styles and the influence of local topography in reaches where the channel was incised and floodplain more elevated with well drained soils. A strong trend of single species dominating regeneration of surfaces was also noted. Directly adjacent to the channel, Samphire, Paperbark and various sedge and rush species were equally dominant on undisturbed sites and those with minor and major erosion. In contrast, Paperbark dominated regeneration where sediment was deposited.
Away from the channel, regeneration was dominated by African Love Grass (Eragrostis curvula). African Love Grass is an exotic species that aggressively colonises deposited sand sheets, evident at Sites 9, 20 and 45 (see Figure 7.8). Golden Wattle (Acacia saligna) and very occasionally Paperbark was found in association with African Love Grass, although shrubs were typically well spaced. Paperbark, sedge and rush species are able to grow on surfaces eroded to clay, where the African Love Grass struggled to grow. Similar to the Kent River, revegetation potential was limited by localised controls on plant growth, such as; annual rainfall, species competition, substrate and local land management (i.e. whether the riparian zone is cleared). Grazing is also an important factor, and is known in other settings to alter the structure of the riparian vegetation communities in favour of open (or absent) understorey, reduced overstory, and over-dominance of unpalatable vegetation (Trimble and Alexander, 1995; Hughes, 1997; Jansen and Robertson, 2001). On the valley sides and areas bordering the floodplain, stands of remnant vegetation and productive tree and cereal crops were found, indicating that when sufficiently elevated above the channel, groundwater salinity has little influence on their growth potential.
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3 9 14 Site
~ 200 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes June 2005 2005 June May 2000 May 2000 245 250 255 Site June 2005 2005 June May 2000 May 2000
45 51 199 Site Figure 7.8 Vegetation recovery along the lower Dalyup River and West Dalyup Rivers from 2000 to 2005.
~ 201 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes The colonisation of deposited sediment by weeds at the expense of shrubs with a deeper root system has potentially serious implications for channel evolution at these reaches. African Love Grass has shallow and fibrous roots concentrated in the top decimetre (Animal and Plant Control Commission of SA, 2001; Natural Resources Mines and Water Qld, 2006). In contrast, Paperbarks have a particularly dense root system and in places forms dense root mats that fortifies the surface from erosion and entrainment and adds tensile strength to the soil (see Figure 6.19C). Paperbarks and other native tree species are significant in preventing bank failure and erosion (Abernethy and Rutherfurd, 2000; Abernethy and Rutherfurd, 2001; Tickner et al., 2001).
The patterns of vegetation regeneration on floodplains following the March 2000 flood on the Dalyup River further demonstrate how local land management practices can control vegetation patterns. A previous example from the Kent River showed the role of grazing and the potential to improve vegetation conditions by allowing regeneration through stock exclusion. In this catchment, managing weeds is a key management factor that controls the types of vegetation regeneration along the river banks, with the potential to affect the long-term stability of reaches. Field observations and aerial photography found that almost all channel avulsion on the Dalyup River during the March 2000 floods occurred on sites dominated by pasture or weeds. Weed management strategies may be important, particularly for river styles storing large sediment volumes at the channel margins and on the floodplain, where weeds out compete species that are likely to better stabilise these reaches. This is further investigated in the following section of this chapter.
Upstream of the repeat photography sites, where salinity increases have been more severe, aerial photography from of the Dalyup River from before clearing to present (1969 - 2004) and field observations sites, show that the conceptual model of vegetation growth potential can explain most of the observed patterns of contemporary vegetation growth. Approximately 3 km upstream of the final repeat photography site, only Paperbark and Samphire are able to regenerate in the wetted channel zone. Further upstream again, the growth and regeneration of Paperbark ceases where snapshot salinity measurements of 40,000 to 60,000 mg l-1. In the far upper catchment, the growth of Samphire stopped where salinity approached 100,000 mg l-1.
These data identify limited potential for river rehabilitation on the wide, saline valleys that dominate the upper catchment in both the Kent River and Dalyup River. Waterlogging and high salinity across the entire valley-floor, clayey material underlying the channel, and subdued topography across river cross sections, limits revegetation options for these reaches. While the data presented above provide a model from which to assess locations where species can grow, the effectiveness of different vegetation types in stabilising reaches remains unknown. The
~ 202 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes following section applies this conceptual model of vegetation growth potential for different river styles, to consider a series of river management scenarios. A channel hydraulics model is used to evaluate whether different vegetation-based river management can stabilise reaches in degrading, saline landscapes.
7.4. Modelling Geomorphic Effectiveness of Revegetation Options A series of vegetation-based river management scenarios were applied to selected river styles. These considered the potential vegetation growth under the current salinity regime for that river style, based on data presented earlier in this chapter, summarised as the conceptual model in Figure 7.7. The first scenario considered the effects of replacing pasture on the floodplain and valley margins with a tree plantation for the low sinuosity bedrock (Dalyup Site 2), low sinuosity planform controlled discontinuous floodplain (Dalyup Site 10), and channelised fill (Dalyup Site 7) styles (see Figure 7.9). In the second scenario, the potential for native vegetation rehabilitation through stock exclusion and replanting across the channel and floodplain was investigated for a multi channel sand bed (Dalyup Site 13) and bedrock controlled discontinuous floodplain (Dalyup Site 8) river styles (see Figure 7.10). The final scenario evaluated the potential that channelising the floodplain to lower groundwater and allow regeneration of Samphire in bare areas has as a river management tool for valley fill river styles in the upper catchment. This vegetation growth potential in channelised reaches is based on the discussion in Chapter 5.2 on the effects of ad-hoc channelisation. This was modelled at Dalyup
Sites 15 and 21 (see Figure 7.11).
Table 7.2 Vegetation roughness (Manning’s “n”) values for different vegetation types, based on data from Chow (1959), Arcement and Schneider (2003), Ladson et al. (2003), and Lang et al. (2004). Vegetation Type Manning’s n Bare channel with no vegetation 0.03 Bare channel with Samphire 0.035 Bare channel with dead tree stumps 0.0375 Complete Samphire coverage 0.0375 Paperbark 0.05 Wetted Channel Rushes with Paperbark 0.055 Bare Clay 0.025 Pasture 0.03 Samphire 0.035 Love Grass 0.04 Love Grass with occasional shrubs 0.05
Floodplain Paperbark with mixed understorey 0.07 Dense Paperbark thicket, tree plantation 0.08 Dense, closed eucalypt forest 0.1 Flood stage reaching branches of shrubs and trees increases by Manning’s n by 0.01 – 0.02 depending on the density of branches and leaves.
~ 203 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes The influence of different vegetation types was modelled based on roughness values (see Table 7.2), derived from the tables of published Manning “n” values (Chow, 1959; Arcement and Schneider, 2003; Ladson et al., 2003; Lang et al., 2004). Changes in vegetation roughness are noted on Figure 7.9, Figure 7.10 and Figure 7.11 in bold. River cross section data was collected during field trips (presented in Chapter 6) using standard surveying equipment, and input into a channel hydraulics model, WinXSPRO (Version 3.0), which has been applied in other research to estimate flow velocity under different vegetation conditions (see Birkeland, 2002; Faustini and Jones, 2003; Stromberg et al., 2005; Sable and Wohl, 2006). Stream power was calculated based on the Equations 5.1 and 5.2. Sediment transport and erosion thresholds were estimated from Hjulström curves (Hjulström, 1935), based on mean sediment diameter data presented in Appendix 3O, and as has been applied elsewhere (e.g. Hooke, 2003). Results from modelling report velocity values at the floodplain surface rather than mean velocity for the entire cross section.
Three vegetation-based management questions were tested, based on the scenarios described above: x Can replacing pasture with tree plantations prevent channel avulsions and the loss of agriculturally productive land? x Are Paperbark, native sedges and rushes better able to stabilise reaches in comparison to weeds and pasture? x Will improvements in vegetation condition associated with channelising the valley floor (i.e. constructing open deep drains) reduce erosive potential, and what is the effect of channelisation on flow velocity and erosion potential?
Figure 7.9 considers the potential of replacing pasture with tree plantations on the floodplain at Dalyup Sites 2, 10 and 7. Scenarios A and B consider two sites where channel avulsion events occurred during the March 2000 flood. Scenario C focuses on a site where sediment was deposited in this flood. Given the volume of sediment stored laterally to the channel, and the association of most channel avulsion events during the March 2000 flood with this river style, this is now a potential avulsion site and a significant risk to downstream reaches. This scenario considers management whereby a landowner would be encouraged to replace pasture with a commercial tree crop on the floodplain, maintaining a viable agricultural crop, rather than enlarging the riparian corridor with uneconomic species. Areas throughout the Kent River and Dalyup River have commercially viable tree crops growing on floodplain areas (elevated from the saline channel), including Eucalyptus globulus, various Pinus sp. (typically radiata and pinaster) and there is potential for growth of many other species (see Stolte et al., 1997; Dunin, 2002; Barrett-Lennard et al., 2003; Tonts and Selwood, 2003; Barrett-Lennard et al., 2005). Replacing pasture with trees significantly increases vegetation resistance and will affect flow
~ 204 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes velocity, with potential to reduce velocity below critical thresholds and so avert avulsion and loss of productive agricultural land.
Figure 7.9 A and B estimate the pre-flood cross section and vegetation from aerial photography and field surveys. The position of avulsion and current channel dimensions is identified by the grey polygon. Modelling found that replacing pasture with a tree plantation reduced channel velocity and stream power significantly at both sites. At Dalyup Site 10 (Figure 7.9 B), predicted velocity of 0.75 – 1 m sec-1 caused an avulsion event (less than the erosion thresholds suggested from the Hjulström curve). When the replacement of pasture by a tree plantation was modelled, floodplain velocity was reduced to 0.5 m sec-1 At Dalyup Site 2, flood velocity was 1.65 – 1.9 m sec-1 with pasture, close to the Hjulström erosive thresholds. Under a plantation, floodplain velocity was reduced to 1 m sec-1.
Predicting the occurrence of avulsive events based on critical erosion thresholds from the
Hjulström curve, using mean (D50) channel bedload, appears a poor predictor in this setting, as avulsions occurred when velocity was half the erosion threshold suggested by the Hjulström curve. The avulsion at Dalyup Site 10 occurred for a modelled velocity value of 0.75 – 1 m sec- 1. Based on the modelling results and avulsion events at velocity values well below those suggested for erosion on the Hjulström curve, a value of 1 m sec-1 was adopted as a value that indicated potential for avulsion. Using this value, a plantation on the floodplain at Dalyup Site 10 would most likely have prevented the channel avulsion, and at Dalyup Site 2 floodplain velocity would be reduced to 1 m sec-1, and therefore would approximate the threshold value.
Figure 7.9 C highlights the effect of replacing the existing pasture with a 75m wide tree plantation on the floodplain, and fencing the river to exclude grazing, leading to improved vegetation condition away from the wetted channel. Modelling suggested that a flood of similar magnitude to the March 2000 event would produce a floodplain velocity of 2 – 2.5 m sec-1, while with a tree plantation floodplain velocity would be reduced to 1.1 – 1.25 m sec-1. This suggests that the tree plantation is likely to reduce flow velocity close to the critical avulsion thresholds; though it is uncertain as to whether it would be able to prevent an avulsion. Under the “do-nothing” scenario, where pasture was left on these areas, most of the productive agricultural land is likely to be lost. Tree plantation offers one solution for these river styles, for stabilising reaches and maintaining productive economic returns from the floodplain. This scenario also highlights the importance of maintaining wide riparian corridors and that viable farming systems can co-exist with river management strategies that offer potential for stabilising potentially avulsive reaches.
~ 205 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes The second scenario considered the use of native species in a wide, vegetated riparian buffer zone of native, not-for-profit species. At Dalyup Site 13 (Figure 7.10 A), there is potential to revegetate sandy mid-channel islands, as they are locally elevated above saline baseflow. Paperbark regeneration is evident in the field and photographic surveys of this reach in Chapter 6. The potential to revegetate mid-channel islands with Paperbark was investigated in this scenario. The improvement in vegetation condition through revegetation had little effect on modelled stream velocity, which remained between 0.45 – 0.7 m sec-1, with bedload transportation thresholds exceeded for all flow conditions. In this river style and under this stream salinity gradient, there is little potential for rehabilitation to stabilise the channel and sediment stores. While this reach is a major downstream contributor of sediment, little can be done to fortify the sediment source under the current salinity regime, and therefore, it would be assessed as a low priority rehabilitation reach. The potential for river management in these reaches and many other river styles with wide, flat floodplains, lies in catchment-scale efforts to return hydrology and stream salinity to pre-clearing levels.
At Dalyup Site 8 (Figure 7.10 B), there is greater potential to carry out management in this river style, as sediment bodies are elevated higher above the wetted channel. At this reach, the management scenario investigates the improvement of vegetation condition across the existing 80m wide river corridor through stock exclusion (the reach is already fenced, though grazed) to encourage Paperbark regeneration. This management action reduced velocity from 0.9 – 1.0 m sec-1 to 0.45 – 0.5 m sec-1 at stages corresponding to where the major sediment stores were located. This suggests that maintaining a wide and well vegetated riparian corridor for river styles such as the bedrock controlled discontinuous floodplain reach investigated in Figure 7.10 B and similar reaches, can be successful in stabilising lateral sediment storages. Investment in river management in these reaches is likely to be relatively successful and more likely to reduce downstream sediment loads than targeting sand-bed reaches set in wide, flat and saline valleys such as in Figure 7.10 A.
Figure 7.11 A and B highlight the effectiveness of channelisation as a management option for wide floodplain valley reaches such as the valley fill, fine grained multi channel and fine grained low sinuosity river styles found in the mid to upper catchment on both rivers. In these reaches there are no vegetation-based river management options due to high salinity levels. Chapter 5 outlined the effects of channelisation in locally lowering saline watertables and allowing vegetation regeneration. By pursuing a remediation strategy through re-engineering the channel from a valley fill, to a channelised fill, low sinuosity fine grained or fine grained multi channel, there is potential for vegetation-based strategies to be effective.
~ 206 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes Channelisation was used extensively for moderating flood impacts during the mid twentieth century, and many rivers reported reduced channel stability, widespread erosion, bank collapse and increased sediment yields (Harvey and Watson, 1983; Hupp, 1992; Thorne et al., 1997; Downs and Gregory, 2004). The Avon River in Western Australia, which was “trained” from 1957 until early 1973 (J Davies & Assoc and Ecoscape Pty. Ltd., 1996), has resulted in sedimentation of pools (Pen, 1999; Water and Rivers Commission, 1999). Given these previous negative experiences with channelisation and drainage, there is considerable public and scientific debate as to whether deep drains are an appropriate management tool, or are creating a negative environmental legacy (Speed and Simons, 1992; Keen, 1998; Coles et al., 1999; Deep Drainage Taskforce, 2000; Fitzpatrick et al., 2000; Dogramaci and Degens, 2003; Ali et al., 2004). As yet, there is no data available on the effect of using open deep drains as a management tool on wide valley floodplains in saline landscapes. This scenario investigates this potential purely from a channel hydraulics perspective. Changes in stream velocity were modelled for two such reaches in the upper Dalyup River (Dalyup Sites 15 and 21), comparing a channelised condition with improved vegetation cover (Samphire) on the valley floor, against bare areas with an unchannelised main channel.
Modelling predicted that channelisation caused a minor increase in velocity for the actual channel (Figure 7.11 A & B), and small reduction of velocity away from the channel under the better vegetation conditions. The most significant change was at the margins of the deep drain at Dalyup Site 21 (Figure 7.11 B). Under the unchannelised scenario, the area directly adjacent to the channel was bare, but under the channelisation scenario when vegetated by Samphire, velocity reduced from 0.25 m sec -1 to 0.15 m sec-1, below the critical transport threshold. At Dalyup Site 15 (Figure 7.11 A), the changes were relatively insignificant, with velocity approximating the transportation threshold for bedload (0.25 m sec-1) once the bankfull stage was exceeded. Channelisation caused a small decrease in velocity (to 0.175 m sec-1) once the bankfull stage for the deeper (channelised, box-shaped) channel was exceeded. This indicated the potential for deposition at the channel margins and further entrenchment of the channel through vertical accretion.
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Figure 7.9 Potential for stabilisation of floodplain sediment stores using tree plantations to prevent channel avulsion.
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Figure 7.10 Potential for rehabilitation of in-stream and lateral sediment stores using native vegetation, through fencing and stock exclusion.
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Figure 7.11 Use of channelisation as a river management technique to increase vegetation condition, but increasing velocity in the channel.
~ 210 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes Table 7.3 Table of the function and rehabilitation potential for different river styles found in the study catchments.
~ 211 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes Table 7.3 continued
~ 212 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes Reduced velocity at the channel margins caused by channelisation indicated a potential for these sites to trap sediment, and also the sensitivity of these sites to disturbance (given their proximity to transport thresholds). Overall, model results suggest that changes in velocity are relatively minor. Very low channel gradients are the overwhelming control on flow velocity and stream power, as has been found in other research (e.g. Reinfelds et al., 2004), and Chapter 5 of this thesis. Abernethy and Rutherfurd (1998) found that the upper catchment was the location where vegetation and the reintroduction of LWD was most effective in reducing stream power due to the stream power hump caused by steepest slopes and a confined channel. The “backwards” catchments in southwestern Australia, with low-gradient headwaters set in wide valleys, are an interesting contrast to other catchments with steep-slope upper catchment areas. Under the current regime, low channel gradient and high salinity limit potential for vegetation-based management in river styles such as the valley fill and low sinuosity fine grained reaches.
7.5. Discussion: River Management Potential in Saline Landscapes This chapter addresses the final research question posed in the introduction of this thesis, “what river management strategies might be effective in modulating adverse channel responses in light of the new set of boundary conditions, thresholds and resistance factors that now control channel evolution”? This chapter has identified where different plant species can be used for river management in a saline landscape, identifying the variable and in some locations, limited options across a range of river styles. The strong downstream salinity gradient has an impact on the potential effectiveness of vegetation-based river management strategies in a saline landscape. These are factors that were unknown prior to this research and the implications are significant findings for the management of saline rivers in both study catchments, across the south coastal rivers region, and in potentially saline dryland landscapes elsewhere. The potential for rehabilitation is summarised in Table 7.3, with reference to all river styles identified across the two catchments, and how the modelling results from these seven reference reaches can be applied across a range of reaches and salinity gradients. The method applied in this chapter has potential to be applied in other landscapes to similar research questions. An example would be landscapes where agriculture or industrial activities have altered water quality and resulted in downstream degradation of vegetation and concomitant changes in channel stability.
The first two scenarios considered rehabilitation, identifying where vegetation can be used within the new controls on river channel processes. Within the limitations dictated by the salinity gradient, vegetation-based strategies that are applied to existing river styles were assessed. Both scenarios highlighted the importance of wide riparian zones that cover an area inundated during large flood events. In the first, pasture was converted to a tree crop, significantly lowering floodplain velocity. This example offered land managers agriculturally productive options that are likely to improve river management and river health. Under the
~ 213 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes second scenario, river style had a significant influence on the potential for revegetation to stabilise reaches that store large volumes of coarse sediment. Where sediment is stored on mid- channel islands, across wide, flat floodplain valleys, there is limited potential for rehabilitation (e.g. see Figure 7.7). In more dissected terrain with a narrow valley and sediment bodies at the channel margins elevated above the saline watertable, revegetation options are improved. Higher stream gradient in these reaches also means that there is a greater potential for increased vegetation roughness to cause a greater reduction in velocity, and to stabilise reaches.
The third scenario considered a remediation-based management approach, re-engineering the channel in an attempt to create a morphology more suitable to the current conditions. Field observations and data in Chapter 5 (see Figure 5.11) identify that, while under a scenario of channelisation and improved vegetation, velocity across the floodplain reduced, changes were relatively minor. Under the current salinity regime, revegetation of mid-channel islands to trap coarse sediment has little potential for success as a rehabilitation method. Better vegetation condition in multi channel reaches, particularly those with deeper and more incised channels, suggests that re-engineering single channel reaches in broad and flat valleys, towards a more braided morphology, may have ecological benefits for river reaches with some potential to trap sediments on mid-channel islands in low-gradient reaches. There are, however, serious geomorphic questions and consequences to be considered before this approach could be applied as a river management tool.
The legacy of channelisation schemes in other rivers (e.g. Avon River) stresses the potential for river engineering schemes to have unplanned consequences. Further investigation is required to resolve issues such as the suitability of reaches (i.e. stream gradient and stream power thresholds) and a sustainable channel design, in light of the current and future hydrologic, sediment and salinity controls. Existing deep drains have high management costs and, given their limited effectiveness and high costs, much previous work on their use on hillslopes has concluded that overall they deliver little benefit (see Chapter 5.2). Data from Chapter 3 identified that these reaches are not large sources of sediment; and Chapter 5 that the clay valley fill underlying the channel is limiting the response of these reaches to the new regime. While there is potential for river engineering strategies to offer solutions for these reaches and further research into these limitations and cost-benefit analysis is required, to consider issues such as; x the large costs of remediation versus the potential benefit, x high potential maintenance costs, x the minor contribution of sediment from these reaches to downstream processes, x their success in reducing sediment loads, x strategies and success of ecological restoration in an engineered channel, and x optimal channel design suitable for the new hydrological and hydro-chemical regime.
~ 214 ~ Chapter 7: Recovery and Management of Rivers in Saline Landscapes Beyond this geographic region, these results have wider implications for understanding human interactions with landscapes and how rivers are managed. This chapter underscores the importance of understanding options for river management in a landscape context, where river rehabilitation is affected by both local and off-site processes. This is a concept applicable to saline or non-saline, cleared landscapes elsewhere, where human disturbance has altered landscape stability. This work also stresses the importance of managing vegetation recovery pathways, such as the invasion of weeds over less competitive but more effective vegetation such as shrubs and trees, and how this has the potential for divergent river management outcomes. The enduring lesson of this chapter is, however, that vegetation-based river management strategies offer potential solutions in saline landscapes. There is the potential through appropriate use of the techniques modelled in this chapter to stabilise many river reaches and to reduce downstream impacts. Given the strong similarities in response and behaviour of particular river styles, these tools provide a basis for developing regional-scale river management strategies in a saline landscape.
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~ 216 ~ Chapter 8: Conclusion
Chapter 8: Conclusion
In the upper Kent River the entire valley floor is waterlogged with water more saline than seawater for many months of the year (such as this site near Lake Carrabundup). This research identifies that the rate of change in reaches such as this one and many others in the upper catchment are slow. This is despite large changes to hydrology, sediment loads and vegetation degradation that have been caused by land clearing and salinisation.
8.1. River Response to Land Clearing and Salinisation Dryland rivers are noted for their sensitivity to change (Tooth, 2000). In these settings, hydrologic, climatic and ecologic perturbation have resulted in significant adjustment of river channels (Wolman, 1967; Knox, 1977; Klimek, 1987; Starkel, 1987; Starkel, 1988; Mei-e and Xianmo, 1994; Brooks and Brierley, 1997). In southwestern Australia, recent extensive land clearing has fundamentally changed the landscape. Streamflow has become more perennial, discharge volumes have at a minimum doubled, and bankfull discharge happens more frequently under the new hydrologic regime. Sediment loads have increased by 50 – 500 times the pre-clearing rate, although they remain low on a global-scale. Humans have also changed the landscape through diverting and storing water, which has decoupled large areas of hillslopes from the main channel. Riparian vegetation has been severely degraded by increased streamflow salinity that is now up to three times more saline than seawater in some reaches. Consequently,
~ 217 ~ Chapter 8: Conclusion vegetation roughness has decreased significantly and the erosive thresholds of coarse sediment have been lowered. Despite these extreme changes, many river reaches have not responded catastrophically to the new hydrologic, sediment and hydro-chemical regime. This thesis identifies the strong control that landscape gradient and material thresholds have on changes in landscape stability. Downstream variation in these factors results in spatially variable changes in landscape stability with the potential for catastrophic response. The areas of greatest change in hydrology, and greatest vegetation degradation are not necessarily associated with the areas of greatest change in river morphology.
Prior to this research, the responses of rivers to rapid and extensive land clearing and severe degradation of vegetation caused by secondary salinity was largely unknown. This work has successfully applied a landscape sensitivity approach to investigate changes in landscape stability, focusing on quantifying changes in boundary conditions, connectivity and thresholds. In determining the river’s morphologic response to changed landscape stability, a reach-based classification method (based on the RiverStyles® Framework (Brierley and Fryirs, 2005)) was successful in identifying consistent patterns. Within particular reach styles, the potential for, and limitations of, vegetation-based management schemes to offer solutions for stabilising reaches are now better understood. Given the success of the reach-style approach across two catchments, this research provides the foundations of a regional tool for river management.
The findings of this research have been summarised above and discussed in previous chapters. The purpose of this final chapter is to discuss critically the research methods used, the wider implication of the research, and to suggest how this work can be further extended and applied.
8.2. Thesis Methods, Research Implications and Further Research This research has applied a top-down or data-based analysis of hydrologic, sediment transport and salinity data to determine specific changes in landscape processes resulting from land clearing and salinisation. Spatially comprehensive, but temporally-limited discrete data has been collected and used to complement, or to supplement longer-term continuous data. This approach has centred on determining landscape response from the data, rather than making a-priori assumptions as to how the systems functions and applying models of hydrology, salinity or erosion that may not be applicable to this setting. This approach (rather than a reductionist or mechanistic, upward numerical modelling approach (see review by Sivapalan et al., 2003) was applied due to the dearth of previous research on the fluvial geomorphic characteristics of these rivers. The findings of this research provide a foundation from which a more quantitative extension of this work can be undertaken. This could be through a more informed, upward strategy based on the knowledge of system behaviour gained from this research to select
~ 218 ~ Chapter 8: Conclusion suitable existing models, or in a downward approach to model development following on from this research.
Data availability, quality, resolution and their time span are key factors in quantifying the response of landscape processes to land clearing and salinity. While rainfall data has been extensively recorded over a long period of time, gauging data of discharge and salinity is more limited, and sediment transport data are scarce. Within the Kent River, one gauging station (that covers 1,830 km-2 of cleared and uncleared catchment) has operated from around the time of land clearing, however, no data is available before land clearing. This catchment currently has four automated gauging stations logging discharge and salinity, though two of the stations have been operational for less than five years. While present day patterns of runoff and salt movement are now relatively well understood, a two year backlog in processing data from these gauging stations prevented presentation of data from the flood in April 2005 (aside from pre- and post-flood field observations). Despite these limitations, the approaches used to analyse hydrologic response through flow diversion on the basis of land clearing, enabled the development of a reliable response model using varying degrees of land clearing over the data record.
The lack of gauging data from the Dalyup River necessitated a regional approach using data from the Lort River and Young River, located 50 – 80 km to the west. While these catchments include the Mulglinup and Meleleuka cleared and control catchments, giving a valuable insight into changes in hydrology and salinity following land clearing, it was not possible to calibrate this information with flow and salinity data from the Dalyup River. Results for changes in salinity from discrete sampling corresponded well to known trends from the gauged rivers, and are in line with expected results. Analysis of changes in discharge patterns predicted extreme changes in flow perenniallity (flow was for 20% of the year before clearing, but is now 80%), peak discharge (increasing by two orders of magnitude from 10,000 m3 day-1 to 1,000,000 m3 day-1), and bankfull discharge frequency (increasing from around once every 20 years to several times per year). The response of the Dalyup River is unlikely to be as extreme as predicted by up-scaling data from the Lort River and Young River would suggest. This analysis would benefit from calibration with at least limited gauging data from the Dalyup River. It is noted that there is currently a proposal to establish two gauging stations on the lower Dalyup River and West Dalyup River (S. Janicke, pers. comm.). There is potential to revisit this analysis within a few years to test the accuracy of this model of hydrologic response in the Dalyup River.
Data for changes in sediment transport from the two catchments were very limited. There had been no previous sampling of event bedload sediment transport, nor was this possible for this study due to the physical isolation of catchments and the unpredictable nature of flow events.
~ 219 ~ Chapter 8: Conclusion Analysis of bedload sediment characteristics from grab samples was effective in establishing the downstream trends in size characteristics. Analysis of discrete suspended sediment data collected by the Department of Environment made it possible to determine mean suspended sediment concentrations for each catchment. Sediment transport modelling results from the NLWRA were used to complement the scarce data. Previous investigations (see Radke et al., 2004), and more recent research by others (Wilson, 2005) have concluded that the magnitude of sediment transport is correctly predicted. This research has shown that the model over predicts the influence of diffuse hillslope erosion (sheet flow) and gully erosion. The major sediment source is highly-coupled sediment stored on mid-channel islands and lateral bars that becomes mobile under the vegetation degradation. Removal of highly-coupled sediment stores following land clearing has been described elsewhere (see Wasson et al.1998). Field data suggests that some reaches have been depleted of sediment, and post-clearing sediment transport rates may have already peaked 50 years after land clearing. Overall, the understanding of sediment movement in this landscape remains poor, and a program of event sampling would greatly improve the understanding of transport rates and spatial variability.
Attempts to analyse the changes in runoff mechanisms were relatively unsuccessful. Other studies in cleared and control catchments with more detailed metering, have found significant changes in runoff mechanisms with rising groundwater and expanding saturated source areas (Stokes and Loh, 1982; Williamson et al., 1987; Ruprecht and Schofield, 1989; Ruprecht and Schofield, 1991; Ruprecht and Stoneman, 1993; Stoneman, 1993). Daily discharge data and a baseflow filter was unable to demonstrate any difference in the proportions of overland flow and baseflow in the Kent River since 1979. Finer-scale hydrological metering of sub-catchment would greatly assist in understanding the changes in water movement through these landscapes. This would also provide an opportunity to quantify the effects of farm dams on runoff from small sub-catchments. The investigation methods applied in Chapter 4 focused on a terrain analysis approach, finding that large area of hillslopes are hydrologically decoupled from the main river channel. Attempts to quantify this effect on streamflow using gauging data were not possible as the data was too generalised, for example one gauging station covers 1125 km2 of farmland, tree plantation and remnant forest.
Chapter 4 raised fundamental questions as to how hydrology is represented and modelled in landscapes disturbed by human activity. Although most research and modelling is concerned with hydrologic prediction in landscapes disturbed by humans, to protect humans and associated infrastructure, the hydrologic connectivity and pathways of landscapes have been altered by constructing roads, diverting water and building farm dams. While some research considers these, standard procedures for using DEMs as input for hydrologic models forces all flow out of catchments and ignores human infrastructure. Given that the fundamental input of any
~ 220 ~ Chapter 8: Conclusion hydrologic model is the catchment area, the implication of this research is that efforts to improve prediction through any other means may offer limited potential. This research also identified that methods used to replicate hydrology in disturbed landscapes may also fundamentally compromise the ability for DEMs to accurately represent landscape processes. The application of DEMs for landscape analyses, and the effect of unknown, known and introduced error remains poorly understood and further study is required to better understand these effects and how they propagate through models so as to design better tools and methods for representing landscapes using DEMs.
The RiverStyles® Framework (Brierley and Fryirs, 2005) was successfully applied in a different landscape setting to the coastal catchments of New South Wales for where it was developed. This approach relies on a supervised classification of quantitative data, and while the field-based approach was required for this research and is the method described by Brierley and Fryirs (2005), there is the potential to extend this analysis towards a more quantitative approach. Development of a model to predict river-styles, behaviours and allow modelling of channel hydraulics offers potential, both as a regional management tool, and for analysing the effectiveness of different vegetation-based strategies to stabilise channels. This study modelled the impact of vegetation roughness on reach stability using a channel hydraulics tool and erosive thresholds based on the Hjulström curve. This research provides a foundation for using more spatially and numerically complex hydraulics models to improve factors such as prediction of erosion and to account for the influence of roots when modelling the effectiveness of native shrubs and sedges in comparison to invasive grasses.
Chapter 7 identified the limited potential of river restoration in this environment and the potential for remediation of the most salt-affected reaches. While this study has focused on management timescales, longer term simulations of channel evolution in the most severely degraded, low-gradient reaches may offer interesting insights into sustainable channel designs that can be implemented for river remediation. The potential for these is identified by the ad-hoc channelisation that has occurred. However, the impact of these features on channel processes and downstream reaches remains poorly understood. The hydrologic changes associated with land clearing are analogous to a shift towards a significantly wetter climate. Many rivers overlay sediments that have filled valleys since the climate became more arid during the Oligocene. Understanding channel patterns and evolution processes over geologic timescales may offer interesting insights into how these rivers may function over management timescales under a higher runoff regime.
~ 221 ~ Chapter 8: Conclusion 8.3. Research Contribution Based on research on river response to land clearing and vegetation degradation in other settings, it may have been expected that rivers would have undergone rapid and catastrophic response to changes in hydrology, salinity and associated vegetation degradation. This research demonstrated the significant controls that landscape gradient and erosive thresholds place on river response in landscapes. The areas that have undergone the greatest vegetation degradation have shown surprisingly minor morphological response.
This research also makes a contribution towards understanding the role that humans play in disturbing landscapes. The critical analysis of the ways that humans alter hydrologic pathways and disconnect landscapes also identified the limitation of current methods for representing landscapes. The ability for DEMs to represent the hydrologic reality of landscapes due to error and the actions of humans in altering hydrologic pathways are often compromised by the tools used to correct these issues. This research identified the effect that hydrologic correction methods have on the nature of the DEM and how these differentially compromise subsequent terrain analyses.
Through the successful application of a reach-based approach across fifty two sites on two rivers in southwestern Australia, similar response behaviour of morphologically similar reaches was found. By combining the understanding of river response for particular river styles, with a conceptual model of where vegetation can grow across a range of salinity gradients, the potential for river management strategies was investigated, finding that vegetation-based strategies offer solutions for some river styles in a saline landscape. While there is limited potential for river rehabilitation of flat and wide reaches, which are some of the most severely salt-affected, this research identified the potential that river remediation strategies might offer for these reaches through a river engineering approach. The research identified the response of particular river styles and the potential for vegetation-based river management strategies to offer solutions in a saline landscape, potentially to be applied as a regional-tool for more strategic river management. The approach adopted in this research offers potential for investigating system response elsewhere, to upslope and catchment scale processes that affect vegetation and channel stability, such as changes in water quality caused by agricultural or industrial activities.
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