Jordan River: Environmental Flow Regime Assessment
Peter Davies, Lois Koehnken, Phil Barker, Laurie Cook
Freshwater Systems, Technical Advice on Water, North Barker and Associates.
July 2005
ISBN-13: 9780724669806 fs FFrreesshhwwaatteerr SSyysstteemmss Aquatiic Enviironmentall Consulltiing Serviice
0 Table of Contents
1. Introduction and aims ...... 2
2. Jordan River Hydrology...... 4 2.1 Hydrological characteristics and trends ...... 4 2.2 1974 – 2004 Flows...... 7
3. Jordan River: Water quality...... 10
4. Aquatic ecology ...... 12 4.1 Fish...... 12 4.2 Macroinvertebrates ...... 12 4.3 Aquatic mammals ...... 14 4.4 Aquatic plants ...... 14
5. Geomorphology ...... 15 5.1 Geomorphology – introduction...... 15 5.2 Geomorphic context of Jordan River...... 15 5.3 Catchment changes ...... 16 5.4 Geomorphic characterisation ...... 19
6. Riparian and Aquatic Vegetation...... 32 6.1 Vegetation – introduction ...... 32 6.2 General Characteristics of vegetation ...... 32 6.3 Descriptions of study sites ...... 33
7. EFlow Assessment Methods...... 44 7.1 Key environmental values...... 44 7.2 Instream habitat-flow assessment ...... 44 7.3 High flow/flood events ...... 47
8. Flow-ecology relationships...... 48 8.1 Instream habitat and flows...... 48 8.2 Relationship between flow regime and geomorphology...... 49 8.3 Relationship between flow regime and vegetation ...... 52 8.4 Flows and Herdsmans Cove...... 53
9. Environmental flow regime recommendations...... 58 9.1 Baseflows and Cease-to-flow events ...... 58 9.2 High/flood flows ...... 58 9.3 Conclusions...... 59
10. References...... 61 Jordan River Environmental Flow Regime Assessment
P Davies, L Koehnken, P Barker, L Cook Freshwater Systems, Technical Advice on Water, North Barker and associates.
1. Introduction and aims An assessment of an environmental flow regime requirement was requested for the Jordan River, south east Tasmania. This project was required to: • Summarise key information on the environmental status of the Jordan River; • Identify relationships between key instream habitat features/processes and discharge for representative reaches of the Jordan River; • Conduct a geomorphic characterisation of the Jordan River and recommend key flow events to maintain channel processes and habitat; • Assess riparian and floodplain vegetation communities and their flow/flood requirements; • Identify key flow events required to maintain environmental values in Herdsmans Cove; • Integrate the findings of the above studies into a single environmental flow regime which include base flows and a seasonal set of flood/high flows.
The primary aim of the project was to derive an environmental flow regime for the middle and lower Jordan River and Herdsmans Cove which is designed to maintain existing environmental values and processes.
The Jordan River system, with a catchment area of ca. 1,240 km 2, has a distinctive geomorphology, geology and catchment form in a Tasmanian context. The nature of the landscape and vegetation is strongly linked to the river flow regime. Large sections of the catchment historically constituted floodplain/swamp systems which were naturally highly connected during flood and high flow periods. The Jordan River is now a highly modified system, with a modified, willow-infested channel and a cleared and drained floodplain. The valley, and the river’s tributaries and mainstem are subject to a growing salinisation problem. Wastewater discharge was also a feature of the lower reaches until recently. Overall, the condition of the middle and lower Jordan is considered poor, and there are few remaining natural values. The estuary of the Jordan River, Herdsmans Cove, also forms a component of, and is largely controlled by, the much larger Derwent estuary.
This assessment recognises the changed state of the aquatic system of the Jordan, and the resulting modified environmental values. It also recognises that flow is only one of a number of significant drivers of environmental processes and outcomes in this system. Consideration is given to the degree to which environmental flow management can be effective for the maintenance of riverine ecosystem values in the absence of other substantial management initiatives. EFlow objectives for the Jordan River
The overall objective of an environmental flow regime for the Jordan is to maintain existing environmental values within both the Jordan River mainstem and Herdsmans Cove ecosystems.
A number of values have already been identified through the PEV and Water Quality value setting processes under the State Policy on Water Quality Management (1997) and the Water Act 1999, and in the Jordan River Catchment Management Plan, the Jordan River Rivercare Plan (Ecosynthesis 2001) and the Jordan River State of the Rivers reports (Wilson et al. 2003).
Those values directly and indirectly affected by river flows and identified consistently are as follows:
Water quality – maintain and restore, with emphasis on salinity, turbidity and bacterial quality, ecosystem health and primary contact.
River health – maintain and restore, with emphasis on fish, invertebrates and aquatic plants;
Riparian vegetation - manage/control weeds and restore native vegetation.
This report focuses on these values, including aquatic ecosystem values, geomorphic processes, and estuarine values. 2. Jordan River Hydrology
2.1 Hydrological characteristics and trends The flow regime of the Jordan River is characterised by very low to zero flows during the summer-autumn period, interspersed with occasional flood flows, with higher baseflows during the winter-spring period accompanied by a higher frequency and intensity of high/flood flow events. In comparison with other rivers, the Jordan experiences a combination of typical summer-winter seasonal dry-wet sequences, superimposed with unpredictable flooding (more typical of east coast rivers) and a semi-ephemeral dry season flow pattern. These primary features of the flow regime apply to both the historical flow regime (as recorded at the currently active gauging station at Mauriceton and the past station at Bridgewater) and the natural flow regime.
The hydrology of the Jordan has been described by Gurung and Dayaratne (2002). Modelling of the natural and historical flow regime for the Jordan was conducted by Hydro Tasmania (unpub. data), with the entire modelled flow record from 1917 to 2004 shown in Figure 1.
The Jordan catchment is one of the driest catchments in the state. The annual average rainfall across the catchment is approximately 500 – 600 mm, with little spatial variation in rainfall conditions over the catchment. Average monthly rainfall distribution is relatively uniform between 32 mm to 58 mm, and there is no significant difference in the winter and summer rainfall across the catchment (Gurung and Dayaratne 2002).
The river ecosystem fluctuates between very dry conditions in summer-autumn with frequent periods of channel dry-outs and relict pools with little connectivity, and high- volume floods superimposed on intermittent to continuous baseflows in winter. The catchment experiences some of the lowest rainfalls and highest evaporation rates in the state (Fallon et al. 2000). The river can be characterised as a having highly ‘mediterranean’ hydrological environment.
Equations developed by Colwell (1974) can be used to predict: 1) flow predictability: a measure of how tightly an event is linked to a season, 2) flow constancy: a measure of how uniform the event occurs through all seasons, and 3) flow contingency: a measure of the repeatability of season patterns. Colwell’s Indices based on monthly modelled natural mean daily flow for the Jordan at Mauriceton are all low: predictability = 0.205, constancy = 0.101, contingency = 0.105, indicating a very unpredictable and ‘event based’ flow regime, with weak and variable seasonality. The seasonal pattern in mean daily flows for the record at Mauriceton from 1965 – 2001 is shown in Figure 3.
The historical record of historical mean daily flows is shown in Figure 2 for the Mauriceton gauge (the only gauging site with a sufficiently long record and that is still operating). Since 1965, the annual flow volume at Mauriceton has fallen between 800 and 75,000 ML (Figure 4), with an annual average of 21,000 ML (falling to ca 12,000 ML since 1990), and a total catchment yield of around 35,000 ML. Mean daily, monthly flows and annual yields at Bridgwater are highly correlated with those at Mauriceton. 200
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0 1/01/66 1/01/68 1/01/70 1/01/72 1/01/74 1/01/76 1/01/78 1/01/80 1/01/82 1/01/84 1/01/86 1/01/88 1/01/90 1/01/92 1/01/94 1/01/96 1/01/98 1/01/00 1/01/02 1/01/04
Figure 1. Mean daily flows recorded for the Jordan at Mauriceton (record available for the period 1965 to 2004, DPIWE).
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Figure 2. Natural mean daily flows modelled for the Jordan at Mauriceton (for the period 1917 to 2004, Hydro Tasmania).
The catchment has experienced a decrease in overall yield since the mid 1970’s. Both natural and historical annual catchment yield from 1985/86 to 2004 were statistically significantly lower than for the 1917 to 1984 period (all p < 0.05 by student’s t and Rank sum tests, using TREND software, Chiew and Siriwardena 2005, www.toolkit.net.au/trend), indicating that the recent low flow period was primarily due to decreases in rainfall yield. Climate change predictions for eastern Tasmania are for ongoing reductions in yield (Nunez 2004), especially in the summer-autumn seasons.
The drier period from the mid 1970’s is therefore more likely to be representative of the future flow regime than an earlier historical or modelled flow record. This environmental flow assessment is therefore based only on considerations of the flow regime present in the Jordan from the mid 1970’s onward (1974 to 2004). 5 Jordan River at Mauriceton (#4201)
4 /s) 3 3
2 Stream Flow (m StreamFlow
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Figure 3. Monthly pattern in mean daily flows at Mauriceton (Gurung and Dayaratne 2002), showing the overall pattern and the high level of variability. Boxes indicate central range of data with horizontal line = median values. Crosses indicate extreme outliers.
80000 Jordan River at Mauriceton (#4201)
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0 1965 1969 1973 1977 1981 1985 1989 1993 1997 2001
Figure 4. Annual flow volumes at Mauriceton (Gurung and Dayaratne 2002) 2.2 1974 – 2004 Flows
Historical Flows The overall mean and median of mean daily flows recorded at Mauriceton gauge for the 30 year period 1974-2003 inclusive, were 0.705 and 0.081 cumec, while the 90 th percentile flow was 1.589 cumec. The median of mean daily flows by season was 0.343 cumec in spring (Sept – Nov), 0.067 in summer (Dec-Feb), 0.039 in autumn (Mar - May) and 0.474 in winter (Jun - Aug). The seasonal pattern of baseflows was extracted using the RAP baseflow routine, and was 0.186 cumec in spring, 0.038 in summer, 0.021 in autumn, and 0.146 in winter. Baseflows are therefore consistently low throughout the summer and autumn periods.
Extreme low to zero flows are a feature of the Jordan’s recent flow record. During this period there were 1086 zero or near zero flow days (within the error margins of the gauging station), comprising 14.4% of the time. The mean number of zero or near- zero flow days was 2.90 days in spring, 8.72 in summer, 15.79 in autumn, and 8.24 in winter.
There were 126 low flow events less than 1 cumec in magnitude observed between 1974 and 2003, with the seasonal average number of these events being 2.6 for spring, 1.5 for summer, 1.3 for autumn, and 2.3 in winter. These events lasted an average of 37.8 days in spring, 74.7 in summer, 79.4 in autumn and 50.1 in winter.
Eighty three high flow events greater than 1 - 2 cumec in magnitude were observed between 1974 and 2003, with the seasonal average number of these events being 1.21 for spring, 0.38 for summer, 0.21 for autumn, and 1.14 in winter. These events lasted an average of 4.8 days in spring, 6.2 in summer, 4.9 in autumn and 7.2 in winter.
Natural flows and abstraction effects The median mean daily flows for the natural record at Mauriceton for 1974-2003 are estimated to be 0.155 cumec in spring (Sept – Nov), 0.136 in summer (Dec-Feb), 0.091 in autumn (Mar - May) and 0.558 in winter (Jun - Aug). These are all significantly higher than the same figures derived from the historical record, suggesting that the impact of abstractions is significant, especially during the spring to autumn period. The overall pattern of high flows does not differ significantly under natural or historical flow records, given the uncertainties in the flood routing model, suggesting only a slight loss in flood frequency due to water use (farm dams) in the catchment.
The overall impact of abstraction, farm dam operation and irrigation in the Jordan appears to be a reduction in flows between 0 and 2 cumec, with an increased total duration of near-zero to zero flows by some 2 – 3.5% (Figure 5). 2.00
Figure 5. Flow duration curves for historical (grey line) and modeled natural (black line) flows in the Jordan at Mauriceton for the 30 years of 1974 and 2003. Note y axis expanded to exaggerate differences for flows between 0 and 2 cumec.
The flood history of the Jordan catchment was modeled by Hydro Tasmania, under both natural and historical (ie with abstractions) conditions. The peak size of floods with varying return intervals over the entire modeled record period (1917 to 2004) are shown in Table 1, and the flood exceedance probability plot is shown in Figure 6. Comparison of the flood peaks for the different return periods indicates little difference between natural and historical values for floods of 2 – 200 year return periods, with historical flood peak discharges being 94 – 98% of natural. This suggests that water abstraction and farm dam development have little influence on larger floods. Historical annual flood peaks are, however, significantly lower, by 21% than under natural conditions.
Water abstraction and farm dam development in the catchment have therefore having a significant influence on baseflows and on floods and high/flow events of the order of 1 – 2 cumec and less.
Inspection of mapped farm dams reveals a total of 2015 dams upstream of the Bridgewater gauging station, incorporating 1009 dams in the catchment upstream of the Mauriceton gauge. The total farm dam storage and associated potential annual demand at Bridgewater is estimated to be around 4500 ML (assuming 1 farm dam volume used per year). Note that this does not include Lake Tiberias (and upstream), as this sub-catchment is assumed to yield predominantly no discharge downstream (ie is a net sink due to high evaporation). Licensed takes from the Jordan catchment are estimated to total 7530 ML per year (Gurung and Dayaratne 2002), with 7061 ML being for irrigation.
The current level of direct water abstraction, coupled with farm dam flow capture and use in the catchment is therefore high. The annual yield (ML) at Mauriceton from 1990 to 2001 was estimated to be 9919 ML, with 2789 ML yield for the irrigation season (Oct – Apr). The total licensed take and farm dam interception is estimated to represent potentially up to 70 - 75% of the natural total annual flow yield in the period 1990 – 2001. Not all of these takes are used in every year, and not all farm dam volumes are fully used every year. A significant proportion of the total takes would occur in winter, with farm dam filling, but the impact on summer-autumn baseflows and events < 2 cumec is likely to be substantial.
Table 1. Annual return intervals (ARI) and exceedance probabilities (AEP) of floods in the Jordan River (1917 – 2004).
Natural Historical ARI AEP Flow (m3/s) Flow (m3/s) 1 0.99 1.77 1.40 2 0.5 7.69 7.24 5 0.2 19.08 18.56 10 0.1 33.80 33.14 20 0.05 57.22 56.10 50 0.02 110.07 107.17 100 0.01 176.82 170.60 200 0.005 280.52 267.45
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1.0 1.01 1.111 1.252 5 10 50 100 AEP (1:Y years)
Figure 6. Flood peak discharge vs return probabilities for the Jordan River (1917 – 2004), Hydro Tasmania unpub. data. 3. Jordan River: Water quality A detailed assessment of the water quality status of the Jordan was conducted by Wilson and Foley (2003). From their observations it can be concluded that: • Surface water salinities are high with median conductivity values at most mainstem sites significantly greater than the ANZECC (2000) guideline for upland rivers of 350 µS/cm -1 , and with values often > 1500 µS/cm -1 , a threshold above which impacts occur on aquatic fauna (Nielsen et al. 2003, Figure 7). • Wide ranges in dissolved oxygen concentrations from very high (> 100% saturation) to low (2 - 5 mg/L) were due to the locally high levels of primary production and respiration, enhanced by high nutrient concentrations and low flows, especially in pools (Figure 8). Night time DO levels at some sites were less than 5 mg/L, which adversely affects some aquatic species. • Monthly nutrient concentrations at the majority of sites were in excess of ANZECC (2000) trigger values for upland rivers, and at the higher end for Tasmanian rivers surveyed to date. High total nitrogen and phosphorus, coupled with low nitrate levels, indicate a highly eutrophied and rapidly cycling ecosystem in pools under low flow. • pH levels in the Jordan mainstem tended toward alkaline conditions, with frequent observations of pH > 8 (Figure 9). Such levels result from high levels of salinity and aquatic plant production especially under low flow conditions, and may have adverse effects on aquatic biota. • The ephemeral flow regime of the system exacerbates degraded water quality due to reduced baseflows from high water use and dam construction, as well as fertiliser application, unrestricted stock access, lack of riparian vegetation and instream works.
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0 J1 J5 J6 J7 J9 J10 J11 J13 J15 J16 J17 J18 J22 Site No.
Figure 7. Box plots showing statistics for water conductivity from monthly monitoring at mainstream sites on the Jordan River (from Wilson and Foley 2003). Site numbers increase upstream. Mid line indicates median values, box edges indicate 25 and 75 percentile values, and black circles and whiskers are outliers. Most sites fall in the range 7.5 to 8, with a relatively high number of samples at pH’s > 8. Several sites downstream of J15 (Apsley) have a significant proportion of their values > 1500 microS/cm (dashed line), a threshold value above which aquatic fauna is negatively impacted by salinity. 200
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0 J1 J5 J6 J7 J9 J10 J11 J13 J15 J16 J17 J18 J22 Site No. Figure 8. Box plots showing statistics for monthly dissolved oxygen at sites on the Jordan River (from Wilson and Foley 2003). Sampling occurred at different times of the day for each site. The wide range in values for most sites is symptomatic of high local levels of productivity and respiration (the latter dominating at night), indicative of an enriched, pool-dominated system.
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6 J1 J5 J6 J7 J9 J10 J11 J13 J15 J16 J17 J18 J22 Site No. Figure 9. Statistics of pH variation at mainstream monitoring sites in the Jordan Catchment (February 1999-December 2001), from Wilson and Foley (2003). Site numbers increase upstream. Mid line indicates median values, box edges indicate 25 and 75 percentile values, and black circles and whiskers are outliers. Most sites fall in the range 7.5 to 8, with a relatively high number of samples at pH’s > 8.
The majority of the 19 sites surveyed by DPIWE (Read and Krasnicki 2001) were found to have degraded water quality, with high conductivity (salinity) and high pH. Low dissolved oxygen was also observed at over half of the sites surveyed in autumn. Such poor water quality reduces macroinvertebrate and fish community diversity and abundance. 4. Aquatic ecology Data on the aquatic fauna of the Jordan catchment has been summarised by Read and Krasnicki (2000).
4.1 Fish The Jordan River is characterised by: • reduced abundances, diversity and biomass of native fish; and • high relative biomass and abundance of exotic fish species, particularly redfin perch ( Perca fluviatilis ), brown tout ( Salmo trutta ) and tench ( Tinca tinca ).
Exotic fish dominate both biomass and abundance in fish assemblages upstream of Broadmarsh (Figure 10). Native fish populations in the Jordan catchment upstream of Broadmarsh are depauperate and dominated by shortfin eel ( Anguilla australis ), a species tolerant of habitat degradation and low flow conditions. Annual migrations of a number of juvenile and adult native fish species, particularly Galaxias maculatus and Lovettia sealii , are observed at the tidal limits in lower Jordan (Davies unpub. obs). Three native fish species (the jollytail, Galaxias maculatus , A. australis and the sandy, Pseudaphritis urvillii ) were observed in the lower Jordan by DPIWE in 2003. These same species were observed by Sloane (1976) and Bennison (1976) in the mid 1970’s, but at higher biomass relative to exotic fish.
A limited brown trout fishery occurs in the Jordan, particularly in the middle and some lower reaches, with less than 100 anglers participating in any year, and expending less than 150 angler days effort. Redfin perch are a common by-catch. A larger number of anglers is known to fish in the estuary (Herdsmans Cove and the vicinity of the Ford), targeting bream and occasionally migrating sea and estuarine forms of brown trout. There are no data available on the numbers of bream fishers or the effort expended.
4.2 Macroinvertebrates 19 sites sampled were in the Jordan River and associated tributaries by DPIWE (Read and Krasnicki 2001), and analysis of macroinvertebrate samples (using the AUSRIVAS or Australian River Assessment Scheme protocol) indicated that the river was in poor condition in terms of river health (Figure 10). Many of the impacts on macroinvertebrate communities were related to habitat degradation especially stock access, willow infestation and riparian vegetation clearance at other sites, with many Jordan River sites affected by all three impacts.
A slight improvement in river health was observed in spring, with a shift of some sites from a significant to a moderate AUSRIVAS impairment rating. This was believed to be due to increased flows during winter-spring, but may also reflect a seasonality in colonization and dispersal of flowing water taxa. The poor condition of river health at many sites in autumn are believed to reflect the combination of: • natural low flows and high water demand from irrigation; • reduced habitat availability; and • water quality degradation.
Unimpaired Significantly impaired Severely impaired
Jord10
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Figure 10. Map of Jordan catchment showing AUSRIVAS macroinvertebrate assessment scores for 19 sites sampled in autumn 1999 as small coloured circles (with accompanying site code name list); and the relative biomass of native (blue) and exotic fish (purple) observed in 2002/03 (derived from DPIWE unpub. data). A total of 52 families were identified from edgewater habitats in autumn and 51 families in spring by DPIWE (Read and Krasnicki 2001) from 19 sites sampled in the Jordan. The five dominant families in edgewater habitats in autumn were Ceinidae (amphipods), sub-family Chironominae (midges), Leptoceridae (caddisflies), Corixidae (aquatic bugs), Planorbidae (freshwater snails) and Coenagrionidae (dragonflies) and Baetidae (mayflies). The majority of these taxa are typical of slow flowing or still waters usually in silty habitats or habitats dominated by aquatic macrophytes and/or algae. These samples were mainly derived from pool habitats, due to the frequent absence of flowing water riffle habitats during very low flows.
4.3 Aquatic mammals There are no data available on the population status and condition of platypus or water rats in the Jordan catchment, though both species are known from the main channel and larger tributaries (S. Munks pers. comm.).
4.4 Aquatic plants There are no data sets documenting aquatic plant (macrophyte) assemblages in the Jordan River. See section 6 for further information. 5. Geomorphology
5.1 Geomorphology – introduction
The requirements of the Jordan River Environment Flow Regime Assessment Project include a geomorphic characterisation of the Jordan River and recommendations regarding key flow events to maintain channel processes and habitat.
The 2001 Draft Rivercare Plan (Ecosynthesis, 2001) for the Jordan River includes a geomorphic assessment of the river and a set of ‘actions’ designed to achieve the goals and objectives identified in the Rivercare Plan. This present work seeks to augment the fluvial geomorphology information contained in the RiverCare plan by focusing on how the present river channel has been shaped by flows and alterations to the catchment, and how future flow changes might affect the fluvial geomorphology of the river. This paper does not provide management recommendations for improving the environmental condition of the river, as this is beyond the scope of the project and have been well summarised in the Draft Rivercare Plan and in other investigations (North 1993, Read and Krasnicki 2000).
Because fluvial geomorphic processes are strongly linked with the riparian vegetation of a river, field investigations were completed in conjunction with vegetation investigations. Field work focused on identifying river regions where riparian vegetation and fluvial geomorphic values were high, and identifying what flow regime would be required to maintain the present condition of these reaches. Unfortunately, few areas were identified which possessed values of even marginal conservation significance due to the highly impacted status of the catchment.
The following sections present a historic geomorphic context of the Jordan River; a discussion of how flow and catchment changes have affected the large scale functioning of the river; and a geomorphic characterisation based on the reaches previously identified in the Draft RiverCare Plan.
5.2 Geomorphic context of Jordan River
Geologically, the Jordan River catchment and midlands in general reflects relatively recent events in Tasmania with the oldest exposed rocks in the catchment being Late Carboniferous to Early Permian in age. These sandstone and mudstone sequences reflect a long period of marine and terrestrial sedimentation associated with glacial retreat (Hand 1993). Uplift in northern and Western Tasmania during the Late Permian to Late Triassic was accompanied by a marine regression, resulting in the deposition of fluvial and lacustrine sandstones and mudstones on a broad floodplain in Central and Southern Tasmania (Hand 1993, Sharples 1997).
During the mid Jurassic, normal faulting associated with the intrusion of dolerite disrupted the sedimentary sequences, with the dolerite spreading as thick horizontal sheets at various sub-surfaces within the sedimentary layers (Sharples 1997, Burrett and Martin 1989). The erosion of the overlying sedimentary rocks has led to the exposure of Jurassic dolerite throughout the Jordan catchment. Additional faulting in the Cretaceous and Tertiary associated with the separation of Tasmania from Antarctica resulted in the creation of horsts and grabens, with horsts becoming tabular mountains capped with dolerite, and grabens developing major river systems, such as the Derwent. The present day drainage patterns within the Derwent catchment were established during this period (Sharples 1997).
The fluvially modified graben-controlled valleys were partially infilled by Tertiary basalts which altered the position of rivers within some valleys. The river valleys continued to be modified during the Early Tertiary by energetic fluvial processes during this relatively warm, humid period (Sharples 1997). Stepped Tertiary erosion surfaces in the region indicate repeated fluvial erosion leading to planation of the landscape alternated with episodes of uplift (Davies, in Sharples 1997).
Modification of the landscape continued during Tertiary glacial periods, due to reduced vegetation cover and high river flows associated with glacial outwash. The oversized valleys compared to the size of present day rivers in the midlands today are the result of these processes (Sharples 1997).
Aeolian processes were also active during the last glacial phase (25,000 – 10,000 B.P.) resulting in the creation of aeolian landforms in the Jordan catchment, including a low dune east of the river at Glenfield (Sigleo and Colhoun 1982).
5.3 Catchment changes Since European settlement, the Jordan River catchment has undergone substantial change. Clearing of the floodplains and riparian zone began early in the 1800’s with the development of agriculture, and has continued, resulting in a severely cleared and modified landscape. Where vegetation is present in riparian zones and floodplains it is dominated by invasive weed species. Stock have direct access to the river channel for most of the river. In-channel works (straightening, re-directing, excavations, willow removal) have altered the bed characteristics and riparian zones in many reaches. Instream and offstream storages in tributaries and along the Jordan have also modified the flow regime.
These changes to the floodplains, riparian zone and flow regime directly affect how water is delivered to and transported by the river and have altered the fluvial geomorphic processes operating in the river.
Superimposed on these man-induced catchment changes is a ‘natural’ change in rainfall patterns documented for the region. The Jordan River occupies one of the driest regions in Tasmania, situated in the rain shadow from prevailing rain bearing winds, resulting in average annual rainfall of <600 mm (Fallon et al , 2000). Historically, the catchment was characterised by relatively frequent large flood events, generally occurring at 1 – 4 year intervals (Fallon et al. , 2000). Since the early 1980s, these flood events have become much less frequent. This decrease in flooding has also contributed to a decrease in perennial flows and small flood events, which has been exacerbated by damming and extraction. It is highly probable that even in the absence of catchment clearing and water extraction the river would not be ‘stable’ with respect to geomorphology, but adjusting to the new, reduced flood – flow regime. The alterations to the catchment have directly affected the components of flow that regulate geomorphic and ecological processes in a river ecosystem, namely the magnitude, duration, frequency, timing and rate of change of hydrologic conditions (Poff et al., 1997). Table 2 summarises how these catchment alterations and rainfall trends are likely to have impacted the hydrologic and geomorphic processes in the Jordan River.
The next section discusses these impacts on a zone by zone basis. Table 2. Summary of how changes in the Jordan catchment since European settlement have altered the geomorphic processes operating in the river. LWD = large woody debris.
Change to Catchment Impact on Hydrology of River Impact on Floodplain Impact on River Channel -Increase volume and rate of run off -Increase floodplain erosion (stripping) -Increased risk of bank erosion or bed delivered to channel during flood due to higher water velocities during incision due to higher inflows events; floods; -Shorter flood events can lead to -Decrease inflows to river following -Less deposition during floods due to channel contraction due to Clearing of floodplains large floods and following wet winters higher water velocities colonisation; leading to lower perennial flows -Decrease input of LWD to floodplain -Increased sediment delivery from and channel floodplains -Decrease input of LWD -Increased volume and rate of run off -Allows water to exit floodplain and -Increased susceptibility to scour and delivered to channel during flood return to river channel more rapidly; undercutting due to decreased events; -Increases floodplain erosion roughness of banks; -Higher flow velocities due to -Higher temperatures due to Clearing of riparian vegetation decreased roughness of channel decreased shading lead to desiccation of banks and subaerial erosion of bank and increases susceptibility to erosion during flood events -Decrease input of LWD -Decreased frequency of small floods -Reduced flooding frequency -Channel narrowing and side channel -Alter timing of small floods (first floods -Increased floodplain stripping during narrowing is promoted due to Water extraction and storage captured in dams) large floods due to channel contraction decreased high flow events; -Decrease / eliminate flows during dry -Decrease in sediment movement; periods -Increased flow velocities due to -Increase gully erosion associated with -Increased susceptibility to scour and decrease in channel roughness initiation of knick points undercutting due to reduced Channel modifications roughness; (straightening, excavation) -Initiation of knick-points which propagate upstream -Lower annual flows -Increased floodplain stripping during -Channel and side channel contraction Natural trend of decrease in -Smaller proportion of flows occurring large floods due to channel contraction due to decreased high flow events and large flood events as flood events less movement of sediment
18 5.4 Geomorphic characterisation
5.4.1 Overview Ecosynthesis (2001) identified 14 reaches along the length of the Jordan River on the basis of river characteristics. The following sections group these reaches into four zones and discuss geomorphic characteristics, and how the present flow regime is affecting the geomorphology of the river.
The headwaters of the Jordan River begin at greater than 500 m elevation in Stonor Cr, which drains the high level dolerite hills dividing the Jordan and Coal River valleys. Lake Tiberias was created in the 1800s in a broad alluvial basin about 4 km downstream of the Stonor Creek headwaters. The Lake has developed into a wetland system of high ecological value. Lake Tiberias and Stonors Creek are not considered in this environmental flow project.
For the purpose of this investigation, the Jordan River originates at Lake Tiberias and flows generally northwestward through the undulating dolerite hills near Jericho. The river has a generally low gradient, and is naturally an ephemeral stream. Numerous other small intermittent tributaries drain the surrounding hills in this area. Approximately 9 km downstream of Lake Tiberias near Jericho, the river has cut through the dolerite into older Triassic sediments of the Parmeener group and turns sharply north as it enters a relatively large area of resistant Triassic sandstone, forming Burnt Log Gully. At the entrance to the gorge is the confluence of the Jordan River and Dulverton Rivulet, which drains the Oatlands area. The Jordan has formed a deep gorge within the sandstone, which continues for about 12 km, trending first north, and then southwestward. The river drops about 50 m over the length of the gorge in a series of pools and riffles. About two-thirds of the way down the gorge, a major tributary, the Exe Rivulet, which has also carved a gorge in the sandstone, enters the Jordan from the northwest.
Below the gorge the river enters a series of broadwaters or marshes, separated by gorge sections, formed predominantly by dolerite, although some quartzite topography is present. The broad valleys have developed on the Triassic sediments (Parmeener) in the mid-catchment, and in a basalt filled valley in the lower catchment. In downstream order, these marsh and valley controls include Lower Marshes, Glenmore Sugarloaf / Sandy Toms Rocks, Cooks Marsh and Black Marsh, Memorial Hill / Coal Hills / Nannygoat Hill, the Kempton floodplains, the Tang, Elderslie floodplains, Broadmarsh valley, the Brighton floodplains and the Brighton Gorge. The floodplains and marshes are characterised by low topography, low channel slopes (< 0.05) and increased channel sinuosity.
The Jordan enters the Derwent Estuary at Bridgewater.
19 5.4.2 Description of reaches Jericho Floodplain The ‘Jericho Floodplain’ zone is defined as the reach between Lake Tiberias and the entrance to Burnt Log Gully, located near Jericho. Immediately downstream of Lake Tiberias, the Jordan River flows through relatively flat topped dolerite hills of similar elevation for about 3 km (Photo 1). Downstream of the hills, the Jordan enters a broad alluvial basin, the Woodford Plain (Photo 2), where the river joins Ringwood Creek near Jericho. Downstream of this confluence, the Jordan flows over alluvium and Tertiary sediments for the remainder of the reach. Upstream of the confluence, Ringwood Creek flows northward through a broad alluvial valley draining Bisdee Tier. Woodford Plain ends at a valley constriction formed by Sandhill Spur and Northumbria Hill. Through this stricture, the river turns sharply to the north, around Northumbria Hill and the valley narrows and deepens to the north, the start of the Burnt Log Gully zone.
Photo 1. Jordan River below Lake Tiberias. View from Colebrook Rd looking upstream.
Photo 2. Jordan River downstream of L. Tiberias, flowing into Woodford Plain.
The Jordan and its tributaries are intermittent streams in this zone, with very low or no flow during the dry summer months. The flow regime has been modified through the creation of Lake Tiberias, in stream and off stream storages in tributaries and direct extractions from the river.
20 Evident in Photo 1 and Photo 2 is the highly modified nature of the Jordan River catchment in this zone. Extensive clearing of the riparian zone, floodplain and hills has occurred. The riparian zone is dominated by woody weeds, with the lower bank typically devoid of vegetation cover. The low summer flows have allowed colonisation of the channel by grasses and weeds in many areas.
Burnt Log Gully The Burnt Log Gully zone extends ~25 km from Northumbria Hill through a large inverted U-shaped gorge, emerging at Red Sugarloaf approximately 4 km northwest of the beginning of the zone. The zone is largely confined to Tertiary quartz sandstones with the gorge characterised by steep sandstone walls and valley fill (Photo 3, Photo 4). The river meanders through the valley fill with discontinuous floodplain pockets and partial bedrock control of the channel common (Photo 3, Photo 4). This morphology has led to the development of riffle and pool sequences, as shown in Photo 4 and Photo 5. Where small intermittent tributaries drain the valley walls, fans have developed on the floodplain (Photo 6).
Photo 3. Burnt Log Gully showing floodplain pocket within gorge. Note poor riparian vegetation, weeks and cleared floodplain.
Photo 4. Burnt Log Gully zone-pool in partially bedrock controlled section of river.
21 Photo 5. Riffle section in Burnt Log Gully.
Photo 6. Alluvial fan at mouth of tributary in Burnt Log Gully.
Photo 7. Undercutting and erosion of Jordan bank in Burnt Log Gully.
The floodplains and riparian zone have been cleared throughout the gorge zone, and there is ample evidence of stock access to the channel. The clearing has exposed the sandy river banks to scour during high flow events, with erosion and undercutting common throughout the zone (Photo 7). Where vegetation is present on the banks, it is largely composed of noxious weeds, including impenetrable gorse thickets.
Field investigations of the gorge were completed shortly after a moderate flood event in late January 2004. Medium to coarse sand deposits were present on the floodplain, occurring as shadow deposits behind the occasional grassy tussock (Photo 8).
22 Photo 8. Shadow deposit on floodplain of Jordan River following moderate flow event in January 2004.
The isolated sand deposits suggest that during the flood event, most of the sand being transported by the river passed through the gorge without being deposited. Prior to the clearing of the floodplain and riparian zone, more material would have been trapped by the vegetation, ‘feeding’ the floodplain. In contrast, the exposed floodplains are now more likely to be a source of sediment due to increased water velocities and increased sheet wash erosion.
Upper catchment basin / valley control sequence Between the end of Burnt Log Gully and the Brighton floodplain, there is an ~80 km section consisting of alternating alluvial basins (floodplains / broadwaters / marshes) developed on Permian and Tertiary sediments separated by valley control reaches comprised of quartzite or dolerite. The flat laying alluvial basins have slopes <0.0025 and range in reach length from about 5 to 20 km, with the Elderslie floodplains being the longest. The intervening valley control sections have slopes of ~0.005 to 0.01 and are typically 5 – 8 km in length. A schematic of this zone is shown in Figure 11. Typical examples of an alluvial basin (Lower Marshes) and valley control reaches (Sandy Tom’s Hill) are shown in Photo 9 and Photo 10 .
The first basin, Lower Marshes, is a fault bounded structure within the Triassic sandstone, and is controlled by Glenmore Hill and Sandy Toms Rocks which are sandstone hills similar to the upstream Burnt Log Gully area. The other basins, Black and Cooks Marsh, the Kempton floodplains and Elderslie floodplains, are generally underlain by alluvial and Permian sedimentary sequences. The sandstone or dolerite valley control units present downstream of Lower Marshes include Nanny Goat Hill (Kempton floodplain), The Tang (Mauriceton floodplain), Sandy Hills (Elderslie) and the Broadmarsh valley control.
23 Sandy Toms Rocks/Glenmore Suga rloaf
Downstream Lower Marshes
Nanny Goat/Memorial Hill
Kempton
Floodplain
Tang
Mauriceton
Sandy Hills
Elderselie
Billy Goat/Terry Hill (Broadmarsh)
Brighton Floodplain
Figure 11. Schematic of fluvial geomorphological basin / valley control sequence in Jordan River.
The broad flat basins in this zone have been extensively developed for agriculture. Land clearing, the destruction of the riparian zone and channel alterations have combined to produce river reaches with very poor natural values. The loss of riparian vegetation has increased the exposure of the banks to scour and undercutting during high flow events, resulting in lateral erosion. The instability of the river banks is augmented by the lack of large woody debris supplied to the channel due to the lack of vegetation on the floodplains and banks. Stock access has also increased erosion in many areas. Photo 11 and Photo 12 show some of these features.
Three valley control reaches were investigated, Nanny Goat Hill / Memorial Hill at the upstream of the Kempton floodplain, Sandy Hills separating the Mauriceton and Elderslie floodplains, and Billy Goat Hill at the downstream end of the Broadmarsh floodplain.
The Nanny Goat Hill valley control is generally dolerite bedrock controlled, with discontinuous floodplain pockets. The riparian zone is highly degraded due to weed infestation. Water flow in the river was minimal, even though a moderate flood event occurred 2-weeks prior to field visit, and water clarity was poor (Photo 13, Photo 14).
24 Photo 9. View from the downstream end of Lower Marshes looking up western basin across Pleasant Place
.
Photo 10. View downstream from Black Bridge at the downstream end of Lower Marshes where Jordan re enters valley control reach. Sandy Toms Hill on right.
25 Photo 11 (left). Elderslie floodplain showing poor condition of channel, riparian zone and floodplain. Photo 12. (right) Elderslie floodplain showing lack of riparian zone, and development of floodplain.
Photo 13. Jordan River in Nanny Goat Hill valley control reach.
26 Photo 14.Jordan River in Nanny Goat Hill valley control reach.
The Sandy Hill valley control is created by the passage of the Jordan through undulating sandstone hills with discontinuous floodplain pockets. The river consists of riffle and pool sequences (Photo 15, Photo 16), with large sandstone boulders in the channel. The reach is also highly degraded, with poor riparian vegetation, and extensive lateral erosion in the channel as shown in Photo 17 and Photo 18. The absence of vegetation on the bank promotes desiccation of the fine-grained bank materials, leading to a friable surface susceptible to erosion during high flow events. The banks are higher than the upstream Nanny Goat gorge, due probably to a greater ability of the river to incise into the valley fill.
The final valley control reach investigated, Billy Goat Hill downstream of Broadmarsh, was in slightly better, although overall poor, condition. This reach is composed of riffle and pool sequences (Photo 19) developed in valley fill between two steep dolerite hills.
Photo 15. (left) pool in Sand Hills
Photo 16. Riffle sequence in Sand Hill reach.
27 Photo 17. Jordan River in Sandy Toms Hill, showing lack of riparian vegetation, poor condition of floodplain and extensive lateral erosion.
Photo 18. Jordan River in Sandy Tom Hills Gorge. Bank collapse due to lateral erosion in bank channel.
28 Photo 19.Pool and riffle sequence in Billy Goat ‘gorge’. Large tree on right is undercut.
The riparian zone is in poor condition with a high proportion of invasive weed species and denuded bank faces. Evidence of bank erosion was widespread, with long undercuts common, and extensive exposed tree and plant roots (Photo 20).
Photo 20. Undercutting of denuded bank Billy Goat ‘gorge’. Flood debris on banks indicated high flow level associated with January flood event was well above height of undercut.
Photo 21 and Photo 22 show the transition of the river from Billy Goat valley control section into the Brighton Floodplain.
29 Photo 21. Jordan River exiting valley control at Billy Goat Hill. View upstream into gorge.
Photo 22. Jordan River entering Brighton floodplain from Billy Goat gorge.
Brighton Floodplain and gorge The final 25 km of the Jordan River mainstem comprises the Brighton Floodplain and Brighton Gorge. In this zone, the river is partially confined by Winton Hill upstream of Pontville. Downstream of Brighton, the floodplain diminishes, with the river incised in sandstone, basalt and dolerite.
Photo 23 (left). Bully erosion on Brighton floodplain downstream of Billy Goat Hill. Photo 24 (right). Detail of gully erosion, showing headcut and scale of gully.
30 Similar to the upstream zones, it has been highly modified, with removal of riparian vegetation, in channel works and floodplain clearing. Photo 23 and Photo 24 show extensive gully erosion on the Brighton floodplain.
Near the mouth of the river, the Jordan emerges from ‘Horse Head’ gorge and widens somewhat, as shown in Photo 25.
Photo 25. Jordan River downstream of Brighton, upstream of estuary.
31 6. Riparian and Aquatic Vegetation
6.1 Vegetation – introduction The assessment of the Jordan River Environment Flow Regime includes consideration of the vegetation values, particularly those that that may be dependent upon specific flow events to maintain ecological processes.
Two main studies of vegetation have been undertaken in the Jordan catchment, the first was by North (1999). This study detailed the extent of native vegetation types throughout the catchment and the distribution of records of threatened flora species. A few of these species are associated with the river and its tributaries but none are reported as significant populations in native vegetation. North described the vegetation of the riparian zone as substantially degraded with few areas of intact vegetation. He cited a range of pressures as the source of degradation and weeds as a serious management problem. North also noted that most of the vegetation types of high conservation significance are confined to freehold land and thus limits the potential for protection of such values on public land. Other than a detailed survey of the Brighton area (Zacharek 1998) and a few riparian sites in the catchment (Daley 2001), no formal surveys of remnant vegetation have been undertaken.
Ecosynthesis (2001) produced a Rivercare Plan for the Jordan River. They concluded that the Jordan River is a highly disturbed system displaying poor water quality, degraded ecosystem health with only sparse native riparian vegetation and extensive woody weeds. They recommended a focus for vegetation management on rehabilitation and weed removal. They also recommended an improvement in water quality to help to improve aquatic health. The plan includes a number of floristic species lists from remnants of native riparian vegetation within the catchment; these were collected by Daley (2001).
Because fluvial geomorphic processes are strongly linked with the riparian and aquatic vegetation of a river, field investigations in the present study were completed in conjunction with geomorphic investigations. Field work focused on identifying river regions where riparian vegetation and fluvial geomorphic values were present, and identifying what flow regime would be required to maintain the present condition of these reaches. Unfortunately, few areas were identified which possessed values of even marginal vegetation conservation significance due to the highly impacted status of the catchment.
6.2 General Characteristics of vegetation The vegetation of the Jordan River catchment has been mapped in a number of projects. Firstly during the Regional Forestry Agreement (RFA) and since adopted by Tasveg (a statewide mapping program). This project uses aerial photography of 1:42 000 and 1: 25 000 scale. The Tasveg data include the earlier RFA mapping and are not ground-truthed but provide a general catchment wide impression of the major vegetation types. North (1999) provided a Jordan catchment vegetation map that included substantial corrections to the previously mapped data.
32 The predominant vegetation types in the catchment in order of extent are agricultural land, forest, woodland, native grassland (much derived from clearance of trees), scrub, wetland, weeds, plantation and finally riparian vegetation and heath. Only 107 ha or 0.1 % of the catchment is mapped as riparian vegetation.
The mapped riparian vegetation is restricted to the deeply incised country near Burnt Log Gully in the upper catchment with less extensive area near Glenmore, Nanny Goat Hill and Terry’s Hill in the lower reaches.
The large areas of continuous agricultural land are used for grazing and cropping. In the most intensively utilised areas, such as the relatively fertile alluvial flats adjacent to the Jordan River, the riparian vegetation has been removed. The banks bare of native vegetation and characterised by pasture grasses.
In the less intensively utilised gorge areas some riparian vegetation remains. However, the extent mapped in these areas is over estimated and has been replaced by gorse and willows.
6.3 Descriptions of study sites
Jericho Floodplain The ‘Jericho Floodplain’ zone is defined as the reach between Lake Tiberias and the entrance to Burnt Log Gully, located near Jericho.
The native vegetation is restricted to land away from the river on steeper slopes. The floodplain is cleared of vegetation and the riparian zone is generally unprotected and dominated by Gorse and or Willow.
While the aquatic vegetation is sometimes dominated by native species the collapse of banks and heavy siltation of the stream combined with low summer flows has produced a semi aquatic bog like vegetation characterised by Carex , rushes and exotic agricultural weeds. Some pools do exist and support Triglochin in deeper sections (Photos 27, 28).
The structure, composition and habitat of the original native vegetation has been so severely altered that the values that remain are insignificant in terms of conservation. The imposition of any particular flow regime to “sustain” the remaining native vegetation would not reflect the natural flow regime that would have occurred when the vegetation was in tact. The native riparian vegetation would have been linked to the surrounding non riparian vegetation and may have been sustained by a combination of a flow regime and disturbance by fires. The aquatic vegetation would have existed in a habitat characterised by lower light and lower water temperature (due to shading riparian vegetation) and lower turbidity (with less slope, bank and channel erosion).
33 Photo 27. Riparian and in stream bog vegetation.
Photo 268. A pool with margins dominated by Arrow Grass and Carex with Triglochin in deeper water.
Burnt Log Gully The Burnt Log Gully zone extends ~25 km from Northumbria Hill through a large inverted U-shaped gorge, emerging at Red Sugarloaf approximately 4 km northwest of the beginning of the zone. This zone is mapped by Tasveg as riparian vegetation. However, the narrow valley bottom has been cleared and is now semi-native pasture on sandy alluvial soils. The narrow floodplain vegetation and the riparian scrub has been removed. In areas that are cleared but fenced from stock and areas less accessible to sheep are dominated by Gorse.
On the alluvial fan in Photo 31, near a confluence with a tributary, a remnant of native Poa grassland suggests that this would have been the natural community on the narrow floodplain behind a riparian scrub. Occasional remnant shrubs are still present on the banks including Leptospermum lanigerum, Acacia dealbata, A. melanoxylon
34 and Lomatia tinctoria as well as Asterotrichion discolor . Graminoids and grasses such as Lomandra longifolia, Carex lynx, Juncus spp. and Poa labillardierei are scattered but never as dense as to form a native community with the shrubs and trees.
The aquatic vegetation is sparse with occasional Triglochin and Myriophyllum species present but no diverse aquatic beds of vegetation apparent. Small pockets of emergent stream margin aquatics, particularly Eleocharis sphacelata are occasional where the banks have not collapsed. Such occasional small pockets along more extensive banks with steep erodable faces highlight the extent of loss of the aquatic stream margin habitat. This type of habitat usually dominates broad runs and pools where riparian scrubs do not shade the stream margins. The terrestrial stream margin flora is predominantly a dense cover of grasses and herbs, many introduced. In a native system this area could be expected to grade from aquatic species to semi aquatic graminoids ( Carex spp, Baumea Sp, Lepidosperma spp and Eleocharis spp.). Under riparian scrub the vegetation would normally be sparse and restricted to species such as Poa, Acaena, Lomandra and occasional herbs.
The bank faces are generally unstable and eroding but protected areas still support some native and introduced herbs. The continuing bank instability and collapse demonstrates that the major components of the “system” (flow, sediment and vegetation characteristics) are not in equilibrium. This system change is also demonstrated by the lack of sediment deposition on the floodplain. The potential for deposition is evident as shadow deposits behind sparse remnant tussock grasses. The reduced sediment environment may produce a long term decline in soil fertility, particularly in sandy soils.
A typical species list for the section includes: Grid Reference: 521500E, 5315500N (AGD) Accuracy: within 100 metres Recorder: Phil Barker Date of Survey: 12 Mar 2004 Trees: Acacia melanoxylon, Salix fragilis Tall Shrubs: Acacia dealbata, Asterotrichion discolor, Leptospermum lanigerum Shrubs: Lomatia tinctoria, Rosa rubiginosa, Ulex europaeus Herbs: Acaena novae-zelandiae, Cirsium vulgare, Cotula australis, Lilaeopsis polyantha, Myriophyllum sp., Oxalis perennans, Potamogeton ochreatus, Verbascum thapsus Graminoids: Carex gaudichaudiana, Carex longebrachiata, Carex sp., Eleocharis acuta, Eleocharis sphacelata, Isolepis fluitans, Juncus astreptus, Juncus australis, Juncus sp., Lomandra longifolia, Schoenus sp., Triglochin procerum Grasses: Dactylis glomerata, Holcus lanatus, Poa labillardierei Ferns: Azolla filiculoides, Blechnum penna-marina alpina, Pteridium esculentum
35 Photo 29. LHS Gorse invading areas protected from sheep. RHS riparian vegetation cleared and banks continuing to collapse.
Photo 30. Impact of fencing with gorse dominating inside fenceline.
Photo 31. Alluvial fan with Poa grassland.
36 Upper catchment basin / valley control sequence This is an alternating sequence of alluvial basins (floodplains / broadwaters / marshes) separated by valley control reaches over a length of about 80 km of river. The basins include Lower Marshes, Black and Cooks Marsh, the Kempton floodplains and Elderslie floodplains. The valley controlled reaches are at Nanny Goat Hill, The Tang and the Broadmarsh valley.
Vegetation in the basins has been extensively cleared for agriculture and cropping is predominant on the best soils. The riparian vegetation has also been cleared from the river banks. The loss of riparian vegetation and stock access to the river has resulted in banks being under cut and continuing to collapse. Today the riparian scrubs are replaced with either grassy banks, gorse or willows (Photo 32).
Some pools within the basins still retain aquatic vegetation (Photo 33) but the stream margins and lower banks are highly modified. All submerged aquatic vegetation carried a very high load of algae on the leaves during the survey time. Emergent aquatic vegetation that normally forms a distinct community along the shallow margins is missing. Taxa that normally dominate this niche include Eleocharis and Villarsia.
The vegetation of riffles is severely degraded due to riparian clearance, stock access and impacts of land use (Photo 34). Most of the stream margin is dominated by introduced herbs.
Photo 32. Downstream of Lower Marshes, looking back up western basin across Pleasant Place. The river is indicated by the line of Gorse meandering through the centre of the view. Willows in distance.
37 .
Photo 33. Remnant vegetation downstream in a pool from Black Bridge at downstream end of lower Marshes where Jordon re enters valley control reach. Note Azolla fern.
\
Photo 34. Riffle section Elderslie floodplain
38 Photo 35. Bank scouring in valley controlled section
Controlled reaches The narrow valleys of the controlled reaches have either been cleared or else are subject to stock grazing with little or no control of stock access to the river. Where there have been cleared invasion by woody weeds, particularly Willow and Gorse, has resulted in dense and extensive stands (Nanny Goat Hill). This area is mapped as native riparian vegetation by Tasveg. Only where there are openings in the gorse or willow can native vegetation persist (Photo 37).
Photo 36. Nanny Goat Hills, complete invasion of riparian area with Willow and Gorse.
39 Photo 37. Sparse remnant riparian and stream margin aquatics among weeds.
At Billy Goat (gorge) the native riparian vegetation has not been cleared and appears to be structurally intact although Gorse is increasing in density. Riparian vegetation away from the banks forms a mosaic of scrubs and grassy patches that reflect the depositional pattern of variously shallow and deep alluvial sands. Backwaters and over bank flood filled ponds persist. The riffles are in better condition than those of the floodplains basins. However, examples of collapsing banks are still present in this section. So despite the retention of native riparian vegetation the impact of the combination of stock access and a modified sediment and flow regime upstream is still evident. To stop or reverse this erosion the floodplain would need to be revegetated.
Aquatic vegetation is sparse in the controlled sections and pools are rare or absent. These sections are predominantly river runs. Aquatic margin habitat is also sparse or absent as this generally establishes where flow energy is low on the margins of pools of deeper and slow runs. The gradient and hence flow energy through Billy Goat gorge must be too high to allow emergent aquatics to establish.
40 Photo 38. Billy Goat gorge with the best examples of riparian vegetation. Note high turbidity limiting aquatic vegetation.
Photo 39. Undercut bank at Billy Goat gorge. Such banks normally support graminoid and herb rich vegetation. Constant erosion prevents plants re establishing.
The endangered Blue Devil Eryngium ovinum was found adjacent to this site at 511401 5276965 (GDA). There are less than 1000 plants present here, in cutover E. pulchella forest.
A typical species list for the section includes: Grid Reference: 511808E, 5276822N (GDA) Accuracy: within 100 metres Recorder: Phil Barker Date of Survey: 12 Mar 2004 Trees: Acacia melanoxylon, Eucalyptus ovata, Eucalyptus pulchella, Eucalyptus viminalis Tall Shrubs: Acacia dealbata, Acacia verticillata, Exocarpos cupressiformis, Leptospermum
41 lanigerum, Pomaderris apetala Shrubs: Ulex europaeus Herbs: Acaena novae-zelandiae, Centaurium erythraea, Cirsium arvense, Cirsium vulgare, Elodea canadensis, Hypochoeris radicata, Myriophyllum sp., Oxalis perennans, Solanum nigrum Graminoids: Baumea sp., Baumea tetragona, Carex longebrachiata, Carex sp., Eleocharis acuta, Eleocharis sphacelata, Isolepis fluitans, Juncus pallidus, Juncus pauciflorus, Lepidosperma ensiforme, Lomandra longifolia, Schoenus apogon, Triglochin procerum, Typha latifolia Grasses: Dactylis glomerata, Ehrharta stipoides, Holcus lanatus, Poa labillardierei Ferns: Azolla filiculoides
As soon as the river leaves the protection of the Billy Goat gorge, the riparian vegetation is again cleared (Photo 40). The high turbidity of the water can be seen in this photo. This was taken more than a week after a moderate flood. The more recent clearance illustrates the nature of the progressive establishment of Gorse and the rapid invasion of exotic grasses into the riffle sections where marginal herbfields would have been natural (Photo 41).
Photo 40. Down stream of Billy Goat gorge.
Photo 41 Jordon River below Billy Goat Hill.
42 Lower catchment marsh / valley control sequence The condition of the lower reaches above the estuary are similar to the upper reaches, with riparian vegetation largely cleared. Instream vegetation is in poor condition with plants carrying a high algal load. Series replacement from aquatic to native terrestrial is replaced by a rapid transition to weeds.
Photo 42. Elderslie floodplain below Billy Goat gorge.
43 7. EFlow Assessment Methods The Eflow assessment was conducted in several steps: 1. Identification of key values 2. Assessment of instream flow requirements 3. Assessment of flow needs for geomorphology and riparian vegetation. 4. Integration of flow needs for all values to an environmental flow regime requirement. 5. Recommendation of specific environmental flow prescriptions.
7.1 Key environmental values From the assessments of environmental features described above, it is apparent that the Jordan River is a naturally semi-ephemeral river with a seasonal baseflow pattern and unpredictable and aseasonal high flows. The river support a series of run-pool habitats during low flows, contracting to pools during prolonged periods of very low to zero flow, often characterised by abundant aquatic plant growth.
However the current status of the river is of poor environmental quality with degraded water quality (salinity, turbidity, bacterial levels), degraded natural riparian vegetation, many and dense riparian weeds, frequent local channel and bank erosion of the channel uncontrolled stock access, locally dammed and pumped waterholes, a stream fish community with few native fish and dense populations of exotic fish, and a degraded benthic macroinvertebrate community.
The key environmental values identified though local community consultation, and of relevance to environmental flows are the maintenance and management of improved water quality, river health and riparian vegetation management.
There are few natural values remaining intact in the river, and environmental flow management should focus on preventing further degradation of the few remaining natural values and features in the system.
Environmental flow management will not achieve this, however, without a concerted effort to manage the other problems, especially water quality, riparian and bank conditions.
7.2 Instream habitat-flow assessment An instream habitat assessment was conducted in order to: • Identify flow-habitat relationships for maintenance of pool habitats; • Identify flows required to scour fine sediment and biofilms, disperse aquatic plants and transport fine organic material; • Identify flows required to move and disturb the stream bed for habitat and channel maintenance.
New field data on instream habitats and hydraulics for representative reaches was used to assess flow-habitat relationships. An initial analysis was used to attempt to develop relationships between habitat availability and discharge for key taxa and biological variables in the Jordan River channel, assuming flowing conditions. This is the basis of assessments conducted elsewhere in the state, derived from a method
44 developed by Davies and Humphries (1995). However, inspection of the flow regime revealed extreme levels of variability in flow at daily to monthly time scales. In addition, channel habitats other than pools were highly degraded and do not support a substantial flowing water macroinvertebrate or fish fauna.
The primary ecological values in the Jordan instream environments occur in pool habitats, which often support dense macrophyte and macroinvertebrate assemblages, as well as fish populations. Protection and maintenance of pool habitats through environmental flow management should focus on: • Maintenance flows – flows to maintain pool levels, connectivity and water quality; • Flushing flows – flows to mobilise sediments and organic material, flush biofilms and dilute high salinity pool waters; • Channel (and pool) maintenance floods – high flows to maintain channel form and present infilling, noting that these carry a risk of erosion associated with destabilised channels and banks.
Exploratory macroinvertebrate sampling conducted by us in 2003-04 indicated that the majority of the fauna was restricted to pools, and colonisation of channel habitat during wet periods was slow and resulted in low and highly patchy abundances of predominantly still-water fauna. The habitat assessment therefore focused instead on flows required to maintain and connect pool habitats, flush sediments and biofilms, and the role of high flow/flood events in maintaining the river ecosystem.
Two study reaches were selected - one each in, and representative of, the middle and lower sections of the Jordan (Figure 12). Transects were established at each site in order to collect instream hydraulic and habitat data that were broadly representative of the range of habitat types in each reach. Six transects were established across the channel in non-pool habitats at both sites. A single datum point was established on one bank at each transect location, consisting of a short steel star peg. Data on water surface elevation (WSE, in m measured from the transect datum peg), water velocity and substrate composition was collected once flows commenced in July. WSE and velocity data were also collected on three further occasions at different flows, at discharges ranging between 0.01 and 2 cumec. Locations of, and distance between, transects are shown in Table 3.
Table 3. Location and distances between transects within Jordan River study reaches (Grid references in AGD datum).
Distance from Reach Transect Northing Easting downstream transect (m) 2 J12 5291148 509816 0 J11 5291177 509834 30.7 J10 5291454 510032 371.1 J9 5291662 510447 844.2 J8 5291677 510475 879.2 J7 5291696 510601 1001.7 1 J6 5280034 508722 0 J5 5280096 508625 130.7 J4 5280181 508412 361 J3 5280265 508372 465.4 J2 5280293 508283 553.5 J1 5280347 508221 661.1
45 Datum pegs were also established for three representative pools at each site for the collection of WSE – discharge data at the same discharge values as above.
Three pools were identified within each of the two study reaches. The water surface elevations of these pools were surveyed on several occasions relative to a fixed steel datum peg on the bank.
Two mean-daily flow series were obtained - the historical flow record at Mauriceton (for the period 1965 to 2004), and the modelled natural flow record (with all estimated abstractions removed, modelled for DPIWE by Hydro Tasmania Consulting).
Lake N Dulverton
Lake Tiberias
2: Mauriceton study reach Gauging station
1: Downstream study reach
3: Herdsman’s Cove Derwent estuary
Figure 12. Location of study sites in the Jordan River catchment, shown as black circles. 1 = Downstream study reach 1; 2 = Mauriceton study reach 2, 3 = Herdsmans Cove. Black triangle indicates location of Mauriceton gauging station.
46 7.3 High flow/flood events Sequences of high and flood flow events were inspected from the historical and modelled natural flow records. Frequencies of high flow/flood events were evaluated using the RAP flow analysis package, and by inspection of annual flood return statistics derived by Hydro Tasmania and Gurung and Dayaratne (2002).
Environmental high flow/flood event requirements for the Jordan were then identified in order to address the following ecosystem roles (Table 4):
Table 4. Roles of high flow/flood events in the Jordan River catchment.
Flood Type Ecosystem Role Median flood Channel and pool form maintenance, sediment transport to estuary. Annual flood Channel and pool maintenance, sediment and LWD transport within river, sediment and nutrient transport to estuary, estuarine flushing.
Triggers Downstream fish migration: native fish. Upstream fish migration: native fish, trout. Coarse organic material transport, estuarine mixing and nutrient delivery. Pool flushing. Freshes Maintain riparian vegetation; flushing of algae and fine organic material; aquatic and riparian plant dispersal and germination. Periodic pool-pool connection and salt flushing.
These environmental high flow/flood events are required to have the following frequency and seasonality (Table 5):
Table 5. Long term frequency and seasonal requirements for environmental high/flood flow events in the Jordan River.
High/flood flow event Long term average frequency Desired season Median 1 per 2 years Winter-spring Annual 1 per year Winter-spring Trigger 2 per year Autumn, spring Freshes 8 per year All year
47 8. Flow-ecology relationships
8.1 Instream habitat and flows
8.1.1 Channel habitat Few channel sections were flowing during the study period due to prolonged dry conditions. Samples collected from channel and run habitats in late winter 2002 and 2003 from the two study reaches contained low abundances of predominantly pool fauna. There was little evidence therefore of a permanent or seasonally-established flowing water invertebrate fauna.
8.1.2 Pool habitats Surveys of the pool water levels during dry and subsequent wet conditions in 2003- 2004 revealed little variation in pool water quality. All six pools stayed within 30 microS/cm of their initial conductivity during 30 days with no surface flow in December 2003 - January 2004. None of the pools surveyed developed low dissolved oxygen or anoxic conditions, even under prolonged no flow conditions.
All pools decreased slightly (by up to 26 cm, with a mean of 12 cm) in level during the 30 day period, but readily returned to the ‘full’ state (water levels equal to the level of the outflow point) within a brief period (1-4 days) of low flows, and then rose monotonically as flows increased (Figure 13).
These data suggested that pool levels were probably sustained by a combination of sub-surface and/or groundwater seepage, and that pools rapidly reconnect once flows are restored. Pool fullness and connectivity in both study reaches 1 and 2 could be maintained at flows of 0.2 cumec (17 ML/day) or greater at the Mauriceton gauge – in the absence of intense abstraction in individual pools.
8.1.3 Sediment flushing Initial field observations suggested that freshes of the order of 1 - 2 cumec would be required to flush silt fines and disturb biofilms that had accumulated under low flows.
Results from RHYHAB modelling of sediment flushing are shown in Table 7 for 1 and 2 cumec events. They indicate that, for both 1 and 2 cumec events: • a substantial proportion of the bed in both reaches is flushed of fines of maximum size ca 2mm; • re-deposition of silts during events was limited at these flows; and that • the maximum size of bedload moved under these flows would be of the order of 20 mm.
These results satisfy the desire to maintain benthic habitat condition in runs and shallow pools by partial flushing and disturbance of bed material. There was little difference in the response between 1 and 2 cumec events at either site.
48 81.4
81.2
81
80.8
80.6 Stage (m) 80.4
80.2
80
79.8 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Q (cumec)
Pool 1 Pool 2 Pool 3 Figure 13. Response of three pools in study reach 1 to changes in water level in February 2004. Note depression in levels after prolonged dry spells by up to 0.2 m, which are readily restored under very low flows (ca. 0.01 cumec); and smooth monotonic response of pool levels as flows increase.
Table 7. Modelled effects of freshes in the Jordan River.
Downstream reach 1 Mauriceton reach 2 1 cumec 2 cumec 1 cumec 2 cumec % area of reach flushed: - surface of bed 56.7 65.3 25.2 30.2 - "deep" ie winnowed 49.6 61.5 20.8 25
Suspended sediment size (mm) 1.8 2.2 2.0 2.2
Bedload sediment size (mm) 17.0 21.5 22.0 21.5
% of suspended silt deposited 25.1 20.4 12.9 7.0
8.2 Relationship between flow regime and geomorphology The present condition of the channel, riparian zone and floodplain throughout the Jordan River is very poor, due to land clearing, channel alterations and land use practices. The natural riverine processes which linked the channel to the floodplain have been severely altered, so the system now responds differently to high and low flows as compared to pre-European settlement. The establishment of an environmental flow regime for the Jordan will not re-establish ‘natural’ processes
49 within the catchment unless catchment management practices are altered and riparian and floodplain rehabilitation programs are implemented.
8.2.1 Impact of low flows on the Jordan Presently, the extended periods of low or no flows expose unvegetated river banks to prolonged drying periods which lead to sub-aerial erosion processes, and also increase the banks’ susceptibility to scour during subsequent high flow events. Prior to clearing, the banks would have had a more extensive plant cover which retained moisture, and increased the roughness of the bank which lowered flow velocities, and physically protected the bank from scour. Returning the natural, or maintaining the present low flow regime within the catchment will not maintain the present catchment status nor enhance it. Given the condition of the riparian zone, the present processes will continue to degrade the channel, with lateral erosion a major issue.
8.2.2 Impact of ‘freshes’ on the Jordan Historically, periodic medium flows or ‘freshes’ would have transported fine sands, silts and organic matter through the system, with deposition occurring on bank toes and in the channel. These small disturbances would have delivered nutrients and sediment to vegetation on the lower banks, enhancing bank toe stability.
The inclusion of ‘freshes’ in an environmental flow regime will increase the transport of fine material through the system compared to present, which will enhance water quality, but may also increase scour of some lower banks and toes, due to the lack of riparian vegetation.
8.2.3 Impact of high flows on the Jordan The impact of episodic high flows in the Jordan would have produced a more regular flow in the river prior to floodplain and riparian clearing. Water from the large storm events would have been stored on and in the floodplains, slowly re-entering the main channel, and extending river flow beyond the storm event. Vegetation on the floodplains would have protected the plains from sheet wash erosion and promoted deposition of suspended material, resulting in the continued aggradation of the floodplain. Floodplain vegetation would also have slowed the rate of water return to the main channel, thus reducing channel erosion. These large events would have also transferred large woody debris from the floodplain areas into the main channel, assisting in the stability of river banks through armouring and increased channel roughness.
In the riparian zone, diverse vegetation would have protected the banks during high flow events, and reduced scour by reducing water velocities and physically protecting the banks
At present high flows produce widespread undercutting and erosion in the gorge sections, due to the rapid delivery of water into the gorges from the upstream denuded floodplains and lack of riparian vegetation. Lateral channel erosion on the floodplains results from high velocity flows attacking denuded, friable banks.
50 Reducing the impact of high flow events is only possible through the improvement of the riparian and floodplain zones through the implementation of rehabilitation projects and the adoption of better catchment management practices (ie, excluding livestock from the river, weed removal, etc) on a catchment wide basis.
51 8.3 Relationship between flow regime and vegetation The 2001 Draft Rivercare Plan (Ecosynthesis, 2001) for the Jordan River includes a vegetation assessment of the river and outlines ‘actions’ designed to achieve the goals and objective identified in the Rivercare Plan. This present work seeks to augment the vegetation information contained in the RiverCare plan by focusing on how the present vegetation has been shaped by catchment clearance and management, flows and the geomorphic responses and how future flow regimes might effect the vegetation.
The condition of the vegetation along this river is generally severely degraded. The degradation is due to the combined impacts of land clearing, land use and weeds and the attendant changes in flow and water quality (turbidity, temperature and nutrient enrichment).
The natural dynamics of the vegetation in the Jordan system have been disrupted along the length of the river and the vegetation has been disengaged from surrounding vegetation resulting in a breakdown of processes related to connectivity e.g. floodplain drainage. The current vegetation is in disequilibrium with the flow environment. While the vegetation types originally present are normally ecologically resilient (able to withstand natural disturbance) they are not particularly stable when the environment is changed (introduction of weeds, clearance and changed flow).
A major driver in the physical instability of the vegetation is the continuing erosion of banks. The vegetation’s relationship to the geomorphology in any river system is an important one and is in a large part reflected in the composition and structure of the vegetation (unpub data, Little Swanport River). This is illustrated by the distribution of different vegetation structures and species on floodplains and river banks, along stream margins, and in riffles and pools. Where the relationship between geomorphology and the flow regime is in disequilibrium, then the vegetation will continue to change. It is unlikely that an equilibrium between geomorphology and the vegetation will be reached until a more natural flow regime is established and maintained and land rehabilitation and or protection measures are implemented for an unknown but long period. An example of equilibrium after significant modification to flow appears to be evident on the Elizabeth River below Lake Leake. This was reached in the absence of land clearance and with a regime of relatively constant high summer flow (irrigation release).
Presently, low flows in the Jordan expose unvegetated river banks to prolonged drying periods which lead to sub-aerial erosion processes, and also increase the bank susceptibility to scour. Prior to clearing, the banks would have had a more extensive plant cover of herbs, grasses and mosses which retained moisture, increased the roughness of the bank which lowered flow velocities, and physically protected the bank from scour.
Historically, a portion of the flood flows would have been stored by the floodplains and slowly re-entered the main channel, extending river flow into the drier season. Vegetation on the floodplains would have protected the plains from sheet wash erosion and promoted deposition of suspended material, resulting in the continued aggradation of the floodplain. Vegetation would also have slowed the rate of water
52 return to the main channel. In the riparian zone, deeply rooted vegetation would have protected the banks during high flow events, and reduced scour by reducing water velocities and physically protecting the banks.
Returning the natural, or maintaining the present low flow regime within the catchment will not maintain the present catchment status nor enhance it. Given the condition of the riparian zone, the present processes will continue to degrade the channel, with lateral erosion a major issue. The consequences of the disruption of the basic processes and vegetation features that contribute to stability are not possible to rectify with any flow regime alone.
8.4 Flows and Herdsmans Cove
8.4.1 Features and values Herdsmans Cove, the estuary of the Jordan River, is a small, ‘daughter’ estuary of the Derwent estuary, and is characterised by a small central channel in its lower reaches adjacent to extensive silt- flats which are exposed at low tide (Figure 14). The lower section of the Cove, east of Green Point, contains an extensive area of tidal ‘seagrasses’. Vegetated mudflats and adjacent saltmarsh characterise the upper reaches, which are shaped by steeper local topography and have little exposed area at low tide, and long open water channels. Environmental conditions in the Cove are influenced by the altered nature of water quality, sediment chemistry and pollution and river flows in the Derwent estuary. Thus, management of environmental flows into Herdsmans Cove from the Jordan will only partially address the condition of the Cove.
No data existed on the morphology, hydrodynamics or salinity behaviour of the estuary. Surveys were therefore conducted of Herdsmans Cove in 2003-04. These revealed two deep basins in the upper and lower estuary, respectively, separated from each other by a shallow ‘bar’ reach, and separated by another shallow bar from the main channel of the Derwent estuary (Figure 15).
There are also no available data on the biology of Herdsmans Cove, other than recreational fishing records of bream, trout, mullet and cod (IFS unpub. data), and the presence of tidal macrophyte ( Ruppia spp.) beds in the mouth and along the margins of the lower estuary (Jordan et al. 2001). A range of waterbirds have been recorded from the estuary, including several wader species feeding or resident on the intertidal mudflats (Tas. Bird Observers, unpub. data), and both platypus and water rats have been observed in the lower and upper estuary.
8.4.2 Salinity response to flow Three longitudinal surveys were conducted in early 2004 in which depth profiles of salinity, temperature and dissolved oxygen were measured at 0.25 m intervals from the surface to just above the bottom at 13 sites (Figure 14). These surveys were timed as follows: • January 21 2004 – after prolonged period of near-zero Jordan River flows; • February 10 2004 – during the later stages of a significant flood event; • February 19 2004 – after flood recession.
53 S1 (2.0)
S2 (4.5)
S3 (5.0) S4 (5.0)
S6 (1.9) S5 (4.5)
S7 (2.0)
S8 (2.4)
S9 (4.0)
S10 (3.5)
S11 (0.9)
S13 (3.4) S12 (2.0)
Figure 14. Herdsmans Cove estuary showing tidal limit (at black bar), sampling sites (S1 – 13) and maximum depth (m) on Feb 10 2004.
54 The timing of sampling and the flood event is shown in Figure 16. Key examples of salinity profiles are shown in Figure 17.
Salinity profiles on Jan 21 indicated a slight salinity gradient, with surface salinities in the upper estuary (stations S2 and S3) being around 15 ppt, and similar to those observed in the surface waters of the lower estuary and the Derwent. Low oxygen levels (< 2 mg/l) were observed in the lower 2 m, with hydrogen sulphide odours in near-bottom water. These data indicate that prolonged low flow conditions are associated with fairly uniform saline conditions throughout the estuary and water column, probably aided by tidal exchange with and wind-driven injection of waters from the Derwent.
Profiles on February 10 showed distinct salinity gradients with sharp haloclines throughout the estuary, a gradient of increasing bottom water salinity toward the Derwent, and mixing in the upper estuary basin to ca 3 m depth.
Profiles on February 19 revealed a partial return to pre-flood event conditions, with increased surface salinities, a more uniform salinity profile in the upper estuary basin (associated with low DO conditions in bottom waters). A distinct halocline was observed again in the lower basin, site S13), which was similar to that observed in the main Derwent estuary. These data suggest a separation of upper estuary waters from those in the Derwent, with these waters returning to pre-flood conditions within a week.
It appears that the estuary is uniformly saline to brackish through periods of low to zero Jordan River flow, with salinities similar to and probably determined by the Derwent estuary. Large flood events cause the formation of a salt wedge, with well mixed low-salinity waters down to 3 – 4 m depth overlying saline water in the two main basins. The bottom waters in the upper basin become isolated from those of the lower estuary and Derwent, probably due to the combination of the effect of the two bar ‘barriers’ in the mid and lower estuary and the prevention of tidal incursion by outflowing surface waters. Under these conditions, the upper basin bottom waters can become partially to strongly deoxygenated (0 - 2 mg/l DO).
The return to pre-flood salinity conditions appears rapid, despite the observed flood peaking at > 300 cumec (at Bridgewater) and lasting for 7 – 10 days. Thus, despite a major flood event, conditions within Herdsmans Cove did not last long, returning toward a pre-flood state within ca 7 days of the flood peak.
The salinity conditions in Herdsmans Cove after the flood were tending to become similar to, and presumably ultimately determined by, the conditions predominating in the Derwent estuary (in turn determined by flows in the Derwent River).
We suggest that while floods in the Jordan have a transient effect on the hydrodynamics of Herdsmans Cove, the dominant conditions in the estuary are controlled by those of the Derwent estuary combined with the localised effects of bottom topography. Future modelling of the Derwent estuary should, ideally include Herdsmans Cove, at an appropriate spatial (model grid) scale.
55 Distance from tidal limit (km) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 E. Derw ent Cove Hill Green Hw ayBridge Rd Bridge Point 1
Derwent 2 Estuary
3 Depth (m)
4
5
6
Figure 15. Depth profile of Herdsmans Cove from the tidal limit to the centre of the Derwent estuary, data collected on 10 Feb 2004. Note the two basins within the estuary separated by shallow ‘bar’ sections.
250
200
150
Q (cumec) 100
50
0 1/01/04 8/01/04 15/01/04 22/01/04 29/01/04 5/02/04 12/02/04 19/02/04 26/02/04 Date
Figure 16. Sampling dates in Herdsmans Cove (filled circles) in relation to summer 2004 flow peak in Jordan River (Mauriceton gauge data).
56 Sal (ppt) 0 5 10 15 20 25 0
1
2
3 Depth Depth (m)
4
5
Jan 21 2004 6
S2 S3 S13 S10
Sal (ppt) 0 5 10 15 20 25 30 35 0
1
2
3 Depth Depth (m)
4
5
Feb 10 2004 6
S2 S3 S10 S13
Sal (ppt) 0 5 10 15 20 25 30 35 0
1
2
3 Depth Depth (m)
4
5
Feb 19 2004 6
S2 S3 S10 S13 Figure 17. Plots of salinity distribution at key sites in Herdsmans Cove on three dates before (Jan 21), during (Feb 10) and after (Feb 19) a flow event in 2004. Stars indicate depths below which DO was < 2 mg/l (grey) and below which hydrogen sulphide was recorded (black).
57 9. Environmental flow regime recommendations The results of field observations and interpretation of channel geomorphology and vegetation were collated. An environmental baseflow and high/flood flow regime was assessed in the light of: • relationships between geomorphological condition and flow; • relationships between vegetation condition and flow; • the role of flow in Herdsmans Cove; • the need to maintain and restore instream habitat features (pool filling and connectedness, sediment and biofilm flushing, water quality); • the need to provide a minimum set of key high flow and zero flow events.
9.1 Baseflows and Cease-to-flow events Key aspects of the non-flood flow regime of the Jordan River are: • Frequent and prolonged cease-to-flow events, especially in summer-autumn; • The occurrence of flushing flows up to ca. 2 cumec which serve to maintain pool habitat quality and connectivity, flush sediments and biofilms and periodically restore water quality.
It is apparent that the latter part of the flow regime is under stress from the current level of abstraction and farm dam storage, and that a number of key environmental values are currently degraded due to the combination of low flows, poor water quality and sediment inputs, and should be protected from further increases in takes.
It is not possible to numerically define a baseflow or cease to flow regime that can be used operationally to unambiguously assess compliance at a gauging station. This is due to the high level of variability and uncertainty in timing, duration and magnitude of low and zero flows in any year or season, especially given the current drying trend (since the 1980’s). We recommend that the natural variability and seasonality of low flows be maintained, as they are intrinsic to the ecology of the Jordan.
We therefore recommend that flows below ca. 1.5 - 2 cumec be protected from further abstraction by prevention licencing of any new takes or farm dams or the activation of existing unused ‘sleeper’ licences (for takes or farm dams) when flows are below 2 cumec at Mauriceton. This recommendation aims to preserve the existing non-flood flow regime while, ideally, further management focuses on issues of salinity, vegetation and erosion management.
9.2 High/flood flows The assessment of geomorphological and vegetation responses to the flow regime supports the need to maintain a minimum set of high/flood flows that are consistent with the criteria described in section 6.3.
The physical condition of Herdsmans Cove is, we believe, largely controlled by conditions in the Derwent estuary, especially during prolonged periods of very low to no flow in the Jordan River. Maintenance of Ruppia beds in Herdsmans Cove would
58 potentially be jeopardised by any significant increase in the incidence of low salinities in the upper 2 – 4 m of the water column in either the Derwent or Jordan.
Periodic high/flood flow events are required to flush the upper basin of Herdsmans Cove, transport organic and algal material, and stimulate fish movement and/or spawning.
The following high/flood flow events (Table 8) are recommended as a long-term minimum set to maintain key instream, riparian and estuarine processes.
Table 8. Minimum set of high/flood flow events recommended for maintenance of ecological values in the Jordan River catchment at two key gauging station sites. Long term mean peak event heights, event durations and timing (frequency and season) are indicated.
Peak ht (cumec) Duration (days) Timing Range Long term mean Mauriceton
Biennial 5.5 - 10.0 7.7 1 1 per 2 years, winter
Annual 1.0 - 3.3 1.8 1 1 per year, anytime
2 per year, spring and Trigger 0.2 - 1.1 0.6 1 autumn
Freshes 1.0 Total of 8 per year
Bridgewater
Biennial 8.2 - 15.5 11.2 1 1 per 2 years, winter
Annual 1.4 - 4.5 2.5 1 1 per year, anytime
2 per year, spring and Trigger 0.45 - 1.45 0.8 1 autumn
Freshes 1.0 - 1.5 Total of 8 per year
9.3 Conclusions The Jordan River is in a severely degraded condition due to a number of causes, only one of which is the altered flow regime. Environmental flow management must protect the river from further change to flows below ca 1.5 – 2cumec.
Provided the recommended minimum set of high/flood flow events is maintained, as a long (5 year or greater) average, and flows below 1.5 - 2 cumec are protected from any additional abstraction than is already active as of early 2005, then we believe the current environmental condition as it pertains to the effects of the altered flow regime, will be maintained.
59 We recommend a moratorium be maintained on any new applications for direct takes or farm dams within the catchment, including the activation of any ‘sleeper’ or unused but approved licences for takes or dams, unless those takes or dam licences are activated when flows at Mauriceton exceed 1.5 - 2 cumec.
Any future major dam development must: • Maintain the current summer-autumn flow pattern (ie be ‘transparent’); • Operate so as to maintain the above minimum set of high flow/flood events.
These prescriptions will not prevent ongoing environmental degradation without concerted, effective, outcome-focused and catchment-wide management action of key riparian and water quality management issues.
60 10. References Burrett, CF and Martin, EL 1989. Geological and Mineral Resources of Tasmania, Geological Society of Australia, Special Publication 15, 574pp.
Colwell, RK 1974 Predictability, constancy and contingency of periodic phenomena. Ecology 5: 1148 - 1153.
Daley, E 2001. Unpublished data cited in Ecosynthesis (2001).
Davies, PE and Humphries, P 1995. An environmental flow study of rivers of the South Esk basin. Report to Landcare. DPIF, Hobart.
Ecosynthesis 2001. Jordan River Rivercare Plan. Report to the Jordan Catchment Committee. 84 pp.
Fallon, L, Fuller D and Graham B 2000. Jordan River Flood Data Book. DPIWE Land and Water Management Branch, Resource Management and Conservation Division. Report Series WRA 00/02, May 2000. 22 pp.
Gurung, S and Dayaratne, S 2002. Hydrological Analysis of the Jordan River Catchment. A report forming part of the requirements for State of Rivers reporting. Hydrology Section, DPIWE Water Assessment & Planning Branch. Report Series WRA 99/##. April, 2002.
Hand, SJ 1993. Palaeogeography of Tasmania’s Permo-Carboniferous glacigenic sediments, Gondwana Eight, Unrun, Banks & Veevers (eds), Balkema, Rotterdam.
Jordan, A, Lawler, M and Halley, V 2001. Estuarine habitat mapping in the Derwent – Integrating science and management. Tasmanian Aquaculture and Fisheries Institute NHT Final Report, 67pp.
Nielsen DL, Brock MA, Rees GN and Baldwin DS 2003. Effects of increasing salinity on freshwater ecosystems in Australia. Australian Journal of Botany 51, 655 – 665.
North, A. 1999. Jordan River Catchment. Assessment of biological Conservation Values and identification of management priorities. Brighton and Southern Midlands Councils.
Nunez, M 2004. Tasmanian future water environments using a climate model. Report to Department of Primary Industries, Water and Environment, Hobart Tas. May 2004. 27 pp.
Poff, NL, Allan, JD, Bain, MB, Karr, JR, Prestegaard, KL, Richter, BD, Sparks, RE, and Stromber, JC 1997. The Natural Flow Regime - A paradigm for river conservation and restoration, BioScience, 47, no 11, p 769 – 784.
Read, M and Krasnicki, T 2000. Aquatic ecology of rivers in the Jordan catchment. DPIWE, Hobart. 20 pp.
61 Sharples, C 1997. A Reconnaissance of Landforms and Geological Sites of Geoconservation Significance in the Western Derwent Forest District, A report to Forestry Tasmania, Forestry Tasmania.
Sigleo, WR and Colhoun, EA 1982. Terrestrial dunes, man and the late Quaternary environment in southern Tasmania, Paleogeography, Palaeoclimatology, Palaeoecology, vol 39, p87-121.
Wilson, K and Foley, A 2003. Water Quality of Rivers in the Jordan Catchment. A Report Forming Part of the Requirements for State of Rivers Reporting. DPIWE. 107 pp.
Wilson, K, Foley, A, Krasnicki, T, Read, M, Gurung, S and Dayaratne, S 2003. State of Rivers Report for the Jordan River Catchment. Water Assessment and Planning Branch, Water Resources Division, DPIWE. Report Series; WAP 03/10. July, 2003.
Zacharek, A 1998. Botanical Report: Brighton Natural Vegetation Strategy. Greening Australia. Hobart.
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