Regional Patterns of Erosion and Sediment and Nutrient Transport in the Goulburn and Broken River Catchments, Victoria

R.C. DeRose, I.P.Prosser, L.J. Wilkinson, A.O. Hughes and W.J. Young

CSIRO Land and Water, Canberra Technical Report 11/03, March 2003

CSIRO LAND and WATER

Regional Patterns of Erosion and Sediment and Nutrient Transport in the Goulburn and Broken River Catchments, Victoria

R.C. DeRose, I.P. Prosser, L.J. Wilkinson, A.O. Hughes and W.J. Young

CSIRO Land and Water, Canberra Technical Report 11/03, March 2003

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ISSN 1446-6163

Table of Contents Acknowledgments...... 3 Abstract...... 4 Main Research Report...... 5 Background...... 5 Project Objectives ...... 8 Methods ...... 8 Sediment Delivery through the River Network...... 10 Contribution of Suspended Sediment to the Murray River ...... 12 Nutrient Delivery through the River Network ...... 13 Model Inputs ...... 13 River Hydrology and Channel Form...... 13 Hillslope Erosion ...... 15 Gully Erosion...... 16 River Bank Erosion...... 16 Nutrient Sources – Total P and N ...... 21 Disaggregation of Mean Annual Loads to Daily Loads...... 24 Results and Discussion...... 25 Hillslope Erosion Hazard ...... 25 Gully Erosion Hazard...... 25 Riverbank Erosion...... 26 Sediment Sources to the Stream Network...... 26 Nutrient Sources...... 28 Sediment Delivery through the River Network...... 28 River Suspended Loads...... 29 Bedload Deposition...... 29 Nutrient Budget...... 30 Contribution to Suspended Sediment Export to the Murray River ...... 41 Comparison of Suspended Sediment Loads...... 41 Comparison of Nutrient Loads...... 44 Disaggregation of Annual to Daily Loads...... 47 Testing of Land Use Scenarios ...... 47 Comparison with NLWRA Results...... 49 Conclusions...... 49 References...... 50

List of Figures (abbreviated titles) Figure 1: Map of the Goulburn and Broken River catchments...... 7 Figure 2: Mean annual rainfall across the Goulburn and Broken River catchments...... 9 Figure 3: A river network showing links, nodes, Shreve magnitude of each link (Shreve, 1966) and internal catchment area of a magnitude one and a magnitude four link...... 10 Figure 4: Conceptual diagram of the bedload sediment budget for a river link. STC is the sediment transport capacity of the river link, determined by Equation 1...... 11 Figure 5: Conceptual diagram for the suspended sediment budget of a river link...... 12 Figure 6: Conceptual diagram for the nutrient budget of a river link...... 13 Figure 7: Distribution of average bank heights and channel widths in relation to upslope contributing area for 48 surveyed sites...... 15 Figure 8: Predicted hillslope erosion hazard in the Goulburn and Broken River catchments...... 17 Figure 9: Average density of gully erosion for 10 x 10 km grid cells for the Goulburn and Broken River Catchments...... 18 Figure 10: Mapped amount of intact riparian vegetation...... 19 Figure 11: Predicted bank erosion...... 20 Figure 12: Pattern of dissolved N input to streams...... 22 Figure 13: Pattern of dissolved P input to streams...... 23 Figure 14: Measured floodplain width...... 31 Figure 15: Predicted depth of floodplain deposition...... 32 Figure 16: Predicted suspended sediment load compared with loads measured at gauging stations within the Goulburn and Broken catchments...... 33 Figure 17: Predicted suspended sediment load expressed per unit area...... 34 Figure 18: Modelled sediment transport capacity...... 35 Figure 19: Predicted bedload deposition...... 36 Figure 20: Predicted Total P load...... 37 Figure 21: Predicted proportion of dissolved P to Total P load...... 38 Figure 22: Predicted Total N load...... 39 Figure 23: Predicted proportion of dissolved N to Total N load...... 40 Figure 24: Predicted contribution of suspended sediment to the Murray River for sub-catchments within the Goulburn and Broken River catchments...... 42 Figure 25: Suspended sediment rating curve (left) and cumulative load distribution (right) for Delatite River at Tonga Bridge (405214)...... 43 Figure 26: Comparison of average annual suspended sediment loads predicted from SedNet with estimated loads from gauging station (Figure 16) sediment rating curves (eg. Figure 25)...... 44 Figure 27: Variation in predicted mean daily loads from April to October 1993 for the Delatite River at Tonga Bridge...... 46 Figure 28: Scenario testing in ArcMap...... 48

List of Tables (abbreviated titles) Table 1: Average concentrations of N and P in runoff from selected land uses within the Goulburn and Broken River Catchments...... 24 Table 2: Components of the sediment budget for the Goulburn Broken River Basins...... 27 Table 3: Components of the nutrient budget for the Goulburn Broken River Basins...... 27 Table 4: Comparison of nutrient concentrations and loads to predicted loads of SedNet at selected gauging stations...... 45

Acknowledgments This study forms part of the NLWRA Communications and Adoptions project for dissemination of NLWRA nutrient and sediment budget information to local catchment management agencies. It further tests the application of sediment modeling approaches to focus catchments such as the Goulburn and Broken Rivers through the use of regional data sources. As such we acknowledge the input from both the Goulburn-Broken Catchment Management Authority, Victoria Department of Natural Resources and Environment, and Goulburn-Murray Water who provided much of the necessary resource information required to complete this work. Theiss Environmental provided river cross-section information. In particular we thank Wayne Tennant and Pat Feehan who helped coordinate the collection of resource information. We also acknowledge the assistance of John Gallant of CSIRO for topographic analyses and Hua Lu who provided updated land cover factor and hillslope erosion assessments for the catchment.

We gratefully acknowledge the funding and support provided by NLWRA that made this study possible. Goulburn Broken CMA also provided funding to assist with the collection and processing of resource information.

ABSTRACT This project was carried out to identify the major processes involved in the delivery of sediment and nutrients to rivers within the combined Goulburn and Broken River catchments. The loss of sediment and nutrients from the land can have impacts downstream on the larger rivers such as the Murray River and the estuarine environments that receive this material. An essential part of minimizing the impact of sediment is to reduce losses from the landscape. In regional catchments, such as the Goulburn and Broken Rivers, there are a wide range of environments only some of which will contribute significant amounts of sediment to streams.

There are also many opportunities for deposition of sediment in the catchment so that not all areas of erosion result in export of sediment from the catchment. The project looks at patterns of sediment transport through the river network, identifying which reaches may be impacted by deposition of sand on river beds, and which sub-catchments contribute the most to suspended sediment loads and export from the river basin. We address these issues by constructing sediment and nutrient budgets for the catchment. A budget is an account of the major sources, stores and fluxes of material in a catchment.

Spatial modelling is the only practical method to assess the patterns of sediment and nutrient transport in a large complex catchment as there are only limited measurements of material transport rates. Modelling can be used to interpolate these measurements and combine them with a basic understanding of transport processes and geographical information on controlling factors. This includes mapping of soils, vegetation cover, geology, terrain, climate and measurements of river discharge. We produce maps and summary statistics of predicted surface wash erosion, gully erosion, riverbank erosion and bedload and suspended load transport across the catchments.

The model results suggest that gully erosion is the dominant erosion process contributing approximately 57% of the total predicted sediment supply. Gully erosion is the dominant sediment source in a SW – NE trending zone through the middle of the catchments. Riverbank erosion also makes a significant contribution with 36% of the total predicted supply to streams being derived from this source. Reaches, where there is a combination of poor riparian vegetation cover coupled with high stream power, produce the bulk of this sediment. Sheetwash and rill erosion contributes 7% of the total predicted sediment supply to channels. While it varies by 3 orders of magnitude, only 11% of the catchment has moderate to high surface erosion potential. Much of this is restricted to steeper slopes on grazing land or to areas of cropping.

Rapid accumulation of sand and gravel on the bed of rivers can degrade aquatic habitat. This is a significant concern in some areas of the Goulburn and Broken catchments. Accumulation of sand occurs along reaches of streams that have relatively gentle and wide channels and above which the upstream catchment area has relatively high erosion rates and sediment transport capacities to mobilize and move coarser sediments. Results presented here are considered to underestimate the extent of streams affected by sand accumulation.

The sediment budget predicts that 42% of suspended sediment and about 1% of bedload delivered to the river network in any year is exported from the river mouth. Lakes, reservoirs and floodplains predominantly along the lower sections of the Goulburn and Broken rivers, provide the greatest opportunity for deposition of suspended sediment. We predict that the mean annual export of suspended sediment to the Murray River is 132 kt y-1.

This figure lies within ± 50% of measured sediment loads based on samples collected at gauging stations along the Goulburn River. This represents a reasonable agreement given that there are significant errors involved in the estimation of mean annual loads from the generally non-event based sediment samples and the limitations of the model. The difference between predicted and observed loads could relate to underestimation of the amount of floodplain deposition or that rates of gully erosion have declined in recent decades relative to the long term average.

Each of the sediment sources described above, together with dissolved contributions from hillslope runoff and point sources, deliver nutrients to the network of streams and rivers in the Goulburn and Broken River basins. We predict that 287 t y-1 of Total P and 2326 t y-1 of Total N, are exported from the

catchment. This represents 59% of Total P and 67% of Total N supplied to streams. The higher proportion of Total N export partly reflects the higher proportion of N in a dissolved form (60 - 80%) compared with P in a dissolved form (20 – 40%) transported by the rivers. As a greater proportion of P is transported attached to sediment there are greater opportunities for deposition before reaching the river mouths and hence the proportion of Total P exported is less than that of Total N.

The spatial patterns of nutrient supply and transport in the catchments differ and this has important implications for catchment management. The pattern of Total P loads in rivers is dominated by sediment- bound sources from areas of gully and riverbank erosion. The proportion of dissolved P in rivers remains low relative to sediment-bound P despite significant inflows of dissolved P from agricultural land. In contrast, the greater levels of dissolved N supplied by runoff from agricultural areas, and in particular irrigation in lowland areas, helps to maintain a high proportion of dissolved N in rivers despite significant inputs of sediment-bound N and denitrification occurring. Hence, while reducing the supply of sediment to streams will go a long way to reducing Total P supply, major reduction in the supply of Total N will not be achieved without attention being payed to dissolved sources as well.

Results from the sediment and nutrient budget for the Goulburn and Broken Rivers have strong potential for guiding further investigation, identifying areas for improved management and setting targets for catchment restoration. The results predict that each erosion process (surface wash, gully erosion, and riverbank erosion) is significant and that these processes are highly focused, with much of the sediment and attached nutrients being generated from relatively small areas. If future efforts at minimising soil loss are targeted towards these hotspots, using management guidelines appropriate to the type of process, then a large benefit in reduced sediment and nutrient loads downstream can be achieved with comparatively less effort.

As part of this project, a catchment scenario testing tool has been developed to assist extension providers and natural resource management agencies to investigate the relative effectiveness of different management strategies on long-term sediment loads and yields from river networks. In the ArcGIS environment, users can select an individual or group of watersheds and interactively change the attributes which effect sediment supply to streams. By rerunning the sediment model for this new land use scenario, the user can then see how these modifications will alter the sediment loads further downstream and finally sediment export to the coast. This scenario tool will help to maximise use of the limited resources that might be available at obtaining the best result for minimising sediment and nutrient export from the catchment.

MAIN RESEARCH REPORT

Background A significant aspect to achieving ecologically sustainable land management is to ensure that the downstream impacts of land uses on streams are minimised. An essential part of minimising impact is to reduce the delivery of sediments and nutrients from land to streams. For many catchments with low input farming systems, the bulk of the nutrient load is transported attached to sediment so that sediment and nutrient transport are intimately linked.

To put a particular land use or sub-catchment in context with the regional catchments in which it occurs requires us to conceptualise the critical sources, transport pathways and sinks of sediment and nutrient in a catchment. We need to identify where sediment and nutrient are derived from, where they are stored within the catchment, and how much is delivered downstream to rivers and the sea. To quantify sources, stores and delivery is to construct a material budget for a catchment or any part of a catchment. This is a critical step to conceptualise the context of land use in a large regional catchment and to focus more detailed studies on the areas of greatest potential impact. To date only a few regional studies of sediment and nutrient budgets have been undertaken.

Most catchments are complex systems, often with considerable variation in land use pressures, and diverse topography, soils, rainfall and vegetation cover. Thus before changing any particular management or even undertaking remediation measures we need to determine its significance and the 5

spatial pattern that land uses impact for sediment and nutrient transport. We also need to put the more detailed investigations of other parts of this project in a broader regional environmental context for the results to be applicable across wider areas.

Some parts of the landscape are inherently more at risk of increased erosion and sediment and nutrient transport than others. It is important to identify these areas for these will be the sites that require the most careful management to ensure a sustainable future. For example, some landscapes have inherently poor soils where grass cover is susceptible to dramatic and long-lasting decline when subjected to grazing pressure or drought. Other factors that contribute to inherent risk of sediment and nutrient delivery to streams include steep slopes, high channel density, and high rainfall erosivity.

Sediment and nutrients are derived from a number of processes which include: • Runoff on the land, termed surface wash and rill erosion or alternatively hillslope erosion; • Erosion of gullies formed as a result of land clearing or grazing; • Erosion of the banks of streams and rivers; • Diffuse dissolved losses of nutrients; • Point sources for nutrients such as towns and industry. In many cases one process dominates the other in terms of delivering sediments and nutrients to streams, and the predominant process can vary from one part of a large catchment to another. Management aimed at reducing sediment and nutrient transport will target each process quite differently. For example, stream bank and gully erosion is best targeted by managing stock access to streams, protecting vegetation cover in areas prone to future gully erosion, revegetating bare banks and reducing sub-surface seepage in areas with erodible sub-soils. Surface wash erosion is best managed by promoting consistent groundcover, maintaining soil structure, promoting nutrient uptake and promoting deposition of eroded sediment before it reaches the stream. Consequently it is quite important to identify the predominant sediment and nutrient delivery process before undertaking catchment remediation or making recommendations for changed grazing practice.

Sediment delivered to streams has several potential downstream impacts. High loads of suspended sediment, the silts and clays that are carried in the flow, degrade water quality in streams, reservoirs and estuaries. This is a result of both the sediment itself and the nutrients that the sediment carries. High concentrations of suspended sediment reduce stream clarity; inhibit respiration and feeding of stream biota; diminish light needed for plant photosynthesis; make water unsuitable for irrigation and require treatment of water for human use. Much of the sediment and nutrient is deposited on floodplains, providing fertile alluvial soils, or it is deposited in reservoirs. The extent of this deposition is highly variable from one river reach to another. Deposition potential must be considered when trying to relate catchment land use to downstream loads of sediment.

The formation of gullies and accelerated erosion of stream banks can supply large amounts of sand and gravel to streams. These are transported as bedload, being rolled and bounced along the bed of streams. Where streams are unable to transmit the load of sand and gravel downstream, it is deposited, burying the bed, and in extreme examples forming sheets of sand referred to as sand slugs (Rutherfurd, 2000). Sand slugs are poor habitat. They can prevent fish passage, they fill pools and other refugia, and are unstable substrates for benthic organisms (Jeffers, 1998).

A reconnaissance level sediment budget for the Goulburn and Broken River catchments will provide an understanding of the critical processes of sediment and nutrient transport that can lead to downstream impact. It will place the major land uses within a regional context. The budget will also identify sub- catchments with the greatest potential for downstream impact on aquatic ecosystems. These are the first steps toward better targeting of remedial and land conservation measures.

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Figure 1: Map of the Goulburn and Broken River catchments.

Project Objectives This report constitutes the final phase of the National Land and Water Resources Audit (NLWRA; 2001) Communications and Adoptions Project on extending nutrient and sediment budgets to state agencies, regional resource managers and community groups. Specific objectives are:

• To use the best available regional data sources and improve techniques to get a more accurate result than the NLWRA national results for the Goulburn and Broken River catchments.

• To compare results with monitored sediment and nutrient loads.

• To examine the time sequence of loads and demonstrate how mean annual loads can be disaggregated into daily loads.

• To develop the ability to examine future management scenarios and their affect on sediment and nutrient export from catchments.

Methods The Goulburn and Broken catchments together cover some 22 800 km2 (Figure 1). Main outflows to the Murray River occur at the mouths of the Goulburn River and Broken Creek. Streams from an area of approximately 900 km2 to the west of Waranga Basin terminate in a number of small lakes. In addition, streams from a 1000 km2 portion of the Shepparton Irrigation District discharge flow into the neighbouring Campaspe Catchment via Waranga Channel before flowing into the Murray. Therefore about 8% of the catchment does not contribute to the sediment and nutrient loads of the Goulburn River or Broken Creek.

The only practical framework to assess the patterns of sediment and nutrient transport across a large complex area such as the Goulburn and Broken River catchments (Figure 1) is a spatial modelling framework. There is a large range of climate (eg., Figure 2), topography, and land use which can strongly affect erosion and sediment transport. There are few direct measurements of sediment transport in regional catchments, and it is unrealistic to initiate sampling programs of the processes now and expect results within a decade. Furthermore, collation and integration of existing data has to be put within an overall assessment framework, and a large-scale spatial model of material transport is the most effective use of that data.

The assessment of sediment and nutrient transport is divided into four aspects: hillslope erosion as a source of sediment and attached nutrients; hillslopes as a source of dissolved nutrients; gully erosion as a source of sediment and nutrients; and river links as a further source, receiver and propagator of the sediment and nutrients.

To calculate the supply of sediment, its deposition and its delivery downstream is to construct a river sediment budget. We calculated budgets for two types of sediment: suspended sediment and bedload. A suite of ArcInfoTM programs were used to define river networks and their sub-catchments; import required data; implement the model; and compile the results. The programs are referred to collectively as the SedNet model: the Sediment River Network model.

The SedNet model calculates, among other things:

• The mean annual suspended sediment output from each river link; • The depth of sediment accumulated on the river bed in historical times; • The relative supply of sediment from surface wash, gully and bank erosion processes; • The mean annual rate of sediment accumulation in reservoirs; • The mean annual export of sediment to the Murray River; • The contribution of each sub-catchment to that export.

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Details of the model and its application to regional catchments in Australia are given in Prosser et al., (2001). The methods used in the construction and implementation of the SedNet programmes are described in detail in a number of CSIRO technical reports which are available at http://www.clw.csiro.au/publications/technical2002/ or through contacting the authors. Consequently only a brief overview of the model is included here and in particular where this regional application differs from the previous NLWRA work.

Sediment Delivery through the River Network The basic unit of calculation for constructing the sediment budgets is a link in a river network. A link is the stretch of river between any two stream junctions (or nodes; Figure 3). Each link has an internal sub- catchment, from which sediment is delivered to the river network by hillslope and gully erosion processes. The internal catchment area is the catchment area added to the link between its upper and lower nodes (Figure 3). For the purpose of the model, the internal catchment area of first order streams is the entire catchment area of the river link. Additional sediment is supplied from bank erosion along the link and from any tributaries to the link.

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2 1

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Figure 3: A river network showing links, nodes, Shreve magnitude of each link (Shreve, 1966) and internal catchment area of a magnitude one and a magnitude four link.

Sediment is processed sequentially through the river network beginning with first order links and terminating at the basin outlet (commonly the ocean or a major river such as the Murray River). The sediment load (yield at the outlet) for each link is calculated from the supply of sediment from tributary links and the local watershed, less losses through floodplain deposition (fine sediment), bed deposition (coarse sediment), and reservoir deposition (coarse and fine sediment).

The branching network of streams for the Goulburn and Broken catchments (Figure 1) was built from a 20 m DEM (digital elevation model) (source: DNRE). The DEM was found to contain significant errors which were particularly evident in the gently sloping lowlands and caused significant misalignments of streams. Consequently a technique was developed to edit DEM elevations where errors occurred in order to generate the correct river network. The river network was defined as beginning at a catchment area of 20 km2. This area was selected to limit the number of links across the assessment area, while providing a good representation of the river network. The physical stream network extends upstream of the limit in most areas and these areas are treated as part of the internal catchment area contributing material to the river link. 10

Two anabranches occur along the Broken River from which flow is diverted down streams into the Broken Creek which flows separately into the Murray River. These anabranches are edited into the river network. For each anabranch a distributary ratio is assigned between 0 and 1 according to the portion of mean annual flow that is diverted down the anabranch. This ratio is determined from gauging records of daily flow. The sediment and nutrients loads entering the anabranching node are also split according to this ratio.

The coarse sediment (bedload) budget is illustrated in Figure 4. The main aim of the bedload budget is to predict the formation of sand slugs. These are predicted to occur when there is an excess of sediment supply to a river link beyond the capacity of the link to transport bedload. This is known as the sediment transport capacity (STC) and is based on Yang’s (1973) relationship to unit stream power (Equation 1).

1.3 1.4 86S x Qx STC = ∑ (1) x ω 0.4 wx

Riverbank Gully erosion (t/y) erosion (t/y)

Tributary supply (t/y) STC (t/y)

Downstream yield (t/y)

If loading < capacity If loading > capacity capacitycpacitycapacity• no deposition • deposit excess • yield = loading • yield = capacity

Figure 4: Conceptual diagram of the bedload sediment budget for a river link. STC is the sediment transport capacity of the river link, determined by Equation 1.

Sediment transport capacity is a function of the river width (wx), slope (Sx), discharge (Qx) settling velocity 1.4 of the bedload particles (ω) and hydraulic roughness of the river. ΣQx represents mean annual sum of daily flows raised to a power of 1.4 (Ml1.4 y-1). Using Yang's (1973) equation, and an average value for Mannings roughness coefficient of 0.025, enabled prediction of STC in a river link (t y-1). The value of ω was determined for particles with a mean diameter of 2 mm, being the average size observed for sediment slug deposits (Rutherfurd 1996). All coarse sediment entering reservoirs and lakes is deposited.

The suspended sediment budget is illustrated in Figure 5. The main aim of this budget is to predict the export of suspended sediment after loss of sediment on floodplains and in lakes and reservoirs. Sediment deposition in reservoirs is calculated in the model as a function of the mean annual inflow into the reservoir and its total storage capacity (Heinemann, 1981). For the Goulburn and Broken catchments the extent of floodplain alongside rivers was determined from a detailed floodplain map equivalent to a 1 in 25 year flood event (FLOODWAY25, source DNRE and Goulburn-Broken CMA).

A relatively simple model of floodplain deposition is implemented in SedNet. Floodplain deposition in this case is simply the proportion of sediment that goes overbank and settles out during a typical flood. It is calculated as the ratio of the median overbank flow above bank full discharge multiplied by the proportion of sediment that would be expected to settle out during overbank flow (see Figure 5). Particle settling is a function of the residence time of water on the floodplain. The longer that water covers the floodplain the greater the proportion of the suspended load that is deposited. The residence time of water on

floodplains increases with floodplain area and decreases with floodplain discharge. This simple model of floodplain deposition assumes a uniform sediment concentration and that the majority of suspended sediment is transported at times of high river flow.

Hillslope erosion (t/y) Riverbank Gully Tributary supply (t/y) erosion (t/y) erosion (t/y) HSDR

Floodplain Af Downstream yield (t/y)  vA   − fx  Q  Q = fx  −  fx  D x I x 1 e Q  tx  

Figure 5: Conceptual diagram for the suspended sediment budget of a river link. HSDR is hillslope sediment delivery ratio. The equation is for the amount of sediment deposited on the floodplain (t/y), where Ix is the sediment load input to the link, Qfx/Qtx is the proportion of flow that goes overbank, Afx/Qfx is the ratio of floodplain area to floodplain discharge and ν is the sediment settling velocity.

Contribution of Suspended Sediment to the Murray River The differentiation of sub-catchments which contribute strongly to total river sediment export is an important aspect of catchment management as this enables catchment managers to target areas for rehabilitation. It is not always possible, or sensible, to implement erosion control works effectively across large areas.

Not all suspended sediment delivered to rivers is exported from the basin as there are extensive opportunities for floodplain deposition along river courses. There are usually strong spatial patterns in sediment delivery to basin outlets because some tributaries are confined in narrow valleys with little opportunity for deposition, while others may have extensive open floodplains. There will also be strong, but different patterns in sediment delivery to streams. Thus a map of contribution to export may be very different to a map of erosion.

The contribution that each sub-catchment makes to sediment export can only be calculated once the mean annual suspended sediment export is known. The sub-catchments are the link internal areas described in Figure 3. The method tracks back upstream calculating from where the sediment load in each link is derived. The calculation takes a probabilistic approach to sediment delivery through each river link encountered on the route from source to sea.

Each internal link catchment area delivers a mean annual load of suspended sediment (LFx) to the river network. This is the sum of gully, hillslope and riverbank erosion delivered from that sub-catchment. The sub-catchment delivery and tributary loads constitute the load of suspended sediment (TIFx) received by each river link. Each link yields some fraction of that load (YFx). The rest is deposited. The ratio of YFx/TIFx is the proportion of suspended sediment that passes through each link. It can also be viewed as the probability of any individual grain of suspended sediment passing through the link. The suspended load delivered from each sub-catchment will pass through a number of links on route to the catchment mouth. The amount delivered to the mouth is the product of the loading LFx from the sub-catchment and the probability of passing through each river link on the way:

YF YF YF = x x+1 n COx LFx x x x...... x (2) TIFx TIFx+1 TIFn where n is the number of links on the route to the outlet. Dividing this by the internal catchment area expresses contribution to outlet export (COx) as an erosion rate (t/ha/y). The proportion of suspended sediment passing through each river link is ≤ 1. A consequence of Equation 2 is that all other factors being equal, the further a sub-catchment is from the mouth, the lower the probability of sediment reaching the mouth. This behaviour is modified though by differences in source erosion rate and deposition intensity between links.

Nutrient Delivery through the River Network The nutrient budget model (Annual Network Nutrient Export - ANNEX) predicts the average annual loads of phosphorous and nitrogen in each link in a river network in a similar way to SedNet, with which it is run in conjunction (see Young et al., 2001 for model details). The model considers only the physical (not biological) stores of nutrients in the river system, and is also primarily concerned with the physical nutrient transport processes. It does, however, consider denitrification - a biological process resulting in loss of N to the atmosphere, and phosphorous adsorption-desorption, a physical process influenced by biological activity.

The main source terms are hillslope erosion, gully erosion, riverbank erosion, dissolved loads in runoff water and point sources (Figure 6). As with SedNet, the model then routes nutrient loads through the river network estimating the losses associated with floodplain and reservoir deposition and in stream denitrification.

Land use map Hillslope erosion (t/y)

Nutrient Soil nutrient conc Riverbank Gully concentration (kg/t) erosion (t/y) erosion (t/y) (kg/ML)

HSDR, nutrient Runoff volume Bank nutrient conc. (kg/t) enrichment (ML/y) Point sources

Deposition, P equilibration, Tributary yield (t/y) denitrification

Downstream yield (t/y)

Figure 6: Conceptual diagram for the nutrient budget of a river link. HSDR is hillslope sediment delivery ratio.

Model Inputs

River Hydrology and Channel Form SedNet combines a number of hydrological parameters into the calculation of river sediment budgets. As such, the correct representation of river hydrology is important for routing sediment and nutrients through 13

the river network. The parameters need to be predicted (interpolated) for each river link across the river basin.

The variables used are:

• The mean annual flow (Qa);

• The median daily flow (Qmd) used in the nutrient budget;

• The mean annual sum of Q1.4 for calculating mean annual sediment transport capacity;

• The bank full discharge (Qbf);

• A representative flood discharge for floodplain deposition (in this case median overbank flow – Qob).

Values for these variables were derived from the time series of daily flows for 26 gauging stations (source: Victorian Water Resources Data Warehouse) within the Goulburn Broken catchments. In general, only gauging stations with reasonably long discharge records (ie., > 20 years) were selected for analysis. Each hydrological variable was regionalized across the catchments by developing a simple empirical rule with catchment area (A in km2) and mean annual rainfall (R in mm) using standard linear least-squares fits on log transformed data points.

Gauging stations lying on streams with unregulated flow conditions (eg., no major extractions, or reservoirs occur upslope) were used to determine the regionalizations for unregulated flow as below: = × −5 × 1.087 × 2.205 2 Qa 3.56 10 A R R = 0.97 1.4 < = × −5 × 1.515 × 2.346 2 ∑Q ( Qbf ) 1.531 10 A R R = 0.97 = × 0.964 × 0.112 2 Qbf 8.65 A R R = 0.92 = × 0.872 2 Qob 6.44 A R = 0.80 = × −14 × 1.125 × 4.296 2 Qmd 1.413 10 A R R= 0.89

It was necessary to also predict regulated flow parameters for river links below Lake Eildon (Figure 1) and Lake Nagambie as both have a major impact on river flows. This was undertaken by back-calculating the effective catchment area that would be required to give the observed flow parameter at each gauging station using the regionalizations for unregulated flow. The average effective reduction in catchment area is then calculated for reaches with similar flow regulation prior to interpolation of the respective flow parameter within SedNet. For example, the effective reduction in catchment area required to produce the 2 observed (regulated) mean annual flows (Qa) along the Goulburn River decreased from 1050 km below Eildon, to 7420 km2 below Nagambie, to 10240 km2 from Broken Confluence to the main outlet on the Murray River.

Gauging records were also used to determine distributary ratios along the Broken River. In this way approximately 10% of the mean annual flow of the Broken River above Casey Weir (near to Lake Mokoan) is diverted down the Broken Creek channel. A second distributary occurs further down Broken River at Gowangardie Weir where flows enter Pine Lodge Creek. No flow data was available for this distributary and as such a default value of 1% was assumed.

The calculations of bank full discharge and median overbank flow are based on the average flood recurrence interval determined from the time series of daily flows recorded at rated gauging stations and existing bank heights. A total of 11 cross-sections were examined across the catchment and these indicated a bank full discharge ranging from 0.5 to 20 years and averaging 5 years.

70 y = 6.0367x0.2591 60 R2 = 0.6605 50

Channel width width (m) Channel 20

0 1 10 100 1000 10000 100000

10 y = 1.3258x0.1521 R2 = 0.3646 8

4 Bank height (m) height Bank

0 1 10 100 1000 10000 100000 Contributing area (km2) Figure 7: Distribution of average bank heights and channel widths in relation to upslope contributing area for 48 surveyed sites.

The channel characteristics of width and bank height are used in SedNet for a number of calculations (eg., bank erosion, sediment transport capacity, flood frequency). It is therefore necessary to estimate channel form for all river links. This is done by regionalizing point measurements of bank height and channel width on the basis of upslope contributing area. This procedure produces an average estimate which is applied equally to all stream links on the basis of contributing area. Coefficients of variation of 71 and 81% for bank height and channel width, respectively, demonstrate how spatially variable channel form is.

For the Goulburn and Broken Rivers, surveyed cross-sections were used to assess bank height and channel width at points along the river network (Figure 7). These were obtained from surveys taken at 19 of the river gauging stations (Theiss Environmental) together with a further 29 very detailed channel surveys undertaken by Goulburn Broken CMA. In the case of the more detailed surveys bank heights and channel widths represent the average of measurements taken over the length of the channel surveyed.

Hillslope Erosion Hillslope erosion from sheet and rill erosion processes is estimated using the Revised Soil Loss Equation (RUSLE; Renard et al., 1997) as applied in the NLWRA (Lu et al., 2001). The RUSLE calculates mean annual soil loss (Y, tonnes ha-1 y-1) as a product of six factors: rainfall erosivity factor (R), soil erodibility factor (K), hillslope length factor (L), hillslope gradient factor (S), ground cover factor (C) and land use practice factor (P):

Y = RKLSCP (3) 15

For soil erodibility (K) and rainfall erosivity (R) we used the NLWRA data (Lu et al., 2001). The length and slope factors (L, S) across the Goulburn and Broken River catchments were derived directly from the high resolution 20 m digital elevation model (DEM). Improved regional land use data (area with annual rainfall > 650 mm only) was also used in the calculation of mean monthly cover factors. Land use codes in this survey were reassigned to one of 20 groups for assessment of subfactors before calculation of the resultant soil loss estimates (Figure 8).

The delivery of sediment to streams from sheet and rill erosion on hillslopes is modified by the hillslope sediment delivery ratio (HSDR). HSDR is determined by calibration of hillslope erosion from runoff plots against stream sediment yields. A average value of 5% was found in the NLWRA to be typical of hillslopes across the region covered by the Goulburn Broken catchment and this was applied to all stream links and watersheds in the present study. All sediment produced by sheet and erosion is assumed to contribute to the suspended sediment load of rivers.

Gully Erosion The spatial pattern of gullies in the Goulburn and Broken catchments was derived directly from a gully map of Victoria produced by Lindsay Milton and others in the 1960’s (Ford et al., 1993). Measured gully lengths were converted to average densities (km km-2) for grid cells of 10 x 10 km2 (see Figure 9) before implementation in SedNet. The methods used in generating average gully densities from gully maps are described in Hughes and Prosser (2002).

In SedNet, gully densities are converted to a sediment supply (kt y-1) by multiplying gully density by sub- catchment area (km2), average gully cross-sectional area of gullies (10 m2), average dry bulk density of eroding materials (1.5 t m-3), and then dividing by the time over which gullies have been active. In this case the gullies are assumed to have been active since the mid 1850’s (ie., a period of 150 years) when many of the gullies developed as a consequence of land use development and forest clearance. Details of the assumptions used in the calculation of erosion rates can be found in NLWRA technical reports.

Sediment generated from gullies contributes to both suspended and bedload sediment. Because of the coarse texture of many of the soil types where gullies occur in the Goulburn and Broken catchments (ie., granitic terrain) we have assumed that on average 70% of eroding sediment contributes to bedload and 30% contributes to the suspended sediment load.

River Bank Erosion River bank erosion is modeled as there are generally few direct measurements of bank erosion over the length of individual river reaches. A global review of river bank migration data (Rutherfurd, 2000) -1 suggested the best predictor of bank erosion rate (BE; my ) to be bank full discharge (Q1.58) equivalent to a 1.58 recurrence interval flow. It was, however, found that this model overestimated the amount of bank erosion along the lower reaches of the Goulburn River by calibration with river loads measured at gauging stations, and by comparison with descriptive reports.

ρ Rutherfurd also found a significant relationship between bank erosion and stream power ( gQx S x ) -1 where p is the density of water, g is the acceleration due to gravity, Qx is the mean annual flow (ML y ) and Sx is the energy slope normally approximated to channel gradient. Subsequently a model based on stream power was used to estimate bank erosion for the Goulburn and Broken catchments (Equation 4).

− = × ρ × × × − − 0.008Fx BE 0.00002 g Qx S x (1 PR)(1 e ) (4)

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Hillslope Erosion (t/ha/y) 0 - 0.1

0.1 - 0.5

0.5 - 1

1 - 5

5 - 10

> 10 Kilometers . 05010025 Figure 8: Predicted hillslope erosion hazard in the Goulburn and Broken River catchments.

NATHALIA KATAMATITE ´

SHEPPARTON TATURA

BENALLA MURCHISON

EUROA NAGAMBIE

SEYMOUR MANSFIELD

ALEXANDRA YEA EILDON KILMORE JAMIESON

BUXTON

Legend Gully Density (km/km2) 0 - 0.01

0.01 - 0.1

0.1 - 0.5

0.5 - 1

Kilometers 05010025 Figure 9: Average density of gully erosion for 10 x 10 km grid cells for the Goulburn and Broken River Catchments.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend % Riparian Vegetation 0 - 0.20 0.21 - 0.40 0.41 - 0.60 0.61 - 0.80 0.81 - 1.00

Kilometers 05010025 Figure 10: Mapped amount of intact riparian vegetation.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Bank Erosion (m/yr) 0.000 - 0.001 0.002 - 0.01 0.01 - 0.05 0.05 - 0.1 > 0.1

Kilometers 05010025 Figure 11: Predicted bank erosion.

Bank erosion is modified by the proportion of riparian vegetation (PR) and floodplain width (Fx). Where there is 100% riparian vegetation cover then no bank erosion occurs. Similarly for very narrow or absent floodplains then the model produces little or no bank erosion. This is component of the model is necessary to take into consideration narrow or rocky gorges where there is often little bank erosion due to the predominance of resistant bedrock materials.

The average proportion of riparian vegetation (Figure 10) was assessed from a combination of the Victorian In-Stream Condition (ISC) Index (data supplied by the Department of Natural Resources and Environment) and the land use cover map of the catchment which contained mapped areas of Crown Frontages and Parks and Reserves. In these areas riparian vegetation is considered to be intact (ie., 100%).

The predicted bank erosion rate (Figure 11) is converted into sediment supply (kt y-1) by multiplying BE by channel length (m), bank height (m), average particle density of bank materials (1.5 t m-3) and dividing by a conversion factor of 1000. Sediment generated from bank erosion contributes to both suspended and bedload sediment supply. In view of the lack of spatial information about bank particle size distributions, a 50:50 split was assumed as per the NLWRA for contribution to the fine and coarse sediment budgets

Nutrient Sources – Total P and N The nutrient load from hillslope erosion is calculated as the product of the hillslope sediment yield (hillslope erosion x HSDR) multiplied by the nutrient concentration of this load (NC). The nutrient concentration of the sediment load is determined from the proportion of clay and nutrient concentration of the bulk soil (SC). ANNEX uses a two-part mixing model that assumes all nutrients are associated with the clay fraction. For internal catchment links where the percentage clay is greater than the HSDR, all sediment delivered to the channel is assumed to be clay. The nutrient concentration is then the bulk soil concentration divided by (‘enriched’) the proportion of clay (Cp) in the hillslope soil (Equation. 5).

SC For Cp > HSDR, NC = (5) Cp

In the unlikely cases where the proportion of clay is less than the HSDR, only a portion of the delivered sediment is clay and so the nutrient concentration (Equation 5) is reduced by the ratio of the proportion of clay to the HSDR. Data on soil clay proportions and nutrient concentrations for P and N were extracted from the Australian Soil Resource Information System (Bui et al., 2001).

The loads from riverbank and gully erosion are calculated as the product of their respective sediment yields times the soil nutrient concentration, which for phosphorous was taken to be 0.25 g kg-1 and for nitrogen 1 g kg-1.

Estimation of dissolved loads due to runoff differs from that used in the NLWRA project. In this instance, the dissolved load of a sub-catchment is determined as the product of the mean nutrient concentration in runoff multiplied by the mean annual volume of runoff. The volume of runoff represents the increase in discharge between the inlet and outlet of each stream link. The nutrient concentration is the average of area weighted nutrient concentrations for the dominant land uses within the sub-catchment area (Figures 12 and 13). Since an average concentration in runoff is used in the model, no distinction is made between surface and subsurface runoff volumes.

The concentration of soluble N and P was assessed from gauging station records (source: Victorian Water Resources Data Warehouse) for regions with dominant land uses (Table 1). Data for major drains in irrigation areas comes from a 5 year monitoring survey (source: Goulburn Murray Water, 2002). Nutrient loads in urban areas was assessed from storm water runoff (Wong et al., 2000).

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Dissolved N input (ug/L) 0 - 100

100 - 500

500 - 1,000

1,000 - 1,500

> 1500

Kilometers 05010025 Figure 12: Pattern of dissolved N input to streams.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Dissolved P input (ug/L) 0 - 10

10 - 50

50 - 100

100 - 500

500 - 1,000

Kilometers 05010025 Figure 13: Pattern of dissolved P input to streams.

Table 1: Average concentrations of N and P in runoff from selected land uses within the Goulburn and Broken River Catchments.

Land use Percentage of Soluble N Soluble P catchment concentration concentration (ug L-1) (ug L-1) Forests 30 287 4 Grazing 52 510 8 Rural/Residential 1 800 8 Cropping 5.6 500 22 Non-irrigated dairy 0.05 700 210 Irrigated fruit/veges 0.3 2250 250 Irrigated crops 1.3 1350 320 Irrigated improved pasture 4 1125 500 Irrigated diary 0.04 600 512 Irrigated annual pasture 3.5 1400 750 Urban 0.4 3450 605

Point sources can also produce significant inflows of nutrients to rivers. For this reason Total N and P in loads in the effluent discharge from sewage treatments plants needs to be taken into consideration. Within the Goulburn and Broken catchments a total of 13 significant points sources were identified (Goulburn-Broken Water Quality Working Group Report, 1995). The load from each point source was treated as an input to the stream link nearest the respective town locality. Measured loads for point sources ranged from 1500 to 96400 kg yr-1 for Total N and from 430 to 22300 kg yr-1 for Total P.

Disaggregation of Mean Annual Loads to Daily Loads Ecologists often consider daily sediment and nutrient loads to be of more importance than mean annual loads for assessing the impacts of water quality on rivers and estuaries. Estimates of mean annual loads produced by SedNet can be disaggregated into mean daily loads if the relationship (ie., rating curve) between concentration and flow is known for river links.

Within the Goulburn and Broken catchments suspended sediment has been sampled at many of the gauging stations since the early 1970’s although sampling tends to me more comprehensive over the last decade. In most cases these are non-event based samples taken on an irregular basis. Only those sites having a reasonable sample size and coverage in terms of the range of flow conditions were subsequently selected for the development of rating curves. These included 14 on unregulated streams and a further 10 stations along regulated channels.

A modified form of the standard rating curve (Equation 6) was adopted to determine the relationship between suspended sediment concentration (C) and daily flow (Qd).

= + b C c aQd (6) where a, b, are coefficients determined from linear regression of log transformed data and c is a coefficient representing the mean concentration at low flow. The coefficients a and b were determined from samples taken at high flows only. It was found necessary to exclude concentrations taken at low flows from the regression as they caused significant bias and underestimation of the predicted suspended sediment concentration at high flows.

Mean daily load (Ld) passing each gauging station is represented by Equation 7.

= × + × (1+b) Ld c Qd a Qd (7)

Since the first term in this equation contributes little to total load, then the mean annual load (La) can be represented as the sum of daily loads for the time series of daily flows (Equation 8).

i=n 365 (1+b) = (8) La ∑aQdi n i=1 where n is the number of days for which flows have been recorded. Hence, in the case where the exponent b has been determined from rating curves (via Equation 6), then the coefficient a can be calculated using Equation 9. This equation enables the disaggregation of mean annual loads predicted by SedNet into mean estimated daily loads for all stream links in the river system. × = La n a i=1 (9) (1+b) 365∑Qdi i=n

Routine monitoring of Total N and Total P has only been conducted since the mid-1990’s and of these too few samples have been taken at high enough river flows to construct reliable concentration flow rating curves. Present sampling records suggest there is no significant variation in Total N or P for the range of sampled flows at most gauging stations. Consequently, the concentrations of Total N and P were treated as being constant for the range of river flows. Sampling records at some gauging stations showed significant increases in nitrates and nitrites with increasing flow suggesting the possibility of developing rating curves through the separate treatment of dissolved and sediment attached components. This is, however, outside the scope of the present study.

Results and Discussion

Hillslope Erosion Hazard The pattern of hillslope erosion (Figure 8) shows that the majority (89%) of the Goulburn and Broken catchment has low soil loss rates of < 0.5 t ha-1 y-1. The areas with soil loss rates > 0.5 t ha-1 y-1 relate primarily to areas of high slope on agricultural lands. Comparatively few areas have high erosion rates exceeding 5 t ha-1 y-1 and these usually relate to steep slopes on areas mapped as grazing land. Some of the land lying to the east of Seymour and south of Euroa falls into this category. Elsewhere areas with high erosion rate relate to cropping land such as to the east of Shepparton and north of Benalla. Overall, however, cropping land is predicted to have relatively low sheet and rill erosion rates due to the low rainfall and negligible slope of land in areas where cropping is undertaken. A number of spots had very high erosion rates > 50 t ha-1 y-1 and these are considered to relate to minor errors in the grid analysis.

It was found that about 1.04 x 106 tonnes of soil is moved annually on hillslopes in the Goulburn and Broken catchments. This equates to an average soil erosion rate of 0.46 t ha-1 y-1, about half the previous estimate of the National Land and Water Resources Audit (NLWRA) project. This reflects the improved quality and accuracy of input information to USLE factors such as the regional land use map, and improved slope angle and slope length measurements derived from the 20 m DEM.

The values of hillslope erosion represent local movement of soil on hillslopes. By applying the 5% HSDR discussed earlier, the total input of sediment from sheet and rill erosion to streams is estimated to be about 52 kt y-1.

Gully Erosion Hazard The spatial pattern of gully density (Figure 9) shows that extensive areas of the Goulburn and Broken catchments have undergone gully erosion in the past. Overall, 37% of the catchment area has an average gully density exceeding 0.1 km km-2 while 38% has little or no gully erosion. The highest density of gullies is in areas to the west and south west of Seymour and in areas trending northeast from Seymour through to the area near Benalla. In these areas, gully densities are typically between 0.1 and 1 km km-2. Gully densities rarely exceed 1 km km-2 .

A typical gully is considered to have on average an eroded depth of 2 m and a width of 5 m giving a total sediment yield of 10,000 m3 per kilometre of channel. Thus, areas with a moderate gully density (0.5 km km-2) would have produced a total sediment yield of approximately 75 t ha-1 , equivalent to 0.5 t ha-1 y-1 assuming a sediment density of 1.5 t m-3 and 150 years of activity. While this amount is similar to the average amount of sediment produced by hillslope erosion, the main difference is that all sediment produced by gullies, enters the river network.

Gully erosion removes 4.36 x 105 tonnes of soil annually from watershed areas in the Goulburn and Broken catchments. This is 75 % of the level of gully erosion estimated for the NLWRA.

Riverbank Erosion The lower Goulburn River is known to have undergone significant lateral migration in historical times with locally high erosion rates of up to 0.5 m yr-1 and an average rate for reaches of about 0.05 m yr-1 (Erskine et al., 1993). The combined effect of reduction in flood peaks through construction of dams and water offtakes and improved riparian condition along some reaches of the river are consistent with lower rates of bank erosion observed in recent decades. Some sections of the river now show little or no bank erosion.

One of the two main factors controlling riverbank erosion in the model is the extent of riparian vegetation as shown in Figure 10. In general the majority of the river network has a reasonable riparian condition with better than 40% riparian cover. Poorest riparian cover tends to occur in an area running from the southwest of Seymour through Euroa to the northeast of the catchment. Here average riparian cover along streams lies in the range 20 to 60%. Areas with good riparian cover (>60%) include forested watersheds in the south east and well established riparian margins along the main rivers, particularly below Shepparton.

Predicted bank erosion (m y-1) is a function of stream power, riparian condition and floodplain width (Figure 11). Results of the model suggest that bank erosion rates are at their greatest along the section of the Goulburn River from Eildon to just below Seymour where a combination of high stream power and poor riparian cover along some reaches makes banks particularly susceptible to erosion. Rates between 0.2 and 0.5 m y-1 are predicted for these reaches, consistent with maximum observed rates for the Goulburn River (Erskine et al., 1993). These rates are equivalent to between 1500 and 3750 t y-1 of sediment being input per kilometre of the river network.

Elsewhere bank erosion rates of 0.002 to 0.01 m y-1 (9 - 45 t y-1 km-1) are predicted along tributary streams where poor riparian cover occurs. Negligible bank erosion (<0.001 m y-1) is predicted for forested catchments or areas with little or no floodplain.

In all, riverbank erosion is predicted to remove 2.79 x 105 tonnes of soil annually from watershed areas in the Goulburn and Broken catchments. This is half the estimate for the NLWRA project and reflects improved modelling of bank erosion as well as local calibration against sampled suspended sediment loads (see below).

Sediment Sources to the Stream Network Each of the sediment sources described deliver sediment to the network of streams and rivers in the Goulburn and Broken River basins. The predicted mean annual amount of sediment supplied to streams from the three processes is shown in Table 2 in comparison with results of the NLWRA project.

Clearly, gully erosion is the dominant process contributing 57% of sediment to the river network. This is a reflection of historical land use practices where vegetation has largely been removed from convergent zones along valley floors, whereas forest cover has remained relatively intact on steep slopes where hillslope erosion rates would be expected to be greatest. In comparison hillslope erosion is a relatively minor process only contributing 7% of sediment to the river network. Despite the ameliorating affect of reduced peak discharge from dams and riparian vegetation, bank erosion remains a significant process, contributing the remaining 36% of sediment to the river.

Table 2: Components of the sediment budget for the Goulburn and Broken River Basins.

Sediment budget item Predicted mean annual rate (kt y-1)

This survey NLWRA

Hillslope delivery 52 93

Gully erosion rate 436 571

Riverbank erosion rate 279 540

Total sediment supply 767 1204

Total suspended sediment stored 186 597

Total bed sediment stored 443 554

Sediment delivery to the Murray River 138 53

Total losses 767 1204

Table 3: Components of the nutrient budget for the Goulburn and Broken River Basins.

Nutrient budget item Predicted mean annual rate (t y-1)

Total N Total P

Hillslope to stream delivery 437 47

Gully erosion 436 109

Riverbank erosion 279 70

Dissolved runoff 2160 213

Point Sources 173 47

Total supply 3485 486

Floodplain and reservoir storage 676 199

Denitrification 483 -

Dissolved export 1820 76

Particulate export 506 211

Total losses 3485 486

While all erosion processes need to be managed to reduce sediment loads in the catchment, it is clear that reducing gully and bank erosion will have the greatest benefit on reducing sediment delivery to channels. It should be noted that sheet and rill erosion contributes only to suspended sediment loads, while gully and riverbank erosion contribute to both bedload and suspended load. When this is taken into account the difference in sediment contribution from each process to the suspended sediment load is much less marked (ie., gully 40%, bank 43%, hillslope 17%) and hillslope erosion becomes relatively more important.

Nutrient Sources Each of the sediment sources described above together with dissolved contributions from hillslope runoff and point pollution sources deliver nutrients to the network of streams and rivers in the Goulburn and Broken catchments. Table 3 shows the predicted mean annual amount of Total N and Total P from particulate (erosion), dissolved (runoff) and point sources.

The spatial patterns of dissolved N and P contribution (Figures 12 and 13) are broadly similar and related to land cover. In general, forested land and grazing of residual native pastures is contributing the least to dissolved input of nutrients per unit area, while cropping, dairying and improved pastures, particularly in the irrigated areas in lowland regions about Shepparton and Tatura, are making the greatest contribution. Overall, dissolved nutrients are contributing 62% of Total N and 44% of the Total P supply to the river network.

Of the 486 t y-1 of Total P supplied to streams 47% is derived from the particulate sources of hillslope, gully and riverbank erosion. Since the same bulk soil concentration is used in the calculation of sediment attached P from gullies and river banks, gullies make the greatest contribution to supply of particulate P to streams followed by bank erosion and hillslope erosion contributing the least.

For Total N the relative contribution from particulate sources is lower at 33%. In this case, although hillslope erosion contributes less total sediment to streams than do gullies, the contribution of particulate N is the same as gullies due to nutrient enrichment by particle sorting and concentration of the clay fraction during sediment transport on hillslopes.

The total supply from point sources (Table 3) show that effluent runoff makes a small but significant contribution to the supply of Total N and P to streams (5 and 10% respectively).

Sediment Delivery through the River Network On-site erosion hazard is of concern for continued productivity of the land but can only be translated to downstream impacts if the eroded sediment is transported through the river network. The modelled sediment budget for the basins predicts that 138 kt yr-1 or about 18% of sediment delivered to streams is exported to the Murray River. Nearly all of this amount is exported as suspended sediment. The rest is stored on floodplains or on the bed of streams, with some storage in the basin also occurring in reservoirs. Lake Eildon, for example, with a storage capacity of 3,390,000 ML has an estimated sediment trapping efficiency of close to 99% so that most of the sediment load supplied by streams above the reservoir will be deposited before reaching the outlet (Erskine et al., 1993).

Of the 322 kt y-1 of suspended sediment estimated to be carried by streams 103 kt y-1 (32%) is deposited in reservoirs and lakes while a further 83 kt y-1 (26%) is deposited on floodplains. River, and floodplain morphology together with the frequency of overbank flooding largely determine the river’s ability to both transport and deposit sediment on floodplains. High stream banks and wide channels have a greater ability to confine flows, producing reduced overbank flooding and higher stream power during floods. Conversely shallower and narrower channels will be subject to more frequent overbank flooding resulting in a greater level of floodplain deposition. More extensive floodplains also allow for greater opportunity for overbank flow to be dissipated, depositing out a greater proportion of the transported sediment. Figure 14 shows that in general floodplain width increases with increasing catchment area. The main rivers usually have floodplains of over 500 meters in width while the smaller tributary streams have narrower floodplains which are almost non-existent in the steeper forested sub-catchments.

The overall pattern of floodplain deposition (Figure 15) is therefore similar to that of floodplain width but is inturn modified by the level of upstream erosion. Thus, while floodplain deposition tends to increase with increasing catchment area, it is also greater in sub-catchments with higher sediment yields (eg., west of Seymour) and lower in sub-catchments without a high level of active erosion (eg., lower Broken Creek). Over the length of the Goulburn River the model suggests an average 0.1-0.2 mm y-1 of deposition across the floodplain. This is a slow rate consistent with the age of floodplain deposits. Even so, the net affect of floodplains and reservoirs is that only 42% of suspended sediment transported by rivers reaches their mouths.

Care should be taken in interpreting results of floodplain deposition, particularly when comparing the smaller local sub-catchments. This is because a number of the parameters used in the model for estimating deposition are only approximated. The frequency of overbank flow for example can vary substantially from one point in the catchment to another (0.5 – 20 years ). This is a consequence of spatially variable channel form with both channel widths and bank heights varying by as much as 100% for streams of similar contributing area (Figure 7).

River Suspended Loads The river budget for suspended load predicts mean annual suspended loads through the river network allowing for deposition on floodplains and in reservoirs. As in most river systems the Goulburn and Broken Rivers show increasing sediment load with increasing catchment area (Figure 16). A more meaningful statistic for comparing river basins, is the specific sediment yield (Figure 17) where sediment load is divided by the upstream catchment area.

The predicted mean annual export of the Goulburn River (124 kt y-1) is equivalent to a specific sediment yield of 0.074 t ha-1 yr-1. Similarly the mean annual export from Broken Creek (13 kt y-1) is equivalent to a specific sediment yield of 0.037 t ha-1 yr-1. Such yields are typical of catchments in drier inland regions and are generally below those of wetter catchments in the north of Australia such as the Mary River in Queensland which has an estimated specific sediment yield of 0.45 t ha-1 yr-1 (De Rose et al., 2002).

While sediment yield increases with catchment area, Figure 17 shows that specific sediment loads vary between 0.01 to 0.5 t ha-1 yr-1 along the main river channels. Higher specific loads tend to occur in regions where there is a higher input of sediment to the channel, especially in areas of gully erosion, then diminish downstream due to losses of sediment through floodplain deposition and dilution by inflowing streams carrying less sediment in the lower catchment area.

Bedload Deposition The bedload sediment budget predicts the accumulation of sand and gravel on the bed of rivers as a result of increased rates of gully and bank erosion. We consider that where historical bed deposition is in excess of 30 cm, there is likely to be some impact on bed habitats. This might be through filling of pools, smothering of cobble beds with finer grained sediment or reduced diversity of bed forms.

Bedload deposition is determined as the difference between the total sediment input and sediment transport capacity. The spatial pattern of sediment transport capacity in the Goulburn and Broken catchments (Figure 18) shows that for the smaller tributary channels transport capacity is much greater in the higher rainfall areas and steeply sloping watersheds to the southeast. Deposition of bed materials is therefore more likely in the more northerly watersheds where sediment transport capacities are several orders of magnitude lower. Figure 18 also shows that high sediment transport capacities are only maintained along the steeper sections of the major rivers.

The results of modelling sand deposition suggest that only a small percentage of the river network length in the basin has bed deposition in excess of 30 cm (Figure 19). As expected localised deposition of bed material of between 0.3 and 2 m deposition is predicted either along gently sloping reaches of the main river channels or in tributary channels where sediment input is high relative to the sediment transport capacity. Such areas include streams to the north west of Seymour predominantly but also the occasional reach along streams in the region to the north east of Murchison. Predicted deposition to

depths greater than 2 m represent deposition of all bedload in lakes and reservoirs, or in the case of some short stream links, underestimation of bed slope from the DEM.

In places SedNet accurately predicts the location of sand slugs (eg., the lower Sugarloaf Creek deposit, near to Seymour). The present map of bed aggradation, however, is considered to underestimate the actual extent because many of the streams and creeks draining the Strathbogie Plateau, such as Hughes, Creightons, Castle and Seven Creeks have either undergone or presently have active sand aggradation (Erskine et al., 1993). This could result from overestimation of the sediment transport capacity or underestimation of river loads. Limited river suspended sediment data suggests that sediment loads (Figure 16) are reasonably well predicted by SedNet in this region. Sediment transport capacity (STC) is therefore being overestimated. By examining Equation 1 it is apparent that either overestimation of channel slope and ∑Q1.4 , or underestimation of channel width could be causing excessive values of STC. Most of these streams are known to be anastomosing (Erskine et al., 1993), that is they are comprised of multiple, often sinuous, anabranches: a feature of streams not presently modelled by SedNet. Channel anastomosis reduces ∑Q1.4 along anabranches and results in increased effective channel widths and lengths and reduced average channel slopes. It would appear that all three parameters have conspired to over-predict STC, and therefore under predict sand accumulation for these streams.

Nutrient Budget The nutrient budget predicts mean annual Total P and Total N loads through the river network allowing for losses on floodplains, in reservoirs and as a result of in stream processes of denitrification and P- equilibrium. The predicted mean annual export to the Murray River of Total P amounts to 287 t y-1 which represents 65% of that produced by erosion processes and hillslope runoff. Of this amount 73% is exported attached to sediment.

The mean annual export of Total N amounts to 2326 t y-1 which represents 70% of that produced by erosion processes and hillslope runoff, similar to Total P. In contrast to P export, however, only 21% of Total N is exported attached to sediment. This reflects the much higher input of dissolved N from runoff and point sources despite there being significant losses in N through in stream denitrification (483 t y-1).

The spatial patterns of Total P and N loads (Figures 20 and 22) are broadly similar and are similar to that of sediment loads. This simply reflects the fact that river loads increase with increasing catchment area. Of more importance to differentiating land use impacts on nutrient loads is the ratio between the dissolved and total loads (Figures 21 and 23). Clear patterns emerge showing that regions dominated by sediment-bound nutrient input (eg., gully dominated catchments) have low ratios while those dominated by dissolved inputs (eg., cropping and irrigated lands, forests) have high ratios. For forested lands the high ratio reflects a very low sediment export. These maps show that the important difference between P and N loads is that the high dissolved ratio and therefore loads for N are maintained by the higher dissolved inputs from agricultural land (northern part of the catchments), and in particular irrigated regions, whereas for P the influx of sediment outweighs that derived from dissolved sources in agricultural lowlands, to the extent that the dissolved P ratio is reduced along reaches of the main rivers and streams.

Results suggest that reductions in P export are best achieved by controlling gully and bank erosion whereas reductions in N export are best achieved by controlling dissolved runoff from agricultural lands and point sources.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Width of Flood Plain (m) 1 - 10 11 - 100 101 - 250 251 - 500 501 - 1000 1001 - 2000 2001 - 10000

Kilometers 05010025 Figure 14: Measured floodplain width.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Floodplain deposition (mm/y) 0 - 0.01 0.01 - 0.05 0.05 - 0.1 0.1 - 0.2 0.2 - 0.4

Kilometres 05010025

Figure 15: Predicted depth of floodplain deposition.

11 RHRPIH3@ 2 3@ RHRPIR ´ 78 RHSPQP3@

100 3@ 11 RHSPHR 9 RHRPPR3@ RHRPIT3@ 0.7 94 5 RHSPRT3@ 3@ RHSPHH3@ RHRPHU 2 1 RHSPQU3@ RHRPHT3@ 0.9 3@ RHSPQR 0.6 3@ 32 RHSPSI 63@ 3@RHSPHP4 RHSPRH 5 RHSPIP3@ RHSPIR3@ 6 3@ 4 RHSPHQ 3 0.9 3@ 43@ 3@ RHSPHW RHSPIW RHSPQI 0.6 RHSPPU3@ RHSPHS3@ 2 RHSPTR3@ Legend Suspended Sediment Load (kt/yr) < 1 1 - 10 10 - 25 25 - 50 50 - 100 100 - 150

3@ Gauging stations

Kilometers 05010025 Figure 16: Predicted suspended sediment load compared with loads measured at gauging stations within the Goulburn and Broken catchments.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Specific Suspended Sediment Load (t/ha/y) 0 - 0.01 0.01 - 0.05 0.05 - 0.1 0.1 - 0.5 > 0.5 Kilometres 05010025

Figure 17: Predicted suspended sediment load expressed per unit area.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Sediment Transport Capacity (t/y) 1 - 100 101 - 1000 1001 - 10000 10001 - 50000 50001 - 100000 > 100000

Kilometers 05010025 Figure 18: Modelled sediment transport capacity.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Bedload Accumulation (m/yr) 0 - 0.1 0.1 - 0.3 0.3 - 1 1 - 2 > 2

Kilometres 05010025

Figure 19: Predicted bedload deposition.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Total P load (t/yr) 0.2 - 1 1 - 10 10 - 50 50 - 100 100 - 150 150 - 200 200 - 270

Kilometers 05010025 Figure 20: Predicted Total P load.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Dissolved P:Total P 0 - 0.2 0.2 - 0.4 0.4 - 0.6 0.6 - 0.8 0.8 - 1

Kilometers 05010025 Figure 21: Predicted proportion of dissolved P to Total P load.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Total N load (t/yr) 0.02 - 10 10 - 100 100 - 250 250 - 500 500 - 1000 1000 - 2000 2000 - 2400

Kilometers 05010025 Figure 22: Predicted Total N load.

NATHALIA KATAMATITE 3 3 ´

SHEPPARTON 3 TATURA 3 BENALLA 3 MURCHISON 3

EUROA NAGAMBIE 3 3

SEYMOUR MANSFIELD 3 3

ALEXANDRA YEA 3 EILDON 3 3 KILMORE JAMIESON 3 3

BUXTON 3

Legend Dissolved N:Total N 0 - 0.2 0.2 - 0.4 0.4 - 0.6 0.6 - 0.8 0.8 - 1

Kilometers 05010025 Figure 23: Predicted proportion of dissolved N to Total N load.

Contribution to Suspended Sediment Export to the Murray River The sediment budget predicts that 42% of suspended sediment and < 1% of bedload delivered to the river network in any year is exported from the river mouth. While bank erosion contributes evenly to the coarse and fine budgets and gully erosion is estimated to contribute 70% to the coarse sediment budget, by the catchment mouth, bedload makes up only 0.7% of the total sediment export, meeting common observations of suspended load dominance in large rivers (Richards, 1982).

Given that around 18% of sediment delivered to streams within the Goulburn and Broken River basins is exported to the Murray River, it can be concluded that increased erosion upstream in a sub-catchment will not result in the same increase at the river mouth. In other words, within the Goulburn and Broken catchments there is a low degree of connectivity between upstream erosion in sub-catchments and contribution to the Murray. Sub-catchments which make a substantial contribution to the export at the Murray are those with high erosion and limited floodplain extent between the source and the Murray. Sub-catchments close to the Murray are more likely to contribute to sediment export because of limited possibilities for that sediment to be deposited along the way.

This overall pattern of suspended sediment contribution to export (Figure 24) shows that the Goulburn and Broken catchments can be divided into 3 roughly contiguous regions of land. In lowland areas of the catchment from below Nagambie contribution to export is low (< 0.01 t ha-1 y-1) as a result of low erosion rates in this predominantly flat landscape. Above Lake Eildon and south of Yea sub-catchments are similarly making little contribution to export due to a combination of the long travel distance for sediment to the river mouth, low erosion rates under forest, and the high sediment trapping efficiency of the reservoir. The third region comprises a broad band of land trending from the south west to the north east in the direction of Seymour, Euroa and Benalla. Here, contribution to export is typically much higher (0.05 to 0.5 t ha-1 y-1) primarily due to much higher erosion rates but also the close proximity to the river mouths. A few sub-catchments along the main river channels where bank erosion rates are high, are making the greatest contribution to export (>0.5 t ha-1 y-1).

If the goal is to reduce sediment delivery to the Murray River then remedial works can be focussed on particular sediment sources and the land uses and erosion processes found there. Obviously targeting the areas with a disproportionately high level of contribution should be a priority, such as areas of bank erosion. Targeting these areas will obviously have the greatest effect on reducing sediment export to the Murray. However, because the majority of the catchment is still making a significant contribution, it is unlikely that major reductions in suspended sediment loads to the Murray will be achieved, without more widespread rehabilitation of problem areas.

Comparison of Suspended Sediment Loads The preceding sections have concentrated on predictions of the river sediment and nutrient budgets and their implications with little discussion on tests of their accuracy. For the Goulburn Broken Rivers it is possible to directly compare the results from SedNet with load estimates based on suspended sediment samples taken at gauging stations and sediment deposition in Lake Eildon. Furthermore the relatively dense network of gauging stations allows a spatial analysis of uncertainties in the model.

Suspended sediment loads were calculated for 24 gauging sites across the catchment by applying sediment rating curves to the maximum length of daily flows measured at the stations (Figure 16). Figure 25 shows one of the better sampling records of suspended sediment taken within the catchment. Equation 10 is the best fit to the spread of data points and provides a rating curve for suspended sediment concentration against daily flow at this station.

C = 3 + 0.031×Qd 1.07 , R2 = 0.47 (10)

NATHALIA KATAMATITE ´

SHEPPARTON TATURA

BENALLA MURCHISON

EUROA NAGAMBIE

SEYMOUR MANSFIELD

ALEXANDRA YEA EILDON KILMORE JAMIESON

BUXTON

Legend Contribution of suspended sediment to the Murray River (t/ha/y) 0 - 0.01

0.01 - 0.05

0.05 - 0.1

0.1 - 0.5

0.5 - 1

> 1 Kilometers 05010025 Figure 24: Predicted contribution of suspended sediment to the Murray River for sub-catchments within the Goulburn and Broken River catchments.

The rating curve (Equation 10) when applied to the time series of daily flow from 1947 to 2001 results in a mean annual load of 5 kt estimated to flow past this gauging station. The same general trend as shown in Figure 25 was observed for the majority of gauging stations analysed in the Goulburn and Broken catchments. Principal differences were the mean suspended sediment concentration at low flows, which were found to increase from 4 to 13 mg L-1 with increasing catchment area, and the slope of the rating curve (equal to the exponent in Equation 10) which tended to vary between 0.3 and 1.2. While there appeared to be no systematic variation in the exponent in relation to catchment area, as stations lacking sampling at high flows tended to have the lower values, it was found that regulated channels such as the Goulburn river tended to have lower values and therefore, flatter rating curves.

1000 100

70 100 60

40 10 30 Cummulative percent load 20

Suspended sedimentconcentration (mg/L) 10

1 0 10 100 1000 10000 10 100 1000 10000 Daily flow (ML) Daily flow (ML) Figure 25: Suspended sediment rating curve (left) and cumulative load distribution (right) for Delatite River at Tonga Bridge (405214).

Care needs to be taken in the interpretation of gauging station samples as there are significant sources of error in the calculation of river loads. For example, the maximum sampled flow (2700 ML day-1) in Figure 25 is less than a 1 in 1 year flow event for this site. Furthermore, only 8 samples were taken at flows above 1000 ML day-1 , and no samples were taken at flows above 3000 ML when 47% of the river load is estimated to be transported based on extrapolation of the rating curve to high flows. The poor replication of event based flows, typical of most gauging stations, produces relatively large errors in the estimation of river loads. In this example, the standard error (53 mg L-1) in the mean suspended sediment concentration measured at flows above 1000 ML (97 mg L-1) suggests an error of at least ± 50%. The standard error of the estimate for the regression curve was not used in this instance because of the large bias caused by the bulk of samples occurring at low flows.

Despite these limitations, the predicted load from SedNet for the Delatite River near to Tonga Bridge of 4 kt y-1 compares favourably with the measured load. Figure 26 compares the predicted load plotted against measured load at the 24 stations. Overall, while there is some variation for individual stations which can be partly attributed to errors in measured loads, there is a good agreement between the loads predicted by SedNet and measured loads. There is however a systematic trend evident in Figure 27 with loads being overestimated along the main channels (eg., loads > 10 kt y-1) and underestimated along the smaller tributary channels.

SedNet suggests a decline in sediment load along the Goulburn River from 143 to 124 kt y-1 from below the junction with the Broken River to the outlet at the Murray River. The same trend is apparent in measured loads which decrease from 100 kt y-1 at Shepparton to 78 kt y-1 at McCoy Bridge. The loss in sediment is largely attributed to overbank deposition and the lack of any significant sediment inflows along the lower section of the River.

For the main reach of the Goulburn River below Seymour, SedNet results are on average about 50% higher than measured loads. While this is similar to the ± 50% approximate error in estimation of river loads, all measured loads are below those predicted by SedNet, suggesting a systematic difference. Therefore, either rates of erosion from the three main sources have been over-predicted in the modelling or there has been insufficient flood deposition modelled along the lower floodplain area. Given that the 43

biggest differences occur for sub-catchments dominated by gully erosion (eg., Sugarloaf River and the Goulburn River at Seymour) this suggests there has been a decline in the rate of gully erosion in recent decades. It is important to keep in mind that SedNet predicts long term average sediment loads while measured loads apply to much shorter periods in time. Some of the difference, therefore, can be attributed to temporal variation in the activity of sediment sources.

1000

100

y = 0.5x1.3 SedNet modelled load (kt/yr) SedNet R2 = 0.8 0

0 0 0 1 10 100 1000 Measured load (kt/yr) Figure 26: Comparison of average annual suspended sediment loads predicted from SedNet with estimated loads from gauging station (Figure 16) sediment rating curves (eg. Figure 25). Dotted line is the 1:1 relationship while the solid line is the least squares regression fit.

Under prediction of loads by SedNet for the smaller tributary channels could be due to either underestimation of erosion inputs or overestimation of floodplain deposition. Sites showing the greatest difference all occur in sub-catchments dominated by forest. As these all have very limited floodplain extents then underestimation of erosion inputs is the likely cause. At present SedNet does not consider bank erosion in forested watersheds. The implication of the results is that significant levels of bank erosion are being maintained in forested catchments, particularly those in wetter and steeper catchments where stream power would be expected to be higher.

Rates of Sediment deposition in Lake Eildon also provides a useful comparison with results from SedNet which predicts that 67 kt y-1 of coarse sediment (100% of load) and 50 kt y-1 of suspended sediment (88% of load) are being stored in the reservoir. This is approximately half the measured deposition (Abrahams, 1972) in the reservoir of 211 800 m3 y-1, which is equivalent to 212 kt y-1 if an approximate dry bulk density of 1 t m-3 is assumed for lake sediments that are subject to periodic drying. As the predicted suspended sediment export from the reservoir (6.8 kt y-1) is close to the measured load based on sediment rating data at the Eildon gauging station (Figure 16), then this suggests that the sediment loads of streams entering the lake have been under predicted, consistent with the comparison of measured loads from gauging stations.

Comparison of Nutrient Loads The estimation of nutrient loads is generally more problematic than for sediment loads as nutrients are mostly sampled at times of low flow and rarely at high flows, let alone during flood events. Relationships between nutrient concentrations and daily flow are usually poor, if not non-existent. Approximate load

estimates can be derived from the limited sampling data available at gauging stations by assuming average concentrations of both sediment-bound nutrients and soluble nutrients and then applying these to the estimated mean annual sediment loads and discharges, respectively.

In the Goulburn and Broken catchments measurement of nutrients at gauging stations has included total Kjeldahl N, total nitrates and nitrites (NOx), soluble P, and Total P. Sediment-bound P can be determined from the difference between soluble and Total P. The sum of Kjeldahl N and NOx is equal to the Total N concentration. Ammonium concentrations are required to calculate sediment-bound N, and since these have generally not been measured, then only an approximate measure of sediment-bound N can be obtained from the relationship between Kjeldahl N and suspended sediment concentrations.

Three gauging sites were selected to compare measure loads against predicted loads (Table 4) for different catchment contributing areas ranging from the larger rivers (Goulburn River at McCoy Bridge) to smaller tributaries (Delatite River at Tonga Bridge). The three sites suggest a similar trend of over prediction of Total N and under prediction of Total P, although differences are probably within the errors involved in calculating nutrient loads from sampling data.

Table 4: Comparison of nutrient concentrations and loads to predicted loads of SedNet at selected gauging stations. Sed = sediment-bound nutrient, Diss = dissolved nutrient.

Site McCoy - 405323 Murchison - 405200 Delatite - 405214

Sampled Predicted Sampled Predicted Sampled Predicted

SS Load (kt y-1) 78 130 94 108 5 4

Sed P conc. (g kg-1) 3.2 1.8 2.8

Sed N conc. (g kg-1) 7 8 5

Sed P load (t y-1) 250 180 170 110 14 2.6

Sed N Load (t y-1) 560 490 710 410 25 11

Discharge (ML) 1600000 1430000 923000 920000 125000 105000

Diss P conc. (mg L-1) 0.020 0.0046 0.0044

Diss N conc. (mg L-1) 0.7 0.53 0.26

Diss P load (t y-1) 30 50 4 20 0.55 1.8

Diss N load (t y-1) 1140 1740 490 1200 33 43

Total P load (t y-1) 280 230 174 130 15 4.4

Total N load (t y-1) 1700 2330 1200 1610 58 54

Diss P: Total P ratio 0.12 0.22 0.03 0.18 0.04 0.41

Diss N: Total N ratio 0.67 0.75 0.40 0.75 0.57 0.80

When comparing dissolved and sediment-bound loads there is systematic under prediction of sediment- bound nutrient loads and over prediction of dissolved nutrient loads for both N and P resulting in larger predicted ratios of dissolved to Total N or P (Table 4). This suggests that the dissolved concentrations used in nutrient grids (Figures 12 and 13) have been overestimated and that the bulk soil concentrations of eroding sediment may be greater on average in the Goulburn and Broken catchments that the default values (see Page 21) assumed in SedNet.

Alternately, the systematic trend could reflect inadequate sampling of the range of river flows, with the average sediment-bound P and N concentrations and dissolved P and N concentrations not being representative of high flow conditions when daily loads are greatest (Figure 27). If increases in dissolved nutrient and decreases in sediment-bound nutrient concentrations with increasing flow were to be assumed (as indicated by the analysis of some, but not all, gauging records), then better agreement would have resulted between the sampled and predicted loads shown in Table 4.

There are also a number of other parameters in the model which affect the nutrient concentrations in streams. For example, overestimation of dissolved N could be due to under prediction of the level of denitrification occurring in the Goulburn and Broken Rivers. To meet the observed dissolved N level along the lower Goulburn River would require about twice the level of denitrification to occur than is presently predicted.

5000 Daily flow (<1yr recurrence flows) 4000

3000

2000

Daily Flow (ML) 1000

0 500 2.07 Suspended sediment Ld = 0.012.Qd /1000 400

300

200

100 Predicted mean load (t/day) load mean Predicted

0 200 Total P Ld = 0.035.Qd 150

100

50 load (kg/day) Predicted mean 0 2000 Ld = 0.43.Qd 1500 Total N

1000

500 load (kg/day)

Predicted mean 0

Date Figure 27: Variation in predicted mean daily loads from April to October 1993 for the Delatite River at Tonga Bridge. Daily loads are disaggregated from the mean annual loads predicted by SedNet using the rating curve for suspended sediment and assuming constant concentrations for nutrients.

The ratio between dissolved and sediment-bound phosphorous is partly determined by the adsorption coefficient for Total P, the value of which has been estimated to lie in the range 20000 to 80000 ML kt-1 (Young et al., 2001). An average default value of 40000 ML kt-1 is currently used in SedNet. Increasing the value of the adsorption coefficient would have resulted in increased sediment-bound P loads and decreased dissolved P loads, more comparable with the measured load estimates from gauging stations. A value for the coefficient between 40000 and 80000 ML kt-1 is therefore indicated by the present results.

The present modelled results for nutrient loads in the Goulburn and Broken catchments (Table 4) should be regarded as first approximations. There is potential to further improve results through calibration of some of coefficients in the model and more accurate estimates of both dissolved N and P in runoff and bulk nutrient concentration in eroding sediment, which appear to have been underestimated. This will rely on more extensive comparisons between observed and predicted loads similar to the limited treatment undertaken above.

Disaggregation of Annual to Daily Loads Figure 27 illustrates how mean annual loads can be disaggregated into mean daily loads for river links. In this case, the example used is for the Delatite River at Tonga Bridge but we could have equally used any river link with the Goulburn and Broken catchments. The figure shows how suspended sediment and nutrients are likely to vary through time for a typical year. It demonstrates how storm-flows (ie., peaks on the graphs) transport the majority of suspended sediment and nutrients. The curve for suspended sediment shows the greatest difference in load between peak storm flow and low flow conditions owing to the observed increase in suspended sediment concentration with increasing flow (Figure 25). Relative to the suspended sediment curve, those for Total P and Total N are more attenuated because a higher proportion of nutrients is transported by low flows.

Constant concentrations for Total N and P are assumed due to the lack of event based observations. In Figure 27, the mean concentrations of 0.035 mg L-1 for Total P and 0.43 mg L-1 for Total N are back- calculated from the mean annual loads predicted by SedNet. These are above the mean concentrations of 0.022 and 0.31 mg L-1 for Total P and N measured at low flows. This would suggest an increase in nutrient concentrations with increasing flow if the SedNet loads are accurately predicted. Therefore, the mean daily patterns shown in Figure 27 are in reality likely to be somewhat more accentuated with a greater difference in daily load between high and low flows.

It is important to realise that Figure 27 shows predicted mean daily loads only. The load curves simply serve to illustrate how average conditions fluctuate through time. Actual daily loads for this period will have differed from mean values depending on the influence of environmental variables which also affect daily fluctuations in sediment and nutrient concentrations (eg., rainfall frequency-duration, spatial pattern of rainfall and source area for flow, antecedent conditions etc).

Testing of Land Use Scenarios The results of SedNet so far presented represent those for the current catchment condition and pattern of existing land uses. It is possible to test the effect that different land management scenarios would have on the long-term export of sediment and nutrients from the catchment by reducing or increasing the levels of erosion inputs to channels to reflect changes in land use. By rerunning the sediment and nutrient calculations in SedNet, new estimates of sediment and nutrients loads can be compared with current conditions, bearing in mind that the changes represent long-term annual averages. Changes to flow hydrology or channel morphology are presently not considered for new scenarios.

To achieve this a catchment scenario testing tool has been developed which enables the land manager to interactively change selected attributes in the model which affect contribution of sediment to the channel network. At present ArcMap software is used as the visual interface with the SedNet suite of programmes. Users will therefore need an ArcMap licence to run this scenario tool. When first opening ArcMap the user is presented with a display which shows the catchment area of interest, stream network and other features such as towns, coastline or roads. Figure 28 shows what the display window would look like for a typical catchment.

The user then interactively selects a group of stream links and associated sub-catchment areas for which changes are to be applied. In the example used here (Figure 28) these areas are highlighted in pink. After the watersheds and streams have been selected a window is opened through which the necessary stream link attributes can be interactively changed. Changing the area of a given land use or land uses and the sediment delivery ratio affects the amount of sediment delivered from hillslopes to the channel. Similarly, changing the proportion of gully erosion and riparian vegetation, changes the contribution of sediment from gully and river bank erosion to the channel. These changes are then applied to the attributes recorded in the stream network coverage, but only for the stream links that have been selected. The changes in area of land use, proportion of riparian vegetation, and proportion of gully erosion are applied to the group of selected streams links and associated catchment areas on a weighted basis depending on the area of land or length of channel available for change. For example an increase in the proportion of riparian vegetation can only be applied to those watersheds within the group selected which have less than 100% riparian cover. Changes made to the sediment delivery ratio, which is presently treated as a global constant in the model, effect all selected watershed areas equally.

Figure 28: Scenario testing in ArcMap. Proportion of land use, riparian vegetation, gully density and hillslope delivery ratio can be interactively changed for an individual or group of selected basins (as shown in pink) to reflect changed land use conditions and how they might impact on the supply of sediment to the channel. Sediment loads and contribution to the coast or major rivers are recalculated once changes are made.

The SedNet model programmes can be rerun and the results from the new scenario directly compared with those for current conditions or those from previous scenario test runs. The resulting differences in sediment and nutrient loads between successive scenarios can be displayed for all stream links in a similar way to Figure 28.

Comparison with NLWRA Results The results of sediment and nutrient exports for the Goulburn and Broken River catchments represent significant improvements over those produced for the national scale National Land and Water Audit Project. This is largely a result of the incorporation of better regional data (eg., land use, floodplain mapping) as well as improved parameterization of the model. Specific improvements are summarized below.

• The reasonably dense network of gauging stations allowed for improved regionalization of flow parameters and their prediction throughout the channel network. These in turn improve estimation of sediment and nutrient transport and deposition.

• The combination of a better land use map, and measurements of slope angle and slope length from a 20m high resolution digital elevation model greatly improved estimation of hillslope erosion. This resulted in a significantly reduced estimate of hillslope erosion of 52 kt y-1 compared with the 93 kt y-1 of the audit project.

• Regionalization of river bank height, use of a better regional map of riparian condition, and a stream- power based rule for estimating bank erosion, provided better spatial representation and rates of bank erosion. As a result the contribution of bank erosion to the stream network was lower at 279 kt y-1 compared with the 540 kt y-1 of the Audit project.

• Improved grid analysis of gully density and recognition that gullies have been active over a longer period (eg., 150 years) provided a slightly better estimate of gully contribution to the river network which was lower at 436 kt y-1 compared with the 571 kt y-1 of the Audit project.

• The availability of a detailed floodplain map and improved local estimates of overbank flood discharge, together with the reduced erosion inputs, provided a better estimate of deposition of suspended sediment, which was significantly lower in the case of this assessment (186 kt y-1) compared with the Audit result (597 kt y-1).

Conclusions The present estimates of sediment and nutrient loads in the Goulburn and Broken River catchment represent significant improvements over those that have been previously undertaken as a direct result of the incorporation of better regional information. It is clear that all erosion processes contribute significant amounts of sediment to the channel network of which a portion is exported to the Murray River with the remainder being deposited on floodplains and in lakes and reservoirs.

All erosion processes are highly variable across the catchment with localised hotspots evident in places. Hillslope erosion, although producing the least amount of sediment to streams overall, is at its highest on the steeper grazing land and to a lesser extent, cropping land. These areas tend to concentrate in central areas of the catchment, particularly between Seymour and Euroa. Both gully and riverbank erosion are the dominant erosion processes in the catchments. Again gully erosion varies significantly across the catchment being worst in a broad SW – NE trending area of land from the southwest of Seymour through to the general region of Benalla. Riverbank erosion is at its worst in river reaches lacking good riparian zone condition and where stream power is greatest. As such bank erosion is predicted to be worst along sections of the Goulburn, and to a lesser degree, Broken Rivers. A number of smaller streams and creeks in the central area of the catchment also appear subject to significant levels of bank erosion.

Coarse sediment generated from gully and bank erosion is deposited in lakes and reservoirs and as sediment deposits along a number of river links where slopes are gentle and upslope erosion rates are high. The present modelled results are considered to under predict the extent of sand deposition due to limitations in the way in which channel morphology is expressed in the model.

Suspended sediment generated from erosion processes is deposited on floodplains and in the larger lakes and reservoirs. Overall 58% of the suspended sediment transported by rivers in the Goulburn and Broken catchments is being stored in these areas. The remainder is being exported to the Murray River. Long term export rates are predicted to be about 50% on average greater than the contemporary rates of suspended sediment passing gauging stations, probably due to a decline in the rate of gully erosion, although the levels of floodplain deposition could have been underestimated in the model.

The patterns of each erosion source differ suggesting that each process is fairly independent and that in each location an assessment needs to be made of the dominant source of sediment. Given that the highest rates of erosion occur in localised patches, this suggests that erosion control measures targeted to specific areas will be effective in reducing sediment supply to, and loads in the river network. Overall gully and riverbank erosion should be of major concern. More effective riparian zone management in some sections of the catchment will go a long way to improving water quality as a whole.

The outputs from this research should therefore assist natural resource management agencies and land managers to appropriately target critical areas, so that a comparatively large benefit in reducing sediment loads delivered downstream can be achieved with less effort. The scenario testing tool developed as part of this project provides a means to evaluate the relative effectiveness of targeting these areas for rehabilitation. For example, depending on available resources, a land manager could consider a number of scenarios for rehabilitating different sections of the rivers riparian zone, and directly compare the effect this would have on reducing sediment export. In this way the most cost-effective strategy could be achieved.

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