Linking Sediment Dynamics, Riparian Vegetation and Aquatic Ecology in the Ovens River. Part 1: Regional Patterns of Riparian Vegetation, Erosion and Sediment Transport in the Ovens River Basin

"December 2005".

Project Number: Linking Sediment Dynamics, Riparian Vegetation and Aquatic Ecology in the Ovens River - M/BUS/79.

MDFRC item.

32 pages. Part 1 of 2 part report + executive summary: (see Executive Summary: Linking Sediment Dynamics, Riparian Vegetation and Aquatic Ecology in the Ovens River) and (see Part 2: Impact of sediment movement on the Aquatic Ecology of the Ovens River).

The SedNet sediment budget approach was applied to the 7200 km2 Ovens River catchment to investigate the spatial variability in sediment loads in relation to the distribution of riparian vegetation. Improved representation of the channel network, use of high spatial resolution remotely sensed land cover and elevation data and localized calibration of riverbank erosion has significantly improved the sediment budget when compared with previous results generated for the National Land and Water Resources Audit (NLWRA). Geochemical and radionuclide tracing of flood deposits is investigated at strategic locations along the Ovens and King Rivers and Buffalo Creek to provide validation and testing of the SedNet predicted loads. The sediment budget predicts that 78 and 2 kt y-1 (equivalent to 12 and 3 t km-2 yr-1) of suspended sediment is exported to the Murray River from the Ovens River and Black Dog Creek catchments. This export of sediment represents 67 % of the input suspended sediment delivered to the river network from hillslope, gully and river bank erosion processes. Much of the sediment load is derived from river bank erosion and the model suggests that the contribution of sediment from gully and river bank erosion increases with distance down stream and dominates export in the lower reaches. The radionuclide and geochemical tracing provide additional lines of evidence that support these conclusions.

Key Messages: The dominance of bank erosion as a source of sediment and its major contribution to loads in the mid-lower Ovens River, suggests that more effective riparian management should be a priority for reducing loads and improving water quality, particularly where riparian cover is poor. - In general, there is good agreement between observed and predicted suspended loads for catchment areas > 1000 km2. Model uncertainty increases at smaller scales due to the inability of model equations to represent fine scale sediment delivery and transport processes. - Geochemical and radionuclide tracing mainly support the SedNet model predictions but suggest there is significant uncertainty in predicted rates of bank erosion. A more comprehensive sampling strategy should be considered to assess contribution from channel sources throughout the catchment. - There is likely to be considerable spatial uncertainty in mapped gully density, producing significant uncertainty in modelled loads, particularly for tributary streams. Similarly loads for some upland, predominantly forested catchments (Fifteen Mile Cr, Rose R, and Reedy Cr) are under predicted and these areas warrant field investigation to ascertain likely sediment sources. - In general there is little variation in specific suspended load or mean annual concentrations predicted along the main rivers in the catchment. Impacts on stream habitat are likely to be historical consequences of basin wide changes in land use over time rather than reflecting the contemporary spatial patterns of suspended sediment distribution within the catchment. - Significant accumulations of sand deposits are predicted in the lower reaches of the Ovens River and this may have impacted on stream health by smothering bed habitat.