landuse change in the Loddon River Catchment
Abstract
The upper Loddon River Catchment is characterised by a complex geomorphology confused by the historical legacies of mining, clearing for agriculture and dam construction. This paper describes the processes acting on the catchment to erode the landscape and deliver sediments to the stream network. Our assessment of catchment processes is based on field observations and interpretation of a catchment scale sediment budget – developed through application of the geomorphic process model, SedNet. Gully erosion has been the most important source of sediment input to the Loddon River (84% of sediment yield) with low combined sediment input from hillslopes and bank erosion (16%). Although the Loddon Catchment’s sediment yield, per unit catchment area, is typical for southern Murray-Darling Basin (MDB) catchments and similar to the national average, the amount of sediment derived from gully erosion is some 50% more than the national average.
Only a very small proportion of the sediment mobilised is exported from the study area. Of the sediment that remains, about half is permanently stored in reservoirs or on floodplains with the rest stored as bedload in stream channels. Most of inchannel sediment stores are sand – that we classified as potentially mobile sand slugs. We predict that approximately 50% of stream length is impacted by sand slugs or similar instream deposition. This is approximately double the average proportion for MDB streams, making the Loddon River one of the most sand slug-impacted streams in the MDB.
Although the effect of the combined disturbances of agriculture and mining has been profound, improvements in land-management practices have complemented the natural slowing of landscape adjustment and continue to assist in reducing sediment yield. The slow and episodic movement of sediment slugs in the watercourses of the upper Loddon Catchment will continue to influence channel form and stability for many years to come.
Authors
Bruce Abernethy, Andrew J. Markham, Ian P. Prosser, Tanya M. Wansbrough