The role of cyclic climatic regimes and riparian vegetation: a qualitative and quantitative study into the cause of river bank slope instability and channel widening on the Macdonald River, New South Wales.

Abstract

Numerous case studies have demonstrated the catastrophic nature of channel change experienced on the Macdonald River, a tributary of the Hawkesbury-Nepean River, New South Wales. However, there is an absence of studies that clearly state how and why the channel changed as dramatically as it did. As a consequence, the magnitude of, and ultimate controls on the changes to the Macdonald River’s form and processes are not fully appreciated. In this study, a comparison is undertaken on the three existing river morphology perspectives with respect to the Macdonald River. The Warner and Erskine Perspective states the sole importance of the cyclic hydro-climatic conditions of the FDR and DDRs on river morphology. The Brierely and co-workers Perspective states the sole importance of anthropogenic influence in the catchment and on the banks on river morphology. The Intermediate Perspective of Hubble and co-workers considers both existing and conflicting perspectives and states the importance of both cyclic climatic regimes and anthropogenic activity in the catchment and particularly on the river banks, in determining river morphology. Resultantly, a selection of Hubble and co-workers Perspective is made to classify the Macdonald River’s morphology. It is clear, from the analysis of historical aerial photographs, archival sketches, photographs and historical documentation, that riparian vegetation was absent from the banks of the Macdonald River from 1941 as a result of land-clearing practices from the early 19th Century. The banks of the Macdonald River would not have experienced this ‘catastrophic’ channel change between 1949 and 1955 had riparian vegetation remained on the banks. Riparian vegetation has been found to increase the soil-shear strength and hence the stability of river bank slopes, in particularly on the Upper Nepean and the Macdonald Rivers in New South Wales, where vulnerable sands to silty-sands predominate the bank material. This has been further proved with geochemical bank stability modeling.