River metamorphosis and environmental change in the Macdonald Valley, New South Wales, since 1949

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Abstract
A significant metamorphosis of channel form occurred on the lower Macdonald River between 1949 and 1955; width and width‐depth ratio increased and depth, sinuosity and weighted mean per cent silt‐clay in the channel perimeter decreased. The channel has remained unstable since 1955, exhibiting frequent variations in bed elevation and a recent period of minor channel contraction followed by slight enlargement. Since 1946 there has been an increase in summer and annual rainfall and an abrupt, upward shift of the annual series flood frequency curve. As a result an increased proportion of the total sediment load of the river is now being transported as bed‐material load. The change in sediment load is a result of greater stream competence and the consequent reworking of sandy sediment temporarily stored within the channel as benches. The observed channel changes are a quasi‐adjustment to the increased flood and bed‐material load discharges.