Channel and Hydrological Controls on Water Quality in a Steep,Bedrock-Confined Stream

Steep, pool-fall channels of the sandstone areas near Sydney, New South Wales, Australia consist of discrete pools of varying sizes separated by bedrock reaches. The pools are linked by low-volume discharges during dry weather. Relatively large in-channel water storage volumes and small dry weather flows result in extended in-channel retention of water, sediments and contaminants and hence delayed delivery to receiving waters. Streams are flushed infrequently by high flow events. Field data and simulation results indicate that the long retention times experienced during dry weather flows are conducive to the attenuation of non-conservative contaminants. Flushing of channels, which occurs approximately once a year, resets the system and transports stored contaminants and sediments to receiving fluvial and estuarine systems. The low frequency of these floods allows the receiving systems time to recover between deliveries of dilute contaminants during high flow events. These findings have important implications for the management of water quality in the Sydney region.