COMPLEX CHANNEL RESPONSE TO FLOW REGULATION - CUDGEGONG RIVER BELOW WINDAMERE DAM, AUSTRALIA

Windamere Dam is located on the Cudgegong River near Mudgee, Australia, and was built for irrigation and town water supply. It is operated in a similar manner to the other large irrigation dams in New South Wales. The response of the Cudgegong River to flow regulation since dam closure in February 1984 was determined for 13 km of river downstream of the dam where the channel is a bedrock-confined gravel-bed stream. Mean daily flows for durations less than 70 per cent have been reduced by between 20 and 73 per cent and those for durations greater than 70 per cent have been increased by between 58 and 496 per cent. The dam has reduced downstream flood magnitudes for all recurrence intervals by between 57 and 72 per cent. The dam's sediment trap efficiency is estimated at greater than 95 per cent of the incoming sediment load. Complex river response was induced by flow regulation, and included simultaneous channel contraction, degradation, aggradation, accommodation adjustment, formation of tributary-mouth bars and in-channel benches, and massive vegetation encroachment at different locations. Mean bed-material size decreased at all aggrading sites. Regulated flows are incompetent for the transport of the bed material but large floods generated in unregulated tributaries below the dam exceed the threshold of motion at channel constrictions. These unregulated tributaries now supply the majority of sediment to the channel. This material is reworked by dam releases which preferentially remove the fines and deposit them in downstream pools and on in-channel benches. Vegetation has invaded these recent deposits.