CHANNEL CHANGES ASSOCIATED WITH 2 ADJACENT WEIRS ON A REGULATED LOWLAND ALLUVIAL RIVER

Survey data from 1906 and 1988 for the lower River Murray, Australia, reveal changes in channel morphology associated with the construction of Locks 2 and 3 (1925-8), which form contiguous pools in the Valley a nd Gorge sections, respectively. Regulated annual flows are less than half the volume of natural flows, and the frequency of maximum flows h as decreased. Upstream weirs have reduced the sediment supply to the s tudy reach by 1.05 x 10(6) tonnes per annum. The trap efficiencies of Pools 2 and 3 from 1906 to 1988 were 8 and 13% respectively, with rete ntion channel of 80 723 and 267 470 tonnes. Responses over the past 60 -70 years have differed between the pools-some channel characteristics have attained a new dynamic equilibrium and others have not. In Pool 3 the average bed slope has been reduced by 0.00004 (44%) and has atta ined equilibrium, but in Pool 2 the supply and redistribution of sedim ent have not been sufficient to reduce the bed slope. There are contin uous areas of degradation and aggradation in Pool 3 but discontinuous areas in Pool 2, reflecting different stages in slope adjustment. Cros s-sections have become wider and shallower in Pool 3 but narrower and deeper in Pool 2. These adjustments are influenced by local boundary c onditions, floodplain morphology and the position of the study reach i n the sequence of weirs. In general, the situation in the lower Murray does not conform to conventional models of the impact of regulation, which are based on data from upland dams. The Murray's gross morpholog y remains as a relict of the natural, semi-arid regime. Low contempora ry stream energies and cohesive bank materials restrict the initiation of channel metamorphosis and prolong the time required for the comple tion of channel adjustment.