GRADIENT IRREGULARITY IN THE HERBERT GORGE OF NORTHEASTERN AUSTRALIA

The seventy-kilometre-long Herbert Gorge of northeastern Australia preserves a record of past floods in slackwater deposits and palaeostage indicators. Step-backwater modelling of water-surface profiles indicates that discharges ranging from 11 000 to 17 000 m3s-1 have occurred six times in the gorge during the last 900 years. These flood reconstructions provide insight into the role of extreme flows in shaping bedrock channel morphology. In particular, the hydraulics of extreme flows can be related to boulder transport, and to the location of large boulder bars. Large boulder bars occur throughout the Herbert Gorge, being best developed at loci of stream power minima along the inside of bends, at tributary junctions, and at obstructions in the channel caused by bedrock highs. Only the flows exceeding approximately 8000 m3s-1 are competent to transport the boulders which constitute the bars. In the straight channel reaches, the boulder accumulations and bedrock highs have a fairly regular spacing which appears to be independent of lithologic or structural controls. The bars provide an efficient means of energy dissipation, and they are interpreted as a result of the inherent high turbulence of flow in a steep channel. The regular spacing of the bars, and their correspondence with the hydraulics of large flows, suggest that the bars and associated bedrock highs may represent a self-regulating mechanism akin to the pool-riffle sequence of alluvial channels. It may therefore be appropriate to view bedrock channels as deformable on the timescale of extreme discharges.