Aspects of the fluvial geomorphology of the eastern Kimberley Plateau, Western Australia

Rivers on the eastern Kimberley Plateau in monsoonal northwestern Australia reveal incised and steepened bedrock and boulder-bed reaches downstream of low gradient mixed alluvial-bedrock sections. This study investigates causes for this downstream steepening, describes forms and processes in the bedrock channel reaches, and examines a variety of alluvial anabranching systems found alternating with non-alluvial channel reaches. It concludes with an investigation of the Quaternary chronology of the region's alluvial deposits flanking the rivers. Bedrock channels cut by hydraulic plucking of joint-blocks into the region's welljointed and only gently deformed sandstones reveal channel morphologies shaped by high magnitude floods and they clearly reflect strong geological control. Along the Durack River, clusters of imbricated boulders comprising rock slabs up to 1 m thick, 8 m wide, and 13 m long provide evidence of bedrock erosion and transport during extreme floods. Based on estimates of balanced resisting and driving forces, flow velocities needed to initiate the motion of such large rock slabs are estimated. In contrast are the region's sand-bed reaches where steep-sided tree-lined ridges subdivide the total channel into well defined anabranches shaped by more frequent lower magnitude floods. It is argued that the ridges form to compensate for the less efficient flow conditions associated with these densely vegetated alluvial reaches. They may also be related to the development of secondary currents. The morphology, sedimentology, and TL chronology of alluvial surfaces flanking rivers on the Kimberley Plateau suggest that the later half of the Holocene was characterized by floodplain construction and channel contraction. Complementary TL chronologies of alluvium and dune sands in the east Kimberley provide evidence of fluvial activity in Isotope Stage 3, aeolian activity during the Last Glacial Maximum, and renewed fluvial activity in the early Holocene.