Anabranching rivers: ridge-form alluvial channels in tropical northern Australia

Sand-dominated ridge-form anabranching rivers on the Kimberley Plateau in northwestern Australia are a new type of alluvial channel system not previously described or explained in detail. This paper examines the morphology, sedimentology and stratigraphy of this type of river, and proposes mechanisms for their formation and maintenance. Alternating with relatively narrow, bedrock reaches of valley are wider, depositional, alluvial reaches that support roughly parallel, very elongated, steep-sided and treed sandy ridges of approximately floodplain height. These ridges subdivide the total flow system into remarkably straight, canal-like, anabranching channels. In this seasonally arid environment there is an abundance of riparian vegetation, in places growing chaotically over wider sections of stream bed. It is proposed here that the formation of ridges results in a reduction of flow resistance with an increase in depth. The ridges concentrate the flow and compensate for the less efficient flow conditions associated with these well-vegetated alluvial reaches; trees generally do not survive on the bed of the higher-energy channels. Enhanced velocities and increased bed shear act to maintain or increase water and sediment flux in these alluvial reaches. The formation of the channels and intervening ridges may be aided by the development of double flow helices in each anabranch, similar to that described for much smaller ridge features observed in laboratory flumes. Anabranches are also observed having formed by channel avulsion scouring new channels into adjacent floodplains, particularly in widened sections of valley near tributary junctions.