Anabranching rivers on the Northern Plains of arid central Australia

Anabranching rivers are a widespread feature of the Northern Plains in the Alice Springs region of central Australia but their unusual characteristics previously have not been described. On the Northern Plains, anabranching o ccurs on rivers transporting bedloads of coarse sand and gravel and is char acterised by channels of variable size and shape which occur within a broad er, typically well-defined, channel-train. Channels are separated by channe l-train ridges-narrow, flow-aligned, vegetated features-or by wider islands . Ridges and islands are either depositional features (formed in situ by ac cretionary processes) or erosional features (formed by excision from once-c ontinuous areas of floodplain). Vegetation plays a key role in the initiati on, survival and growth of depositional forms through its influence on flow , sediment transport and ridge and island stability. Anabranching is also r elated to the influence of tributaries, for some large rivers alternate fro m single-thread to anabranching along their length in response to tributary inputs of water and sediment. Tributary inputs occur during flow events th at are either independent from, or in concert with, floods in the trunk cha nnel. Ridges and islands form in association with tributaries as a result o f various hydrological, depositional and erosional processes, including irr igation of enhanced numbers of in-channel trees and resulting lee-side sedi ment accretion, floodplain scour, and the formation and maintenance of defe rred-junction tributaries. The change from single-thread to anabranching do wnstream of tributary junctions occurs in the absence of any significant ch ange in channel gradient or degree of channel confinement. On the Northern Plains, anabranching appears to be a stable river pattern that helps to mai ntain the throughput of relatively coarse sediment in low-gradient (typical ly 0.0005-0.002) channels characterised by an abundance of within-channel v egetation and subject to declining downstream discharges. (C) 1999 Elsevier Science B.V. All rights reserved.