Downstream changes in dryland river channels: The Northern Plains of arid central Australia

Many dryland rivers undergo marked downstream changes owing to factors such as infrequent floods, flow transmission losses, and typically few tributary inflows beyond the headwaters. Along the Sandover, Bundey (Sandover-Bundey) and Woodforde Rivers on the Northern Plains of arid central Australia, downstream channel changes are broadly similar. In upland zones, small, rocky channels transporting sand and gravel gradually increase in size before entering piedmont zones, where channels and narrow floodplains are confined by bedrock, alluvial terraces, or aeolian dunes. In lower gradient lowland zones, channels and floodplains remain confined and, in the absence of tributary inflows, channel cross-sectional areas and discharges decrease downstream. Confining landforms are not present in floodout zones, which results in splay formation, increased floodplain widths, and marked overall downstream decreases in cross-sectional areas. Eventually, channelised flow and bedload transport terminate, although occasional large floods continue across extensive unchanneled alluvial surfaces termed 'floodouts'. These broad similarities apart, downstream changes along the three rivers differ in detail. The Sandover is largely a single-thread channel, whereas many reaches of the Sandover-Bundey and Woodforde are anabranched. On the small Woodforde River, downstream decreases in parameters such as cross-sectional area and width are roughly linear. On the larger Sandover and Sandover-Bundey, downstream changes are more irregular, particularly through the floodout zones where there are marked fluctuations in widths, depths and bed slopes. The irregular downstream changes typical of the lower reaches of these large rivers may be due to the reduced influence of vegetation on bankline stability and width adjustment relative to that of smaller rivers. On the Northern Plains, as in other drylands, complex interactions between discharge, sediment transport, slope, patterns of tributary drainage, bank sediment type and vegetation result in variable patterns of downstream channel change. (C) 2000 Elsevier Science B.V. All rights reserved.