The role of vegetation in the formation of anabranching channels in an ephemeral river, Northern plains, arid central Australia

As the distribution and abundance of vegetation in drylands is often contro lled by the greater availability of water along river channels, riparian ve getation has the potential to influence significantly dryland river form, p rocess and behaviour, This paper demonstrates how a small indigenous shrub, the inland teatree (Melaleuca glomerata), influences the formation and mai ntenance of anabranching channels in a reach of the ephemeral Marshall Rive r, Northern Plains, arid central Australia. Here, the Marshall is character ized by ridge-form anabranching, where water and sediment are routed throug h subparallel, multiple channels of variable size which occur within a typi cally straight channel-train. Channels are separated by channel-train ridge s - narrow, flow-aligned, vegetated features - or by wider islands. By prov iding a substantial element of boundary roughness, dense stands of teatrees growing on channel beds or atop the ridges and islands influence flow velo cities, flow depths and sediment transport, resulting in how diversion, ban k and floodplain erosion, and especially sediment deposition. Ridges and is lands represent a continuum of forms, and their formation and development c an be divided into a three-stage sequence involving teatree growth and allu vial sedimentation. (1) Teatrees colonize a flat, sandy channel bed, initiating the formation o f ridges by lee-side accretion. Individual ridges grow laterally, verticall y and longitudinally and maintain a geometrically similar streamlined (lemn iscate) form that presents minimum drag. (2) Individual ridges grow in size, and interact with neighbouring ridges, causing the lemniscate forms to become distorted. Ridges in the lee of othe r ridges tend to be protected from the erosive effects of floods and surviv e, whereas individual teatrees or small ridges exposed to flow concentrated between larger ridges, lend to be removed. (3) Ridges lengthen, and coalesce with downstream ridges, eventually subdiv iding the channel-train into well-defined anabranches. This sequence turns a channel, initially obstructed with dense and chaotic stands of teatrees, into a well-organized system of ridge-form anabranches. In the moderate- to low-gradient Marshall River, which is colonized by an abundance of within-channel vegetation and subject to declining downstream discharges, this helps to minimize flow resistance, thereby maintaining an efficient water and sediment flux. Copyright (C) 2000 John Wiley & Sons, Lt d.