Anabranching and anastomosing rivers
Multiple channel rivers characterized by vegetated or otherwise stable alluvial islands that divide flows at discharges up to bankfull are termed 'anabranching'. They consist of a diverse group ranging from low-energy with organic or fine-textured flood plains that have generally been termed anastomosing rivers, to high-energy gravel systems, or even those carved into bedrock. They are not as common as meandering or braiding rivers but occur widely from the subarctic to the tropics and from humid alpine to lowland arid regions. Furthermore, it is the dominant style among the world's largest alluvial rivers. Individual alluvial anabranches develop either as erosional channels scoured into flood plains or from mid-channel bars that grow progressively to form long-lived stable islands or ridges that divide the flow in previously much wider channels. They can also form from delta progradation and subsequent modification of the distributary system. Where rapid vertical accretion leads to frequent channel avulsion and abandonment, the pattern will be relatively unstable but can remain anabranching for millennia. Such aggrading rivers form thick sedimentary sequences, their instability recognizable in the rock record as vertically stacked packages of multiple channels encased in flood plains of accreting clastic and/or organic sediment. Under such conditions of sediment excess, a network of anabranching channels can relatively efficiently redistribute and store the surplus load across wide valleys or advancing deltas. Alternatively, where vertical accretion is slow or nonexistent, very stable anabranching channels can form, which appear to have a long-term advantage over other patterns. Modeling has shown that such anabranches can lead to equilibrium conditions that maintain the mass-flux balance of bedload by confining bankfull flows in transport-efficient low width/depth (w/d) ratio channels. Reduced w/d ratios formed by the introduction of islands enhance bed shear and move sediment over gradients that would otherwise be insufficient. By contrast, in underloaded systems, modeling has shown that anabranching can consume surplus energy by generating greater skin resistance with the addition of channels. On relatively small rivers, vegetation appears to be essential for maintaining the bank strength necessary for anabranching. By contrast, on very large rivers where banks are high and stage varies greatly, vegetation probably has limited or even no effect, and anabranching is probably linked to keeping w/d ratios in a range optimal for sediment transport as discharge increases. Along with meandering and braiding, anabranching provides yet another mechanism facilitating self-adjustment in alluvial rivers.