Anabranching and Anastomosing Rivers
Publication Name
Treatise on Geomorphology
Abstract
Multiple channel rivers characterized by stable islands that divide flows at bankfull are termed anabranching. They consist of a diverse group ranging from low energy with organic and/or fine clastic alluvial floodplains (anastomosing rivers), to high-energy gravel systems, with some carved into bedrock. Anabranching is not as common as single channel meandering or braiding but it nevertheless occurs widely in rivers from the subarctic to the tropics and from humid alpine to lowland arid regions. There are even examples on Mars. Importantly, it is the dominant pattern on the world’s largest alluvial rivers, suggesting that there is a size that a single-channel system cannot exceed and remain stable. Modeling and empirical research have shown that the introduction of islands reduces width/depth ratios and this enhances bed shear stress and associated sediment transport over gradients that would otherwise be insufficient for system stability. The same modeling reveals that in underloaded systems anabranching can consume surplus energy and encourage stability by generating a greater area of boundary flow-resistance. Individual anabranches develop either as erosional channels scoured into floodplains or from the growth of midchannel bars into long-lived islands or ridges that divide previously wider channels. They can also form from the progradation, vertical accretion and subsequent modification of delta distributary systems. Where sediment accretion leads to repeated channel avulsion and abandonment, anabranching can remain a dynamic, somewhat unstable, self-replicating pattern lasting millennium. These accumulating systems form thick stratigraphic sequences, their instability recognizable as vertically stacked packages of multiple channels and crevasse splays encased in floodplains of clastic and/or highly organic sediment. Under these conditions of sediment excess, a network of anabranching channels can, more efficiently than a single channel, distribute and store large volumes of sediment relatively uniformly across wide aggradational floodplains, fans and deltas. Alternatively, where vertical accretion is slow or non-existent, anabranching can develop to maintain mass-balance sediment flux that enable such rivers to remain stable for millennia. On small rivers, vegetation appears to be important for maintaining the bank strength necessary for anabranching whereas on very large rivers, where banks are high, steeply sloping and unstable, vegetation probably has very limited direct effect. Many European rivers have been changed from anabranching to single thread due to Late Holocene human-induced forest clearance that exacerbated floodplain sedimentation and channel filling. Engineering during the Middle Ages and more intensely during the Industrial Revolution was removed anabranches to make rivers and floodplains more suitable for agriculture, settlement, transportation and industry. Recent research has proposed that, for the benefit of wildlife and as a landscape management tool, some of these rivers should be returned to an anabranching state. Meandering, braiding and anabranching are all planform options available to alluvial rivers as they self-adjust to imposed environmental conditions.
Open Access Status
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First Page
544
Last Page
564