Anabranching and Anastomosing Rivers
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 bed...
| Main Author: | |
|---|---|
| Format: | Text |
| Language: | unknown |
| Published: |
Research Online
2022
|
| Subjects: | |
| Online Access: | https://ro.uow.edu.au/test2021/7770 https://doi.org/10.1016/B978-0-12-409548-9.12518-7 |
| Summary: | 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 ... |
|---|