Large alluvial islands of relatively stable floodplain are common. One theory for the formation of such patterns in large rivers holds that large discharges of water and suspended sediment are best accommodated in relatively narrow, deep channels. Although width can easily increase as river size scales upward, depth cannot. The depth of scour is limited by the nature of the alluvial valley fill. Therefore, very large rivers with high suspended sediment loads must split their channels, maintaining several channels of optimum width-depth ratio.
Anastomosed rivers have relatively fine sediment loads and gentle gradients.
Note that anastomosed rivers of this type are very different from braided rivers, despite similar sinuosity and the presence of islands. Braided rivers possess wide, shallow channels that are optimum for transporting relatively coarse sediment as bedload. As the stage drops during recession of the flood hydrograph, the bars become obstructions, splitting the flow into narrower, more efficient secondary channels. The former bars may eventually stabilize to become temporary islands. However, these can be easily reworked during a subsequent major flood.