An alluvial river is river in which the bed and banks are made up of mobile sediment and/or soil. Alluvial rivers are self-formed, meaning that their channels are shaped by the magnitude and frequency of the floods that they experience, and the ability of these floods to erode, deposit, and transport sediment. As such, alluvial rivers can assume a number of forms based on the properties of their banks; the flows they experience; the local riparian ecology, and the amount, size, and type of sediment that they carry.
Natural alluvial channels have a variety of morphological patterns, but can be generally described as straight, meandering, braided, or anastomosing. Different channel patterns result from differences in bankfull discharge, gradient, sediment supply, and bank material. Channel patterns can be described based on their level of sinuosity, which is the ratio of the channel length measured along its center to the straight line distance measured down the valley axis.
Straight channels (sinuosity <1.3) are relatively rare in natural systems due to the fact that sediment and flow are rarely distributed evenly across a landscape. Irregularities in the deposition and erosion of sediments leads to the formation of alternate bars that are on opposite sides of the channel in succession. Alternating bar sequences result in flow to be directed in a sinuous pattern, leading to the formation of sinuous channels (sinuosity of 1.3-1.5).
Meandering channels are more sinuous (>1.5 sinuosity) than straight or sinuous channels, and are defined by the meander wavelength morphological unit. The meander wavelength is the distance from the apex of one bend to the next on the same side of the channel. Meandering channels wavelength are described in section 1.2 Geomorphic Units. Meandering channels are widespread in current times, but no geomorphic evidence of their existence before the evolution of land plants has been found. This is largely attributed to the effect of vegetation in increasing bank stability and maintaining meander formation.