Alluvial river

An alluvial river is one 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. For this reason, 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. An alluvial river is one 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. For this reason, 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. Braided channels are characterized by multiple, active streams within a broad, low sinuosity channel. The smaller strands of streams diverge around sediment bars and then converge in a braiding pattern. Braided channels are dynamic, with strands moving within the channel. Braided channels are caused by sediment loads that exceed the capacity of stream transport. They are found downstream of glaciers and mountain slopes in conditions of high slope, variable discharge, and high loads of coarse sediment. Anastomosing channels are similar to braided channels in that they are composed of complex strands that diverge and then converge downstream. However, anastomosing channels are distinct from braided channels in that they flow around relatively stable, typically vegetated islands. They also have generally lower gradients, are narrower and deeper, and have more permanent strands. The meander wavelength or alternate bar sequence is considered the primary ecological and morphological unit of meandering alluvial rivers. The meander wavelength is composed of two alternating bar units, each with a pool scoured out from a cutbank, an aggradational lobe or point bar, and a riffle that connects the pool and point bar. In an idealized channel, the meander wavelength is around 10 to 11 channel widths. This equates to pools (and riffles and point bars) being separated by an average of 5 to 6 channel widths. The radius of curvature of a meander bend describes the tightness of a meander arc, and is measured by the radius of a circle that fits the meander arc. The radius of curvature is between 2 and 3 times the channel width. Floodplains are the land areas adjacent to alluvial river channels that are frequently flooded. Floodplains are built up by deposition of suspended load from overbank flow, bedload deposition from lateral river migration, and landscape processes such as landslides. Natural levees occur when the floodplain of an alluvial river is primarily shaped by overbank deposition and when relatively coarse materials are deposited near the main channel. The natural levees become higher than the adjacent floodplain, leading to the formation of backswamps and yazoo channels, in which tributary streams are forced to flow parallel to the main channel rather than converge with the main channel.