Alluvial river

An alluvial river has a bed and banks made of loose sediment or soil that can shift. These rivers shape themselves: their channels are carved by the size and frequency of floods, and by how those floods erode, deposit, and move sediment. As a result, alluvial rivers take many forms, depending on their bank material, the flows they handle, the surrounding streamside ecology, and the amount, size, and type of sediment they transport.

Over short timescales and small areas, events like seasonal flooding create patches of soil that range from oxygen-rich to oxygen-poor, each with different nutrients and rates of decay. Over larger areas and longer timescales, features are shaped by glacial events, sea-level changes, tectonic shifts, and other slow processes. Together, these short- and long-term patterns determine an alluvial river’s character. These rivers also include features such as valley-side hillslopes, terraces (old floodplains above the current one), natural levees, meander scrolls, drainage channels, and both temporary and permanent floodplains.

**Alluvial channel patterns** Natural alluvial channels come in several forms, generally described as straight, meandering, braided, or anastomosing. These patterns result from differences in bankfull discharge, slope, sediment supply, and bank material. Patterns are often classified by sinuosity—the ratio of the channel’s centerline length to the straight-line distance down the valley.

**Straight/sinuous channels** Straight channels (sinuosity less than 1.3) are rare in nature because sediment and flow are rarely spread evenly. Irregular erosion and deposition create alternate bars on opposite sides of the channel in sequence. These bars steer flow into a sinuous path, forming sinuous channels (sinuosity 1.3 to 1.5).

**Meandering channels** Meandering channels are more sinuous (sinuosity greater than 1.5) and are defined by the meander wavelength—the distance from one bend’s apex to the next on the same side. Meandering channels are common today, but no evidence of them exists before land plants evolved. This is largely because vegetation strengthens banks and helps maintain meanders.

**Braided channels** Braided channels have multiple active streams within a broad, low-sinuosity channel. Smaller strands split around sediment bars and rejoin downstream. These channels are dynamic, with strands shifting. Braiding occurs when sediment loads exceed the stream’s transport capacity. They are found downstream of glaciers and mountain slopes where slopes are steep, discharge varies, and coarse sediment loads are high.

**Anastomosing channels** Anastomosing channels also consist of complex strands that diverge and converge, but they differ from braided channels in that they flow around relatively stable, often vegetated islands. They have gentler gradients, are narrower and deeper, and have more permanent strands.

**Geomorphic units** **Meander wavelength** The meander wavelength—or alternate bar sequence—is the key ecological and morphological unit of meandering alluvial rivers. It consists of two alternating bar units, each with a pool scoured at a cutbank, an aggradational point bar, and a riffle connecting them. In an ideal channel, the meander wavelength is about 10 to 11 channel widths, meaning pools (and riffles and point bars) are spaced roughly 5 to 6 channel widths apart. The radius of curvature of a meander bend—measured by the radius of a circle fitting the arc—is between 2 and 3 times the channel width.

**Landforms** **Floodplains** Floodplains are the land areas next to alluvial river channels that flood often. They form from suspended sediment deposited by overbank flow, bedload deposited as the river migrates sideways, and landscape processes like landslides.

**Natural levees** Natural levees develop when overbank deposition shapes the floodplain and coarser material settles near the main channel. These levees rise higher than the surrounding floodplain, creating backswamps and yazoo channels—tributaries forced to run parallel to the main river instead of joining it.

**Terraces** Terraces are sediment storage features that record a river’s past sediment delivery. Many changes in boundary conditions can create them. The most basic cause is that the river lacks the transport capacity to move all the sediment supplied by its watershed. Past climate during the Quaternary has been linked to floodplain aggradation and incision, leaving step-like terraces.