Alluvial Systems

The channel is the depression through which water flows from the head to the mouth of a stream. Some channels follow sinuous, or meandering, paths. Erosion occurs along the outside edge of meanders, where water velocity is the greatest, and deposition occurs along the inside edge of meanders, where water velocity is the lowest. The resulting depositional feature is known as a point bar, and alluvial channel deposits are formed by the constant migration of meanders through geologic time. Other channels are braided, meaning that they are composed of two or more interconnected low-sinuosity channels. Braided stream channels form when the sediment load is large compared to the sediment transport capacity of a stream. Braided streams can occur naturally in areas of high sediment supply, for example near the snouts of glaciers or where the stream gradient abruptly decreases. A change from a meandering stream to a braided stream over a few years or decades, however, can indicate an undesirable increase in sediment supply as the result of activities such as cattle grazing or logging.

During times of high discharge, for example shortly after heavy rainstorms or spring snowmelt, streams can rise above their banks and flood surrounding areas. The low-lying and flood-prone areas adjacent to streams are known as flood plains. The velocity of water flowing across a flood plain is much lower than that in the adjacent channel, which causes suspended sediment to be deposited on flood plain. Coarse sediment is deposited close to the channel and forms natural levees that help to control future floods, but silt and clay can be carried to the far reaches of the floodplain. Thus, alluvial systems generally consist of both coarse-grained channel deposits representing bedload and fine-grained floodplain (also known as overbank) deposits.

Meanders can be abandoned, particularly when they become extremely sinuous, to form crescent-shaped oxbow lakes within the flood plain. Abandonment occurs when it becomes more efficient for the stream to transport its sediment load by cutting a short new channel, thereby locally increasing its gradient, than by flowing through a long meander.

Another common feature of alluvial systems is the stream terrace. A stream terrace is simply a floodplain that was abandoned when the stream incised to a lower level due to a change in base level. Careful observation can often reveal several generations of step-like terraces, each of which represents the elevation of an abandoned flood plain, along a stream valley.