Rivers and alluvial fans

Excerpt

A river is a natural stream of water that under the influence of gravity flows regularly or intermittently in a channel or channels toward a receiving basin, commonly an ocean or lake. Rivers are open systems in which energy and matter are exchanged with the external environment (Knighton, 1998). They are supplied with water almost entirely from precipitation (meteoric water) routed to the channel via overland flow, groundwater, swamps, lakes, snowfields, and glaciers, and they acquire most of their sediment load by dissecting uplands or by reworking unconsolidated debris from previous erosional events. Sediment deposited by rivers in subaerial settings is called alluvium. Consequently, rivers are divisible into bedrock reaches with rigid boundaries, and alluvial reaches that have a mobile boundary of relatively unconsolidated material, often with vegetation assisting channel stabilization. Rivers drain catchments (drainage basins or watersheds), are organized into complex patterns of trunk, tributary, and distributary channels, and commonly support adjacent floodplains that are inundated when the channels exceed bankfull capacity. Their headwaters are principally in upland areas and they may flow throughout the year (perennial) or cease for part of each year (intermittent if flow is seasonal and ephemeral if flow is irregular). Due largely to tributary contributions, river systems generally increase in discharge and channel dimensions downstream, however, in areas of permeable sediment or aridity, percolation, and evaporation may cause channels to reduce or disappear. …