Fluvial processes
Description
Fluvial processes are the interaction of flowing water and the natural channels of rivers and streams. Fluvial processes play an essential and conspicuous role in the denudation of land surfaces and the transport of rock detritus from higher to lower levels. Most of the world’s land surface has been sculpted by running water. Running water is one of four flowing substances that erode, transport, and deposit mineral and organic matter (others: wind, waves, ice). Rivers and streams alter the Earth’s landscape through the movement of water and sediment. Streams are powerful erosive agents moving material from their bed and banks. In mountains, stream erosion often produces deep channels and canyons. Streams can also deposit large amounts of sediment on the terrestrial landscape (floodplains) and within lakes and ocean basins. Losses of material carried by the stream system occur through a number of processes. Water is lost by evaporation, seepage, and flooding. Stream flow ends when the water carried by stream enters a receiving basin like a lake or an ocean. Sediment is lost by various types of deposition.
The nature of the stream channel differs along the stream profile.
In the stream headwaters the stream gradient and surrounding topography is quite steep. Stream velocity is at maximum and cuts a narrow and deep channel. The high velocities carry all sizes of sediment load downstream and floodplain is minimal or not present at all. The steep relief provides the energy for rapid erosion and produces a V-shaped river valley. The stream load is thus high.
In the middle reach of the stream profile changes of the relief cause modification of the stream channel. The stream leaves the mountains and enters more gently sloping landscape. The stream velocity is reduced. The stream adjusts to this change by deposition of coarse sediment load by forming the narrow floodplain. The prevailing river pattern is the braided channel. The braided channel continuous change in size, number and location is caused by temporal variations in stream discharge. The temporal fluctuations in flow cause changes in the amount of sediment in the floodplain and the floodplain extent.
The extensive flat floodplain is usually formed near the mouth of the stream. The river pattern is usually meandering stream that deposits point bars and other floodplain fines. The channel is usually quite wide and U-shaped. In reaches like this the stream discharge is at its maximum and sediment load is generally composed of fine material (sand, silt, clay).
The landforms formed by flowing water can be divided in two sub-categories: erosional and depositional.
In erosional landforms can be fitted: gorges, ravines, waterfalls, rapids, gullies, arroyos, canyons, bank outcrops, scars, floodplain channels.
In depositional landforms can be fitted: bars, floodplains, levees, alluvial fans, deltas, crevasse splays.
References:
[1] Goudie, A.S., 2004. Encyclopedia of Geomorphology, Routledge, London, 1156 s.
[2] Strahler A., Strahler, A., 2003. Introducing physical geography, Wiley, New York, 684 s.