Alluvial Systems - Alluvium, Commmon Components, Coastal Alluvial Plains, Alluvial Fans, Deltas
fluvial term streams flowing
An alluvial system consists of sediments eroded, transported, and deposited by water flowing in rivers or streams. The sediments, known as alluvium, can range from clay-sized particles less than 0.002 mm in diameter to boulders greater than 64 mm in diameter, depending on their source and the sediment transport capacity of streams in the system. The term alluvial is closely related to the term fluvial, which refers to flowing water. Thus, alluvial systems are the result of fluvial processes.
Modern alluvial systems can create flat and fertile valley bottoms that are attractive for farming because of their rich soils, which are replenished during frequent floods. The same floods that replenish soils, though, can become hazardous when homes are built on floodplains. Ancient alluvial systems that now lie below Earth's surface can be exceptionally good aquifers and petroleum reservoirs.
This delta formed downstream of a break in a natural levee. Note the marsh growth that developed along the banks of the distributaries. Photograph by Dan Guravich. Photo Researchers, Inc. Reproduced by permission.
Additional Topics
Alluvium is the product of sediment erosion, transportation, and deposition. Therefore, its nature is controlled by the sediment supply and sediment transport capacity of streams in the watersheds from which it is derived. Regions subjected to high rates of tectonic uplift or tropical climates can supply large amounts of sediments to fluvial systems because rocks weather rapidly under those condit…
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 alluvi…
Alluvial fans form where high-gradient mountain streams flow into valleys or onto plains and deposit their sediment load. Such is the case along the foot of the Panamint Mountains bordering Death Valley in California. As with overbank floodplain deposits, the coarsest sediment is generally deposited closest to the mountains and finer sediment can be carried many miles. Geologists often refer to th…
Deltas are formed at the mouths of streams that flow into lakes or oceans. They are fan-like deposits similar to alluvial fans, but located in the water rather than on dry land. Like alluvial fans, coarse sediments are deposited close to shore and fine-grained sediment is carried farther out to sea. The Mississippi River has formed the most prominent example of a delta within the United States. Ot…
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