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In geography and geology, fluvial sediment processes or fluvial sediment transport are associated with and and the deposits and created by . It can result in the formation of ripple marks and , in fractal-shaped patterns of erosion, in complex patterns of natural river systems, and in the development of and the occurrence of flash floods. Sediment moved by water can be larger than sediment moved by air because water has both a higher density and viscosity. In typical rivers the largest carried sediment is of sand and gravel size, but larger floods can carry cobbles and even boulders. When the stream or rivers are associated with , , or , the term glaciofluvial or fluvioglacial is used, as in periglacial flows and glacial lake outburst floods. Wilson, W.E. & Moore, J.E. 2003. Glossary of Hydrology, American Geological Institute, Springer, 248pp. Fluvial sediment processes include the motion of sediment and erosion or deposition on the river bed.
Sediment in rivers is transported as either bedload (the coarser fragments which move close to the bed) or suspended load (finer fragments carried in the water). There is also a component carried as dissolved material.
For each grain size there is a specific flow velocity at which the grains start to move, called entrainment velocity. However the grains will continue to be transported even if the velocity falls below the entrainment velocity due to the reduced (or removed) friction between the grains and the river bed. Eventually the velocity will fall low enough for the grains to be deposited. This is shown by the Hjulström curve.
A river is continually picking up and dropping solid particles of rock and soil from its bed throughout its length. Where the river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium.
Even small streams make alluvial deposits, but it is in and River delta of large rivers that large, geologically-significant alluvial deposits are found.
The amount of matter carried by a large river is enormous. It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea,Mathur, Anuradha; Dilip da Cunha (2001). Mississippi Floods: Designing a Shifting Landscape. New Haven, CT: Yale University Press. the Yellow River 796 million tons, and the Po River in Italy 67 million tons.Dill, William A. (1990). Inland fisheries of Europe. Rome, Italy: UN Food and Agriculture Organization. . http://www.fao.org/docrep/009/t0377e/t0377e00.htm The names of many rivers derive from the color that the transported matter gives the water. For example, the Yellow River in China is named after the hue of the sediment it carries, (2025). 9780300238334, Yale University Press. ISBN 9780300238334 and the White Nile is named for the clay it carries.
where
| Bed load | >2.5 |
| Suspended load: 50% Suspended | >1.2, <2.5 |
| Suspended load: 100% Suspended | >0.8, <1.2 |
| Wash load | <0.8 |
If the upwards velocity is approximately equal to the settling velocity, sediment will be transported downstream entirely as suspended load. If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move (see Initiation of motion), it will move along the bed as bed load by rolling, sliding, and saltating (jumping up into the flow, being transported a short distance then settling again). If the upwards velocity is higher than the settling velocity, the sediment will be transported high in the flow as wash load.
As there are generally a range of different particle sizes in the flow, it is common for material of different sizes to move through all areas of the flow for given stream conditions.
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