Ingeography andgeology,fluvial sediment processes orfluvial sediment transport are associated withrivers andstreams and thedeposits andlandforms created bysediments. It can result in the formation ofripples anddunes, infractal-shaped patterns of erosion, in complex patterns of natural river systems, and in the development offloodplains and the occurrence offlash floods. Sediment moved by water can be larger than sediment moved by air because water has both a higherdensity andviscosity. In typical rivers the largest carried sediment is ofsand andgravel size, but larger floods can carrycobbles and evenboulders.When the stream or rivers are associated withglaciers,ice sheets, orice caps, the termglaciofluvial orfluvioglacial is used, as inperiglacial flows andglacial lake outburst floods.^[1][2] Fluvial sediment processes include themotion of sediment anderosion ordeposition on theriver bed.^[3][4]

The movement of water across thestream bed exerts ashear stress directly onto the bed. If thecohesive strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow. In addition, if the river carries significant quantities ofsediment, this material can act as tools to enhance wear of the bed (abrasion). At the same time the fragments themselves are ground down, becoming smaller and more rounded (attrition).

Sediment in rivers is transported as eitherbedload (the coarser fragments which move close to the bed) orsuspended load (finer fragments carried in the water). There is also a component carried as dissolved material.

For each grain size there is a specificflow velocity at which the grains start to move, calledentrainment 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 theHjulström 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 calledalluvial or flood plains, and the dropped particles are calledalluvium.

Even small streams make alluvial deposits, but it is infloodplains anddeltas 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 theMississippi River annually carries 406 million tons of sediment to the sea,^[5] theYellow River 796 million tons, and thePo River inItaly 67 million tons.^[6] The names of many rivers derive from the color that the transported matter gives the water. For example, theYellow River (Huang He) inChina is named after the hue of the sediment it carries,^[7] and theWhite Nile is named for the clay it carries.

The main kinds of fluvial processes are:

• Transportation - The movement of sediment downstream via bedload, suspended load, or dissolved load.
• Deposition - The settling of sediment when the river's energy decreases.

The major fluvial (river and stream)depositional environments include:

• Deltas (arguably an intermediate environment between fluvial and marine)
• Point bars
• Alluvial fans
• Braided rivers
• Oxbow lakes
• Levees
• Waterfalls
• Crevasse splays
• Natural levees
• Backswamps

Rivers and streams carry sediment in their flows. This sediment can be in a variety of locations within the flow, depending on the balance between the upwards velocity on the particle (drag and lift forces), and thesettling velocity of the particle. These relationships are shown in the following table for theRouse number, which is a ratio of sediment settling velocity (fall velocity) to upwards velocity.^[8][9]

$${\displaystyle \mathbf{\text{Rouse}}=\frac{\text{Settling velocity}}{\text{Upwards velocity from lift and drag}}=\frac{w_s}{\kappa u_{\ast }}}$$

where

• ${\displaystyle w_s}$ is the settling velocity
• ${\displaystyle \kappa }$ is thevon Kármán constant
• ${\displaystyle u_{\ast }}$ is theshear velocity

If the upwards velocity is approximately equal to the settling velocity, sediment will be transported downstream entirely assuspended load. If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move (seeInitiation of motion), it will move along the bed asbed load by rolling, sliding, andsaltating (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 aswash load.^[10]

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.

Sediment motion can create self-organized structures such asripples,dunes, orantidunes on the river orstream bed. These bedforms are often preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the flow that deposited the sediment.

Overland flow can erode soil particles and transport them downslope. The erosion associated with overland flow may occur through different methods depending on meteorological and flow conditions.

• If the initial impact of rain droplets dislodges soil, the phenomenon is called rainsplash erosion.
• If overland flow is directly responsible for sediment entrainment but does not form gullies, it is called "sheet erosion".
• If the flow and the substrate permit channelization, gullies may form; this is termed "gully erosion".

• Body of water
• Channel pattern
• Sorting
• List of fluvial landforms

• Hydrology
• Fluvial landforms
• Sedimentology
• Geomorphology

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