Fluvial sediment

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Typical sediment, here gravel

As fluvial or fluvial (from the Latin fluvius "river", so for example "caused by rivers") sediments , colloquially river deposits , are referred to in geosciences as crushed rock carried along by a river .

Fluvially transported sediments are usually well rounded and can include all rocks that occur in the catchment area of the respective river or stream . They have grain sizes that - depending on the location in the lower or upper reaches of the water - range from fine sand to rubble (0.1 mm to about 20 cm). Individual stones are referred to in southern Germany and beyond as Wacken .


In fluvial sediments transported between solution cargo , suspended matter or suspension load and rubble or solid cargo distinguished.

Solution load is the proportion of the total load that is transported dissolved in the water . The solids load is not dissolved in the water and is transported either jumping ( salting ) or rolling-pushing. Suspended matter is suspended in the water column because of its weight.

Original edges are sanded off during transport. The amount of water and its speed determine the size of the cargo in a stretch of flowing water. A measure for this is the transport capacity of the flowing water. If the transport capacity decreases while the bed load remains the same, the material is deposited (sedimentation).

Whether erosion (removal of material) or sedimentation (deposition) prevails in a river section is expressed by the load ratio. Numbers> 1 denote sedimentation, <1 erosion, and 1 the equilibrium.

Erosion occurs z. B. in the formation of river valleys in mountainous or hilly terrain. The sideways shift of meandering rivers leads to erosion ( scour formation or scouring) on the impact slope and deposits on the slip slope .


Fluvial transport can lead to significant deposition of sediments, e.g. B. accumulate in river terraces and in gravel banks above or below ground - important raw material sources for gravel pits .

Rocks are transported most and farthest in times of heavy rainfall, especially in ice ages and floods . During the Ice Ages, fluvial transport led to the formation of many river terraces that shape vast landscapes - for example in the Danube countries , in the catchment area of ​​the Rhine or in Poland and Russia . Even today, floods can lead to the shifting of rivers if a flood- carrying river deposits its debris in a shallow meander and thus blocks its own path.

In the case of many rivers with low gradients, it is ecologically significant that their sludge means an important addition of nutrients in the riverside regions during floods . Examples are the Nile floods that used to take place annually until the construction of the Aswan Dam , which deposited the fertile Nile mud on the fields near the river and thus helped Egypt to cultivate intensive agriculture . The same applies to the great rivers of China , although this effect is now also eliminated in the Yangtze River due to the Three Gorges Dam ; For similar reasons, the damming up in the upper reaches of the Eufrat is criticized.

The absence of regular flooding can also - such as B. in the Danube regulation - lead to the deepening of the river bed and lower the water table significantly.

See also

Other forms of sedimentation caused by water are called fluvioglacial (mixed form of river / glacier) and limnic and marine sediment ( lake or sea deposits ). See also:

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