Fluvial

Fluvial

Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.

Fluvial processes comprise the motion of sediment and erosion or deposition (geology) on the river bed.Erosion by moving water can happen in two ways. Firstly, the movement of water across the bed exerts a shear stress directly onto the bed. If the cohesive 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. However, if the river carries significant quantities of sediment, 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 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 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 Hjulstrøm 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 the flood plains and deltas of large rivers that large, geologically-significant alluvial deposits are found.

The amount of matter carried by a large river is enormous. The names of many rivers derive from the color that the transported matter gives the water. For example, the Huang He in China is literally translated "Yellow River", and the Mississippi River in the United States is also called "the Big Muddy." It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea,^[3] the Huang He 796 million tons, and the Po River in Italy 67 million tons.^[4]

I. Straight channels tend to develop sinuousity. Any perturbation tends to enlarge, either erosional by bank cutting or depositional by formation of bars attached to channel sides. These rivers will meander if flows sufficiently strong and/or bank material sufficiently weak to allow channel migration. So it is hard to get a perfectly straignt channel in nature.

At the same time there is an upper limit on how much sinuousity can occur because if too sinuous meander loops will touch a get cut off (ox bow lakes can form this way). Hence there is a zone, the meander belt or channel belt, along a river valley where the active meandering channel will tend to be found. The channel freely meanders within this zone through time, but the width of the belt is set by the sinuousity of the channel. Over time the channel belt can migrate, if, for example, the channel tends to migrate to the right or left over time, but generally the belt stays more or less fixed until the river avulses, i.e. abandons its channel at a point, during a flood, and after the flood receeds the river follows a new course.

II. Meandering processes and deposits

A. As meander belts migrate they incise along the cut bank on the outside of a bend and deposit a point bar along the inner part of the bend. The point bars are seen in white in the photo above. As the channel continues to migrate, the old position of a point bar is preserved topographically as a system of ridge and swales referred to as scroll bars that can be seen out across modern flood plains and in ancient sedeimtnary deposits (below).

B. Channel fills tend to fine upward due to decreased flow depth and resultant decrease in shear stress, so that the flow is only capable of carrying finer and finer material as channel depth gets reduced.

C. Levees can build during floods as the river rises, and comes out of its confined channed. As the water flows overbank, there is flow expansion, a reduction in shear stress and any sediment in the flow will start to deposit.

D. At times the levees are breached locally during a flood, a process referred to as a crevasse splay. Water shoots out of this gap and, via flow expansion, slows down and deposits its sediment, referred to as a crevasse splay deposit.

E. Fining upwards sequences take place as the channel migrates and is filled in by progressively finer and finer grained sediment.

F. Avulsion - Over long time scales (centuries to thousands of years) river avulsion takes place whereby rivers leave their channel belt at a point, presumably during a flood, and move to another part of the alluvial basin. This results in the abandonment of channel belts. In the rock record this can be seen by abrupt tops of sand bodies, representing the channel belts.