If a stream is flowing along straight, the strongest, fastest flow will be in the center of the stream well above the bottom of the bed or channel but below the surface. Friction with the bottom and sides of the channel acts to slow the water against it, and that in turn slows the adjacent water, but not as much. The surface water experiences friction with the air, but it is not as great as that with the ground. See the illustrations below of the map view and the cross section of the stream.

stream flow map

stream flow cross section

A bend in a stream is called a meander. On a meander, the main flow of water is forced to the outside of the bend, just like your body is pulled toward the outside of a curve as you go around it in a car. This means the main force of the water is going to be directed against one side of the bank. Here, the water will hit against the bank and create turbulence. Faster, more turbulent water has a greater erosional capacity than slower, less turbulent water. On a meander, then, the stream will actively cut away or erode the outside bank making it steeper. This steepened, eroded slope is called an UNDERCUT SLOPE. On the opposite side of the stream, the water will be less turbulent. This slower, gentler water is less capable of carrying the load or material being transported by the water, so it is a location of deposition. This deposition will fill in part of the bend making a gentle slope called a SLIPOFF SLOPE. Material deposited by a stream is called ALLUVIUM.

cross section of a meander

This relationship between velocity and carrying capacity is an extremely important aspect of the fluvial process. A stream carries or transports a load. Some of this load is just being rolled or pushed along the bottom of the stream (traction), some is being bounced along the bottom (saltation), some is being picked up and carried (suspension), and some of the load has been dissolved (solution). If the velocity increases, the size (defined by weight) of the particles moved by traction, saltation, and suspension will increase. This means that more material will be picked up and moved, if it is available. The load will increase. If, however, the velocity drops, the amount of material being transported by each of these will be reduced. As the velocity rises or falls, so does the amount of load.

transportation of stream load

Consider the situation of where a stream flows into a body of water. As the stream flows downslope, it is carrying a load based on the gradient and velocity of the stream. When the stream empties into the water body, that gradient is lost. The water slows down, and it can no longer carry all of that load. The result is that it drops or deposits this material. The landform that develops is called a DELTA. The stream drops the material which eventually can clog the channel. Part of the water overflows and follows another path. The stream ends up branching apart and extending the deposits further into the body of water. These stream branches are DISTRIBUTARIES.

Another similar situation with relationship to gradient reduction results in the formation of an ALLUVIAL FAN. Where a stream goes from a steep slope to a lesser slope, such as when flowing from mountains onto a plain, the sudden drop in gradient promotes the deposition of material. A delta shaped deposit of material is added onto the plain as the water slows down, drops part of its load, and then branches around the blockage through new distributaries. Note that the first major difference between an alluvial fan and a delta is that the alluvial fan forms on land and the delta forms in water.

Braided streams form in a similar manner. When a stream has a sudden decrease in volume or a sudden increase in load, it may be unable to handle all of this material. Like in the above two situations, it drops the material and then some of the water must seek an alternate route. The stream develops multiple, smaller channels through the area. The channels intersect resembling a braid.

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