<P> As the slope of the river channel decreases, it becomes unstable for two reasons . First, gravity makes the water flow in the most direct course down slope . If the river breaches its natural levees (i.e., during a flood), it spills out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope . Second, as its slope gets lower, the amount of shear stress on the bed decreases, which results in deposition of sediment within the channel and a rise in the channel bed relative to the floodplain . This makes it easier for the river to breach its levees and cut a new channel that enters the body of standing water at a steeper slope . Often when the channel does this, some of its flow remains in the abandoned channel . When these channel - switching events occur, a mature delta develops a distributary network . </P> <P> Another way these distributary networks form is from deposition of mouth bars (mid-channel sand and / or gravel bars at the mouth of a river). When this mid-channel bar is deposited at the mouth of a river, the flow is routed around it . This results in additional deposition on the upstream end of the mouth - bar, which splits the river into two distributary channels . A good example of the result of this process is the Wax Lake Delta . </P> <P> In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition . This results in the smoothing of the planform (or map - view) shape of the delta as the channels move across its surface and deposit sediment . Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan . The more often the flow changes course, the shape develops as closer to an ideal fan, because more rapid changes in channel position results in more uniform deposition of sediment on the delta front . The Mississippi and Ural River deltas, with their bird's - feet, are examples of rivers that do not avulse often enough to form a symmetrical fan shape . Alluvial fan deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape . </P> <P> Deltas are typically classified according to the main control on deposition, which is a combination of river, wave, and tidal processes, depending on the strength of each . The other two factors that play a major role are landscape position and the grain size distribution of the source sediment entering the delta from the river . </P>

How is the formation of an alluvial fan similar to the formation of a delta
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