As explained by Water Encyclopedia, the geometry of a stream channel is controlled by both water and sediment movement, which reflect regional climate, geology , and human land use in a given drainage basin.
During floods, streamflow may exceed the capacity of the channel to contain it. As floodwaters spill out of the channel and across the valley bottom, the water encounters much more frictional resistance from trees and other objects. The velocity of the water decreases dramatically, and the sediment being transported is dropped onto the floodplain . Much of this sediment deposition occurs immediately beside the channel, creating linear mounds parallel to the channel called natural levees.
Some sediment is deposited beyond the levees, creating a floodplain. A floodplain is the portion of the valley bottom dominated by stream processes, rather than by sediment coming directly from the hillslopes as landslides, for example. The floodplain may also be described based upon the average frequency with which it floods.
Because stream channel geometry reflects water and sediment movement along the channel, any change in water discharge and sediment transport will cause a corresponding change in channel geometry. When the increased discharge of a flood brings more sediment downstream, sediment deposition may transform a meandering channel into a braided channel. Or, a flood transporting little sediment may have so much erosive energy that it dramatically erodes the stream banks and scours the streambed, enlarging the width and depth of the channel. These changes may persist for decades after the flood, or smaller flows may quickly reshape the channel to its pre-flood configuration.
Read more about Stream Channel Development at Water Encyclopedia.