The shape of a river channel reflects the pattern of the waterflows than run down through that channel, as well as the rock through which the water has carved its path. A river will carve a channel with clearly-defined banks on each side, in most cases. During occasional floods, the water level will rise to fill the channel. Newspapers may describe the river as running "full to the banks," but the river does not reach flood stage until it rises higher than the river banks and water spills over onto the adjacent land.
When the river level starts to drop, perhaps after the surge of water from an intense thunderstorm has passed, the river is described as having "crested." In many areas of the United States, the spring thaw brings on an annual flood as the winter snows melt. When the water flows over the riverbanks, it encounters obstructions - grass, trees, fences, even barns and houses. The obstructions slow the flow of the water, and some of the silt carried in the floodwaters settles out along the edge of the river. The deposited silt fills in the low places near the river edge. In the resulting flat floodplains, the soil is rich in organic material. Floodplains are desirable farmland - the soil is fertile, enriched by nutrients eroded from upstream, while seeds/plants on the flat land are less likely to be washed away by normal rainstorms.