natural levee

A river spreading over its banks slows and releases some of its burden of sediment. Flooding repeatedly, the river will coat its banks with sediment in the slower, lower reaches. Year after year the layer of alluvial soil is laid down as varve and dries, and over the centuries the banks get higher, keeping the river in its channel except when floods break through the barriers called natural levees. Rivers are dynamic and meandering, an ongoing process. Stream channels are always shifting across their floodplains, and sedimentation occurs along point bars and erosion tears at cutbanks. Along clearer, faster upland streams natural levees are made by seasonal floods in low ridges of sandy floodplain deposits, parallel or adjacent to the stream. Raised shoulders of loam and sand, they confine the normal stream in its channel, in the stretches where the current slows and the floods dump gravel, sand, and soil year after year. The banks may shift, crumble, or erode, but they are raised again by repeated floods and sweeping flash tides from the slopes above.

Robert Morgan