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The plain tract, characterized by a low gradient and significant stream meandering, is developed by aggradation of sediments deposited from the river. Eustatic rise of sea level and downwarping of the earth's crust in the delta area promote the accumulation of sediment. The river is free to meander over the entire plain tract, but usually follows a well-defined meander belt which is characterized by meander scars.
Repeated overtopping of the natural levees results in very thick deposits of clay in the floodplain between meander belts. The clay is very stiff because it is deposited in thin laminae and is often desiccated (dried out) in the periods between the floods. The clay material has a very low permeability, but root holes and burrows may create very permeable conditions.
Oxbow-lake deposits also form in the floodplain. The entrances to a meander become plugged with sand which may form an oxbow lake, and then the lake fills with silt and clay from flood water.
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