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In mature and old-age fluvial valleys, the lateral erosion is generally more prominent than vertical deepening so that the valley tends to be much wider than the river or stream channel. Repeated overtopping of the natural levees results in very thick deposits of clay in the flood plain between meander belts. As flood waters overflow the channel banks, the sudden drop in velocity over the wide plain causes the suspended load to settle with the coaser material (coarse silt and fine sand) deposited closer to the channel and the finer material (fine silt and clay) further away. Although some sand appears in the suspended load, the larger portion of the sediment includes silt and clay as the flood waters originate mainly from the upper water column which may be a considerable vertical distance from the bed load.
The floodplain deposits tend to be very stiff because of the thin laminae and desiccation (water loss) which occurs in the periods between flooding. The clay material has a very low permeability, but root holes and burrows may create very permeable conditions. These floodplain clays may also cover deposits of oxbow lakes, which may contain very soft clays.
The rate of aggradation affects the distribution of the deposits. In areas of fast downwarping or eustatic rise of sea level, aggradation will occur at a very fast rate so that the clay and silt deposits covering the floodplain will not be eroded because the meander belt does not have sufficient time to move over the entire floodplain (valley floor). As a result, the deposits of clay, silt, sand, and gravel can be erratically distributed throughout the sediments. The clay and silt will indicate the former floodplain, and sand and gravel will mark the former course of the stream. During periods of slow aggradation, the river has more time to meander and remove the floodplain deposits (silt and clay). The coarse material will be deposited and underly a thin covering of silt and clay. The extent of the thicker sand and gravel deposits depends on the degree to which the river meanders across the entire valley floor.
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