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During flood stages, the overflow water carries sediment onto the floodplain. A greater portion of the sediment is deposited near the channel thus forming an elevated aggradational deposit called a natural levee. This feature may become a major topographic feature in the floodplain, and can even cause the river to flow at a higher elevation than the floodplain. Tributary streams must often flow parallel to the main trunk for many miles before the tributary breaches the natural levee. This type of tributary is called a yazoo-type river.
The material in a natural levee exhibits evidence of rapid deposition, shallow flow depths, multiple flow cycles, and subaerial exposure. The continual wetting and drying causes the sediment to be compacted, oxidized, and highly leached. The grain size includes sand and coarser silt near the stream and finer silt and clay toward the floodplain.
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