overbank deposit

The silt, sand, and gravel left by floodwaters on lands adjacent to a river are collectively called an overbank deposit. Also called floodplain deposits, they refresh valley soils and raise the lips of riverbanks. When dams prevent the natural passage of sediments downstream and their overbank deposition, as does the Aswan High Dam on the Nile River, agricultural bottomlands suffer soil depletion. The heavy silt load of a river also limits the useful life spans of reservoirs, such as may be the case at Glen Canyon Dam and Lake Powell on the Colorado River. Dam removal requires careful study as to how the release of silt will affect banks and other habitats downstream, as with the planned elimination of the dams on the Elwha River in Washington’s Olympic Peninsula. Undammed rivers, too, can become clogged by silt and gravel when subject to steep-slope logging upstream. This excess effluvium may exacerbate annual floods and exaggerate overbank deposits, swamping instead of renewing pastures.

Robert Michael Pyle