Floodplains BY Rebecca Hinks & Charlotte Bootherstone

What are Floodplains? A floodplain, or alluvial plain, is a piece of flat land on either side of a stream or river, that will experience occasional flooding. It holds the excess water, allowing it to be slowly released into the river system and/or to infiltrate. Flooding is when river discharge exceeds the capacity of the channel, water rises over the channel banks and floods the surrounding low-lying lands. Floodplains also give time for sediment to settle out of floodwaters. Floodplains often support important wildlife habitats and are frequently used by humans as recreation areas.

What are Floodplains? 2 Floodplains generally contain sediments, often accumulations of sand, gravel, loam, silt, and/or clay. Sections of the Missouri River floodplain show layers or areas of varying material. Sometimes the deposits are of coarse gravel, fine sand or silt. During its formation a floodplain can be noted by: meanders, ox-bow lakes, levees and can periodically be completely covered with water. When arresting factors affect the drainage system the floodplain may become an area of considerable fertility. Floodplains can support rich ecosystems (with 100 or even 1000 times more species than a river). When floodplain soil is wet it releases a surge of nutrients: Microscopic organisms thrive and larger species enter a rapid breeding cycle. This makes floodplains particularly valuable for agriculture.

An erosional floodplain is created as a stream cuts vertically and laterally into its channel and banks. A stream with a steep gradient will tend to down cut faster than it causes lateral erosion, resulting in a deep, narrow channel with little or no floodplain at all. Such as the Colorado River, Arizona. As the stream approaches base level, lateral erosion increases, creating a broader floodplain. Floodplains are formed in two ways: by erosion and by aggradation. Floodplain Formation

Floodplain Formation 2 An aggradational floodplain is created when a river deposits thick layers of sediment. This happens when the river's velocity decreases, forcing it to drop sediment. Consequently the lower course of the river valley becomes layered with alluvium. Aggradational floodplains are the most common, and are most usually found near the estuary’s of large rivers, such as the Rhine, the Nile, the Ganges and the Mississippi, where there are occasional floods and the river carries a large load. In times of flood, the larger discharge of water in the upper courses erodes away and carried a larger quantity of sediment resulting in the creation of a flat terrain (plantation) as well as aggradation. Such as one stream in la Plato county, Colorado.

Floodplain uses The main use of floodplains is for agriculture, however the use of the land for settlement has become more popular. Settlement on floodplains, despite occasional flooding, is attractive because: - Water is most available. - The land is more fertile. - Rivers correspond to cheap modes of transport. - Flat or gently sloping land is more suitable for development. Here is an example of how floodplains have been used for settlement and how the river flooding has had to be controlled.

Human influences on floodplains In areas where floodplains have been used for settlement, flooding can be a hazard, and therefore it has had to be controlled. Levees are built or strengthened to prevent flooding, which would help to benefit and protect the urbanised area surrounding the floodplain. Here is an example of a levee in an urban area.

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