India's Kosi River can shift its channel 730 metres in a single year
Floodplains are built by rivers that refuse to stay put. As a channel swings through its meanders, it gnaws one bank and lays sediment on the other, and some rivers move astonishingly fast: India's Kosi has been measured shifting up to 730 metres a year. The flat, fertile land left behind feeds civilisations and periodically drowns them.
Two processes do most of the building. On the outside of each bend, flowing water erodes the bank, while a point bar of sediment grows on the inside, so the channel migrates sideways without much change in width. Geologists call this lateral accretion. Then, when a flood overtops the banks, the water spreads a thin layer of sediment that is coarsest and thickest near the channel, a vertical accretion. In rivers left undisturbed, such overbank floods typically happen every one to two years whatever the climate. A three-day flood on the Meuse and Rhine in 1993 laid down roughly 0.57 to 1.0 kilograms of sediment per square metre across the floodplain and 4 or more on the levees.
Sand drops first, piling into natural levees, ridges along the banks, while finer silt and clay settle farther out as floodplain mud. Occasionally a bank fails and floodwater carves a crevasse that spreads a small delta-shaped fan. Over time repeated floods raise an alluvial ridge above the surrounding plain, and every 10 to 1,000 years the river may abandon it entirely for a new course, a jump known as avulsion. The Yellow River flood of 1855 and the Kosi flood of 2008 were avulsions.
That regular flood pulse makes floodplains among the most species-rich ecosystems. Floods deliver nutrients, fish time their spawning to the rising water, and trees grow quickly and tolerate disturbed roots. European floodplain forests once dominated by oak, elm and hornbeam have shifted toward ash and maple under human pressure, and though they covered only about 1 percent of the continent in the 1800s, they remain important refuges.
People have always crowded onto this rich ground, from the Nile to the Mississippi, and paid dearly. Floods on China's Yellow River in 1887 killed around a million people, and the 1931 China floods, estimated to have killed millions, rank as the worst natural disaster outside famines and epidemics. Some American towns, such as English in Indiana, have been moved wholesale off the floodplain.
Source: Floodplain
