floodplain

FLOODPLAINS HAVE been a focal point of settlement throughout history and of controversy in recent decades. Humans are drawn to floodplains to cultivate the fertile soils that compose them. Floodplains are some of the best lands for growing crops, but they come with a price. Sometimes, rivers exit their banks and inundate the flat lands and everything on them.

A floodplain is defined as a strip of relatively smooth land bordering a stream that overflows at time of high water. Floods build up the bordering plains with organic-rich sediments. While flooding is sometimes disastrous for humans, it is necessary in the development of floodplains. Rivers create floodplains over thousands of years through two processes, vertical and lateral accretion.

Vertical accretion is the deposition of sediments like sand, silt, and clay from floodwaters. As floodwater spreads out over a flood plain, it slows down and loses energy. As energy is lost, sediments held in suspension drop out and are deposited on the plain. The heaviest sediments, like sand, are the first to be deposited and accumulate near the river channel to form a natural levee. Lighter sediments, like clay, stay in suspension longer and are deposited further away from the river in areas called back marshes.

Lateral accretion is the deposition of sediments to the inside curve of river meanders. As water flows around a curve, the current is fastest on the outside and erodes sediments to create a cut bank. The current on the inside of the curve is of low energy and suspended materials drop out. In this way, point bars are created as the river moves laterally. Because of this, land on one side of the river may be much younger than that of the other.

Today, natural floodplain processes have been largely interrupted in the UNITED STATES. Large networks of artificial levees have been constructed to protect crops and property from flood damage along many of the country's rivers. While flooding is decreased, so is the natural fertilization of soils. Thus, farmers have to apply artificial fertilizers to sustain crop yields, adding to pollution.

The effectiveness of flood control levees is now being questioned. While flooding has been decreased, it is becoming apparent that the severity of floods that do occur is being increased. As more levees are built, less floodplain area is available for rivers to spread over; this means that more water is confined between the levees.

When a levee is breached during times of high water, the result is massive local flooding rather than normal flooding over a broad expanse. Steps are now being taken to curb the flooding problem. Floodplain lands are being purchased and returned to their natural cycles. This effort relieves pressure between the remaining levees and lessens the necessity to continually increase the height of levees.