Floodplain

Gravel floodplain of glacial river near Snow Mountains in Alaska, August 10, 1902.

Floodplain along Lynches River Johnsonville, South Carolina Showing high water mark on tupelo and cypress

A floodplain, or flood plain, is flat or nearly flat land adjacent to a stream or river that experiences occasional flooding. Floodplains are formed by the deposition of sediment carried by river as it floods.

The extent of floodplain inundation depends part on the flood magnitude, defined by the return period. Because they border waterbodies, floodplains have been popular sites to establish settlements, which has led to an increase in flood-related disasters.

In some tropical floodplain areas, annual flooding events are a natural part of the local ecology and rural economy.

Physical geography

Floodplains may be extremely broad, as in the case of the Platte River flowing across the Great Plains, where the boundary between river and floodplain is not even clear, or quite narrow, as in the case of entrenched rivers such as the Snake River in the Snake River Canyon or Colorado River in the Grand Canyon. In unmodified drainage systems where the terrain is fairly flat and rainfall intermittent, a floodplain may take the place of a river entirely. Instead of a defined streambed, there is simply a broad flat area where water flows from time to time.

Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream or river. These are accumulations of sand, gravel, loam, silt, and/or clay, and are often important aquifers, the water being drawn from them being pre-filtered compared to the water in the river or stream.

Geologically ancient floodplains are often represented in the landscape by terrace deposits. These are old floodplain deposits that remain relatively high above current deposits, and can indicate former courses of rivers and streams.

Formation

When the slope down which a river runs has become very slight, it is unable to carry the sediment brought from higher regions nearer its source, and consequently the lower portion of the river valley becomes filled with alluvium. Since, in times of flood, the rush of water in the high regions tears off and carries down a greater quantity of sediment resulting in planation, with aggradation. That is, they may be due to a graded river working in meanders from side to side, widening its valley by this process and covering the widened valley with sediment. Or the stream -- by cutting into another stream (piracy), by cutting through a barrier near its head waters, by entering a region of looser or softer rock, and by glacial drainage -- may form a flood plain simply by filling up its valley (alluviation only).

Any obstruction across a river's course, such as a band of hard rock, may form a floodplain behind it, and indeed anything that checks a river's course and causes it to drop its load will tend to form a flood plain. Still, floodplains are most commonly found near the mouth of a large river, such as the Rhine, the Nile, or the Mississippi, where there are occasional floods and the river usually carries a large amount of sediment. "Levees" are formed, inside which the river usually flows, gradually raising its bed above the surrounding plain. Occasional breaches during floods cause the overloaded stream to spread in a great lake over the surrounding country, where the silt covers the ground in consequence.

Sections of the Missouri floodplain taken by the United States geological survey show a great variety of material of varying coarseness, the stream bed being scoured at one place, and filled at another by currents and floods of varying swiftness, so that sometimes the deposits are of coarse gravel, sometimes of fine sand or of fine silt, and it is probable that any section of such an alluvial plain would show deposits of a similar character.

The floodplain during its formation is marked by meandering or anastomosing streams, ox-bow lakes and bayous, marshes or stagnant pools, and is occasionally completely covered with water. When the drainage system has ceased to act or is entirely diverted for any reason, the floodplain may become a level area of great fertility, similar in appearance to the floor of an old lake. The floodplain differs, however, inasmuch as it is not altogether flat. It has a gentle slope down-stream, and often, for a distance from the side towards the center.

Ecology

Floodplains can support particularly rich ecosystems, both in quantity and diversity. These are termed riparian zones or systems. A floodplain can contain 100 or even 1000 times as many species as a river. Wetting of the floodplain soil releases an immediate surge of nutrients: those left over from the last flood, and those that result from the rapid decomposition of organic matter that has accumulated since then. Microscopic organisms thrive and larger species enter a rapid breeding cycle. Opportunistic feeders (particularly birds) move in to take advantage. The production of nutrients peaks and falls away quickly; however the surge of new growth endures for some time. This makes floodplains particularly valuable for agriculture.

Markedly different species grow in floodplains than grow outside of floodplains. For instance, riparian trees (that grow in floodplains) tend to be very tolerant of root disturbance and tend to be very quick-growing, compared to non-riparian trees.

Interaction with society

Historically, many towns, homes and other buildings have been built on floodplains where they are highly susceptible to flooding, for several reasons:

• This is where water is most available
• Floodplain land is usually the most fertile for farming
• Rivers represent cheap sources of transportation, and are often where railroads are located
• The flatter land is easier to develop than hill land

In the USA the National Flood Insurance Program regulates development in mapped floodplains (Flood Insurance Rate Maps) and flood hazard mitigation efforts are being used to reduce flood impacts. The Hazard Mitigation Grant Program (HMGP) is one funding source for mitigation projects. A number of whole towns have been completely relocated to remove them from the floodplain, such as English, Indiana. Other mitigation efforts on a smaller scale include building acquisition and demolition, building elevation, building flood proofing, and outright restrictions to development in flood prone areas.

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. {{cite encyclopedia}}: Missing or empty |title= (help)