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This picture shows the flood plain following a 1 in 10 year flood on the Isle of Wight.

Gravel floodplain of a glacial river near the Snow Mountains in Alaska, 1902.

Entrenched river: The Virgin River at the upper end of Zion Canyon, Zion National Park, Utah, has almost no floodplain at all.

Erosional floodplain with indistinct boundary: The Little Laramie River in Albany County, Wyoming, 1905.

Aggradation and planation: The Laramie River meanders across its floodplain in Albany County, Wyoming, 1949.

Aggradational floodplain: This floodplain of a small meandering stream in La Plata County, Colorado, is underlain by silt deposited above a dam formed by a terminal moraine left by the Wisconsin Glacier.

Oxbow lakes on the floodplain of the White River near Des Arc, Arkansas, 1949.

Riparian vegetation on the floodplain of the Lynches River near Johnsonville, South Carolina. These tupelo and cypress trees show the high water mark of flooding.

A floodplain, or flood plain, is flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding. It includes the floodway, which consists of the stream channel and adjacent areas that carry flood flows, and the flood fringe, which are areas covered by the flood, but which do not experience a strong current. Image File history File links Gnome-globe. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Image File history File linksMetadata Alaska_Floodplain_1902. ... Image File history File linksMetadata Alaska_Floodplain_1902. ... Official language(s) None[1] Spoken language(s) English 85. ... Year 1902 (MCMII) was a common year starting on Wednesday (link will display calendar) of the Gregorian calendar (or a common year starting on Tuesday [1] of the 13-day-slower Julian calendar). ... Image File history File linksMetadata Entrenched_river. ... Image File history File linksMetadata Entrenched_river. ... The Virgin River, a tributary of the Colorado, is shown highlighted on a map of the southwestern United States The Virgin River is a tributary of the Colorado River, 200 mi (322 km) long, in the southwestern United States. ... Zion Canyon as seen from the top of Angels Landing at sunset Zion National Park is a United States National Park located in the Southwestern United States, near Springdale, Utah. ... This article is about the U.S. state. ... Image File history File linksMetadata Little_Laramie_River_1905. ... Image File history File linksMetadata Little_Laramie_River_1905. ... Albany County is a county located in the state of Wyoming. ... For other uses, see 1905 (disambiguation). ... Image File history File linksMetadata Laramie_River_floodplain_1949. ... Image File history File linksMetadata Laramie_River_floodplain_1949. ... The Laramie River The Laramie River is a tributary of the North Platte River, approximately 216 mi (348 km) long, in the U.S. states of Colorado and Wyoming. ... A meander is a bend in a river, also known as an oxbow loop. ... Albany County is a county located in the state of Wyoming. ... Year 1949 (MCMXLIX) was a common year starting on Saturday (link will display the full calendar) of the Gregorian calendar. ... Image File history File linksMetadata Animas_Valley_CO_1903. ... Image File history File linksMetadata Animas_Valley_CO_1903. ... A meander is a bend in a river, also known as an oxbow loop. ... La Plata County is a county located in the U.S. state of Colorado. ... This article is about geological phenomena. ... The Wisconsin (in North America), Devensian (in the British Isles), Midlandian (in Ireland), Würm (in the Alps), and Weichsel (in northern central Europe) glaciations are the most recent glaciations of the Pleistocene epoch, which ended around 10,000 BCE. The general glacial advance began about 70,000 BCE, and... Image File history File linksMetadata Oxbow_lakes_AR_1949. ... Image File history File linksMetadata Oxbow_lakes_AR_1949. ... The White River is a 722-mile-long river that flows through the US states of Arkansas and Missouri. ... Des Arc is a city located in Prairie County, Arkansas. ... Year 1949 (MCMXLIX) was a common year starting on Saturday (link will display the full calendar) of the Gregorian calendar. ... Flood plain: Lynches River, Johnsonville, SC Image taken by me, released under GFDL Pollinator 03:30, 10 Feb 2004 (UTC) The high water mark can be clearly seen on the tupelo and cypress trees. ... Flood plain: Lynches River, Johnsonville, SC Image taken by me, released under GFDL Pollinator 03:30, 10 Feb 2004 (UTC) The high water mark can be clearly seen on the tupelo and cypress trees. ... Lynches River in winter Johnsonville, South Carolina Lynches River, named for Thomas Lynch, Jr. ... Johnsonville is a city located in Florence County, South Carolina. ... Species Nyssa aquatica - Water Tupelo Nyssa biflora - Swamp Tupelo Nyssa javanica - Indonesian Tupelo Nyssa leptophylla - Hunan Tupelo Nyssa ogeche - Ogeechee Tupelo Nyssa sinensis - Chinese Tupelo Nyssa sylvatica - Black Tupelo Nyssa ursina - Bear Tupelo Nyssa yunnanensis - Yunnan Tupelo The tupelos, genus Nyssa, are a small genus of about 9 to 11... Species Taxodium ascendens - Pond Cypress Taxodium distichum - Bald Cypress Taxodium mucronatum - Montezuma Cypress Taxodium is a genus of one to three species (depending on taxonomic opinion) of extremely flood-tolerant conifers in the cypress family, Cupressaceae, one of several genera in the family commonly known as cypresses. ... Floodplain of the Lynches River, Johnsonville, South Carolina. ... Butchers Creek, Omeo, Victoria A stream, brook, beck, burn or creek, is a body of water with a detectable current, confined within a bed and banks. ... For other uses, see River (disambiguation). ... Flooding in Amphoe Sena, Ayutthaya Province, Thailand. ... In physical geography, a channel is the physical confine of a river, slough or ocean strait consisting of a bed and banks. ... A current is fluid flow, especially of water or air. ...

Formation

Floodplains are formed in two ways: by erosion; and by aggradation.^[1] An erosional floodplain is created as a stream cuts deeper into its channel and laterally into its banks. A stream with a steep gradient will tend to downcut faster than it causes lateral erosion, resulting in a deep, narrow channel with little or no floodplain at all. This is the case of entrenched rivers such as the Virgin River in Zion National Park in the U.S. state of Utah and the Colorado River in the Grand Canyon in the U.S. state of Arizona. As the stream approaches its base level, lateral erosion increases, creating an extremely broad floodplain, as in the case of the Platte River flowing across the Great Plains of the United States. There, the boundary between river and floodplain is not clear. 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. For morphological image processing operations, see Erosion (morphology). ... Aggradation in geology is the accumulation of sediment in rivers and nearby landforms. ... For other uses, see Gradient (disambiguation). ... Erosional downcutting by the San Juan River in Utah. ... An entrenched river is a river that is confined to a canyon or gorge, usually with a relatively narrow width and little or no flood plain, and often with meanders worn into the landscape. ... The Virgin River, a tributary of the Colorado, is shown highlighted on a map of the southwestern United States The Virgin River is a tributary of the Colorado River, 200 mi (322 km) long, in the southwestern United States. ... Zion Canyon as seen from the top of Angels Landing at sunset Zion National Park is a United States National Park located in the Southwestern United States, near Springdale, Utah. ... This article is about the U.S. state. ... The Colorado River from the bottom of Marble Canyon, in the Upper Grand Canyon Colorado River in the Grand Canyon from Desert View The Colorado River from Laughlin Horseshoe Bend is a horseshoe-shaped meander of the Colorado River located near the town of Page, Arizona The Colorado River is... This article is about the canyon in the southwestern United States. ... Official language(s) English Spoken language(s) English 74. ... The base level of a river or stream is the lowest point to which it can flow, often referred to as the mouth of the river. ... The Platte River, showing the North Platte and South Platte The Platte River is a tributary of the Missouri River, approximately 310 mi. ... The Great Plains covers much of the central United States, portions of Canada and Mexico. ...

An aggradational floodplain is created when a stream lays down thick layers of sediment. This happens when the stream's gradient becomes very slight and its velocity decreases, forcing it to drop sediment brought from higher regions nearer its source. Consequently the lower portion of the river valley becomes filled with alluvium. 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 (creation of a flat terrain) as well as aggradation. Thus, a stream such as the Laramie River in the U.S. state of Wyoming, widens its valley by working in meanders from side to side and covers the widened valley with sediment. Glacial drainage may also form an aggradational floodplain simply by filling up its valley with alluvium. This article or section cites very few or no references or sources. ... This article is about velocity in physics. ... Alluvium (from the Latin, alluvius, from alluere, to wash against) is soil or sediments deposited by a river or other running water. ... There are very few or no other articles that link to this one. ... The Laramie River The Laramie River is a tributary of the North Platte River, approximately 216 mi (348 km) long, in the U.S. states of Colorado and Wyoming. ... Official language(s) English Capital Cheyenne Largest city Cheyenne Area  Ranked 10th  - Total 97,818 sq mi (253,348 km²)  - Width 280 miles (450 km)  - Length 360 miles (580 km)  - % water 0. ... This article is about the geological formation. ...

Aggradational floodplains are more common than erosional ones. Any obstruction across a river's course, such as a band of hard rock, may form a floodplain behind it. Indeed, anything that checks a river's course and causes it to drop its load will tend to form a floodplain. Aggradational floodplains are most commonly found near the mouths of large rivers, such as the Rhine, the Nile, the Ganges and the Mississippi, where there are occasional floods and the river usually carries a large amount of sediment. Natural levees form 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. It has been suggested that River Rhine Pollution: November 1986 be merged into this article or section. ... The Nile (Arabic: , transliteration: , Ancient Egyptian iteru, Coptic piaro or phiaro) is a major north-flowing river in Africa, generally regarded as the longest river in the world. ... Ganges River Delta, Bangladesh and India The Ganges Delta (or the Bengal Delta) is a river delta in the South Asia region of Bengal, consisting of Bangladesh and the state of West Bengal, India. ... For the river in Canada, see Mississippi River (Ontario). ... A levee, levée (from the feminine past participle of the French verb lever, to raise), floodbank or stopbank is a natural or artificial slope or wall, usually earthen and often parallels the course of a river. ...

Physical geography

Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream. 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 stream.

Geologically ancient floodplains are often represented in the landscape by stream terraces. These are old floodplains that remain relatively high above the present floodplain and indicate former courses of a stream. Remants of former floodplains of the San Juan River in Utah stand as stream terraces above the rivers modern level in this 1927 photo. ...

Sections of the Missouri River 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 Missouri River is a tributary of the Mississippi River in the United States. ... InsertSLUTTY WHORES≤ non-formatted text here{| class=toccolours border=1 cellpadding=4 style=float: right; margin: 0 0 1em 1em; width: 20em; border-collapse: collapse; font-size: 95%; clear: right; |+ United States Geological Survey |- |style= align=center colspan=2| [[Image:USGS logo. ...

The floodplain during its formation is marked by meandering or anastomotic 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, because it is not altogether flat. It has a gentle slope down-stream, and often, for a distance, from the side towards the center. // Anastomosis (plural anastomoses) refers to a form of network in which streams both branch out and reconnect. ... Big Cypress Bayou in Jefferson, Texas off of U.S. Route 59. ... This article is about marsh, a type of wetland. ... Water stagnation occurs when water stops flowing. ...

Ecology

Floodplains can support particularly rich ecosystems, both in quantity and diversity. They are a category of 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. A riparian zone schematic from the Everglades. ...

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; and
• Flat land is easier to develop than hilly land

The extent of floodplain inundation depends in part on the flood magnitude, defined by the return period. A return period also known as a recurrence interval is an estimate of the liklihood of a flood or river discharge flow of a certain size. ...

In the United States the National Flood Insurance Program regulates development in mapped floodplains based on the 100-year flood (1% annual chance of a flood of this magnitude). The Flood Insurance Rate Maps, typically depict both the 100-year floodplain and the 500-year floodplains. Where a detailed study of a waterway has been done, the 100-year floodplain will also include the floodway, the critical portion of the floodplain which includes the stream's channel and any adjacent areas that must be kept free of encroachments that might block flood flows or restrict storage of flood waters. When a floodway is shown on the Flood Insurance Rate Maps, the portion of the 100-year floodplain outside of the floodway is known as the flood fringe. Another commonly-encountered term is the Special Flood Hazard Area, which is any area subject to inundation by the 100-year flood.^[2] The National Flood Insurance Program (NFIP) was created by Congress in 1968. ... A one-hundred year flood is calculated to be the maximum level of flood water expected to occur on average once every one hundred years. ...

In order for flood-prone property to qualify for government-subsidized insurance, a local community must adopt an ordinance that protects the floodway and requires that new residential structures built in Special Flood Hazard Areas be elevated to at least the level of the 100-year flood. Commercial structures can be elevated or floodproofed to or above this level. In some areas without detailed study information, structures may be required to be elevated to at least two feet above the surrounding grade. ^[3] Many State and local governments have, in addition, adopted floodplain construction regulations which are more restrictive than those mandated by the NFIP. The U.S. government also sponsors flood hazard mitigation efforts to reduce flood impacts. The Hazard Mitigation Program is one funding source for mitigation projects. A number of whole towns such as English, Indiana, have been completely relocated to remove them from the floodplain. Other smaller-scale mitigation efforts include acquiring and demolishing flood-prone buildings or flood-proofing them. English is a town located in Crawford County, Indiana. ...

In some tropical floodplain areas such as the Niger Inland Delta of Mali, annual flooding events are a natural part of the local ecology and rural economy. But in Bangladesh, which occupies the Ganges Delta, the advantages provided by the richness of the alluvial soil of floodplains are severely offset by frequent floods brought on by cyclones and annual monsoon rains, which cause severe economic disruption and loss of human life in this densely-populated region. The Niger Inland Delta is a large area of lakes and swamps in Mali, around the bifurcated River Niger and its tributary, the River Bani. ... Ganges River Delta, Bangladesh and India The Ganges Delta (or the Bengal Delta) is a river delta in the South Asia region of Bengal, consisting of Bangladesh and the state of West Bengal, India. ... This article is about the meteorological phenomenon. ... Bold text[[ // [[Image:Media:Example. ...

See also

• Flood-meadow, area of grassland or pasture beside a river, subject to seasonal flooding
• Water-meadow, area of grassland or pasture beside a river, subject to controlled seasonal flooding

Flood-meadow near Hohenau an der March A flood-meadow (or floodmeadow) is an area of grassland or pasture beside a river, subject to seasonal flooding. ... A water-meadow (or watermeadow) is an area of grassland or pasture beside a river, subject to controlled seasonal flooding. ...

References

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain. Encyclopædia Britannica, the eleventh edition The Encyclopædia Britannica Eleventh Edition (1910–1911) is perhaps the most famous edition of the Encyclopædia Britannica. ... The public domain comprises the body of all creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest. ...