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. Alaska ( Аляска Alyaska) is a state in the United States of America, in the northwest of the North American continent Year 1902 ( MCMII) was a Common year starting on Wednesday (link will display calendar of the Gregorian calendar (or a Common year starting

Entrenched river: The Virgin River at the upper end of Zion Canyon, Zion National Park, Utah, has almost no floodplain at all. The Virgin River is a nearly 160 mile (322 km long tributary of the Colorado River in the southwestern United States. Zion National Park is a United States National Park located in the Southwestern United States, near Springdale Utah. The State of Utah (ˈjuːtɔː or) is a western state of the United States.

Erosional floodplain with indistinct boundary: The Little Laramie River in Albany County, Wyoming, 1905. Albany County is a County located in the US state of Wyoming. Year 1905 ( MCMV) was a Common year starting on Sunday (link will display full calendar of the Gregorian calendar (or a Common year starting

Aggradation and planation: The Laramie River meanders across its floodplain in Albany County, Wyoming, 1949. The Laramie River is a tributary of the North Platte River, approximately 216 mi (348 km) long in the U A meander in general is a bend in a sinuous watercourse also known as an oxbow loop or simply an Oxbow. Albany County is a County located in the US state of Wyoming. Year 1949 ( MCMXLIX) was a Common year starting on Saturday (link will display the full calendar of the Gregorian calendar.

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. A meander in general is a bend in a sinuous watercourse also known as an oxbow loop or simply an Oxbow. La Plata County is the fourteenth most populous of the 64 counties of the State of Colorado of the United States. Moraine refers to any glacially formed accumulation of unconsolidated glacial debris (soil and rock which can occur in currently glaciated and formerly glaciated regions such as those "Last glacial" redirects here For the period of maximum glacier extent during this time see Last Glacial Maximum The last glacial period

Oxbow lakes on the floodplain of the White River near Des Arc, Arkansas, 1949. The White River is a 722 mile (1162 km long River that flows through the U Des Arc is a city in and the County seat of the northern district of Prairie County, Arkansas, United States. Year 1949 ( MCMXLIX) was a Common year starting on Saturday (link will display the full calendar of the Gregorian calendar.

Riparian vegetation on the floodplain of the Lynches River near Johnsonville, South Carolina. Lynches River, named for Thomas Lynch Jr, signer of the Declaration of Independence, rises in North Carolina near Waxhaw North Carolina Johnsonville is a city in Florence County, South Carolina, United States. These tupelo and cypress trees show the high water mark of flooding. The tupelos, or pepperidge tree, genus Nyssa, are a small Genus of about 9 to 11 species of Trees with alternate simple leaves Taxodium is a Genus of one to three Species (depending on taxonomic opinion of extremely flood-tolerant Conifers in the cypress Ordinary high water mark refers to the highest level reached by a Body of water that has been maintained for a sufficient period of time to leave evidence on the landscape

A floodplain, or flood plain, is flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding. A stream is a body of Water with a current, confined within a bed and stream-banks "Riverine" redirects here For the use of that term in Maritime geography, see there A flood is an overflow of an expanse of water that submerges land a deluge 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. Physical geography, a channel is the physical confine of a River, slough or ocean Strait consisting of a bed and banks A current, in a River or Stream, is the Flow of Water influenced by Gravity as the water moves Downhill to reduce its

Formation

Floodplains are formed in two ways: by erosion; and by aggradation. Erosion is the carrying away or displacement of solids ( Sediment, Soil, rock and other particles usually by the agents of currents such as wind Aggradation in Geology is the accumulation of Sediment in rivers and nearby landforms ^[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. In Vector calculus, the gradient of a Scalar field is a Vector field which points in the direction of the greatest rate of increase of the scalar Downcutting, also called erosional downcutting or downward erosion or vertical erosion is a geological process that deepens the channel This is the case of entrenched rivers such as the Virgin River in Zion National Park in the U. 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 The Virgin River is a nearly 160 mile (322 km long tributary of the Colorado River in the southwestern United States. Zion National Park is a United States National Park located in the Southwestern United States, near Springdale Utah. S. state of Utah and the Colorado River in the Grand Canyon in the U. The State of Utah (ˈjuːtɔː or) is a western state of the United States. The Colorado River (' Aha Kwahwat in Mojave) is a River in the southwestern United States and northwestern Mexico, approximately The Grand Canyon S. state of Arizona. The State of Arizona ( is a state located in the southwestern region of the United States. 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. 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 is an approximately 310 mi (499 km long river in the Western United States. The Great Plains are the broad expanse of Prairie and Steppe which lie east of the Rocky Mountains in the United States and Canada The United States of America —commonly referred to as the 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.

An aggradational floodplain is created when a stream lays down thick layers of sediment. Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of 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. In Physics, velocity is defined as the rate of change of Position. Consequently the lower portion of the river valley becomes filled with alluvium. Alluvium (from the Latin, alluvius, from alluere, "to wash against" is Soil or Sediments deposited by a river or other running 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. Planation is a geomorphic process which creates nearly flat surfaces by fluvial (river alluvial (wind and marine processes Thus, a stream such as the Laramie River in the U. The Laramie River is a tributary of the North Platte River, approximately 216 mi (348 km) long 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. The State of Wyoming ( is a sparsely populated state in the western region of the United States. Glacial drainage may also form an aggradational floodplain simply by filling up its valley with alluvium. "Glacial" and "Glaciation" redirect here For the geological periods see Glacial period.

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. The Rhine (Rhein Rijn Rhin Reno Rain Rhenus is one of the longest and most important Rivers in Europe at 1320 kilometres (820 mi with an average discharge The Nile (النيل, Ancient Egyptian iteru or Ḥ'pī, Coptic piaro or phiaro) is a major north-flowing River The Ganges Delta (also Sunderban Delta or the Bengal Delta) is a River delta in the South Asia region of Bengal, consisting of Bangladesh The Mississippi River is the second longest River in the United States, with a length of from its source in Lake Itasca in Minnesota to Natural levees form inside which the river usually flows, gradually raising its bed above the surrounding plain. Dike (constructionEmbankmentA levee, levée, dike (or dyke) embankment, floodbank or stopbank is a natural or artificial 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.

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. Stream terraces are relict features such as Floodplains from periods when a Stream was flowing at a higher elevation and has downcut to a lower elevation These are old floodplains that remain relatively high above the present floodplain and indicate former courses of a stream.

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 United States Geological Survey ( USGS) is a scientific agency of the United States government. An alluvial plain is a relatively flat Landform created by the deposition of sediment over a long period of time by one or more Rivers coming from highland regions

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. An anastomosis (plural anastomoses, from gr ἀναστόμωσις communicating opening) is a Network of streams that both branch out and reconnect An oxbow lake is a U-shaped lake water body formed when a wide Meander from the mainstem of a River is cut off to create a lake A bayou (pronounced oʊ or uː is a small slow-moving Stream or creek or a lake or pool ( bayou lake) that lies in an abandoned channel of a stream In Geography, a marsh, or morass, is a type of Wetland which is subject Water stagnation occurs when Water stops flowing Stagnant water can be a major Environmental hazard. 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.

Ecology

Floodplains can support particularly rich ecosystems, both in quantity and diversity. They are a category of riparian zones or systems. "Riparian" redirects here For the legal doctrine see " Riparian water rights. 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. Agriculture refers to the production of goods through the growing of plants and fungi and the raising of domesticated Animals The study of 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;
• River transportation was a key economic factor in the founding of many communities;
• 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 interval of time between events like an Earthquake, Flood or river discharge

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 National Flood Insurance Program (NFIP was created by the Congress of the United States in 1968 through the National Flood Insurance Act of 1968 (P 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]

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. English is a town in Sterling Township, Crawford County, Indiana, United States. Other smaller-scale mitigation efforts include acquiring and demolishing flood-prone buildings or flood-proofing them.

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. The Niger Inland Delta, also known as the Macina or Inner Niger Delta, is a large area of Lakes and Floodplains in Mali. Mali, officially the Republic of Mali (République du Mali is a Landlocked nation in Western Africa. 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. ( Bengali: বাংলাদেশ inc-Latn Bangladesh) officially The Ganges Delta (also Sunderban Delta or the Bengal Delta) is a River delta in the South Asia region of Bengal, consisting of Bangladesh In Meteorology, a cyclone refers to an area of closed circular fluid motion rotating in the same direction as the Earth. A monsoon is a seasonal prevailing wind which lasts for several months

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
• List of alluvial sites in Switzerland

References

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain. The Encyclopædia Britannica Eleventh Edition (1910–1911 is a 29-volume reference work that marked the beginning of the Encyclopædia Britannica The public domain is a range of abstract materials &ndash commonly referred to as Intellectual property &ndash which are not owned or controlled by anyone