A floodplain or flood plain is an area of land adjacent to a stream or river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge. The soils usually consist of levees, silts, and sands deposited during floods. Levees are the heaviest materials (usually pebble-size) and they are deposited first; silts and sands are finer materials.
Floodplains are formed when a meander erodes sideways as it travels downstream. When a river breaks its banks, it leaves behind layers of alluvium (silt). These gradually build up to create the floor of the plain. 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 drawn from them being pre-filtered compared to the water in the river.
Geologically ancient floodplains are often represented in the landscape by fluvial terraces. 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 having been 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. 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 anastomotic streams, oxbow 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 downstream, and often, for a distance, from the side towards the center.
The floodplain is the natural place for a river to dissipate its energy. Meanders form over the floodplain to slow down the flow of water and when the channel is at capacity the water spills over the floodplain where it is temporarily stored. In terms of flood management the upper part of the floodplain (piedmont zone) is crucial as this is where the flood water control starts. Artificial canalisation of the river here will have a major impact on wider flooding. This is the basis of sustainable flood management.
Floodplains can support particularly rich ecosystems, both in quantity and diversity. Tugay forests form an ecosystem associated with floodplains, especially in Central Asia. They are a category of riparian zones or systems. A floodplain can contain 100 or even 1,000 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. River flow rates are undergoing change following suit with climate change. This change is a threat to the riparian zones and other floodplain forests. These forests have over time synced their seedling deposits after the spring peaks in flow to best take advantage of the nutrient rich soil generated by peak flow. 
Historically, many towns have been built on floodplains, where they are highly susceptible to flooding, for a number of reasons:
Excluding famines and epidemics, some of the worst natural disasters in history (measured by fatalities) have been river floods, particularly in the Yellow River in China – see list of deadliest floods. The worst of these, and the worst natural disaster (excluding famine and epidemics) were the 1931 China floods, estimated to have killed millions. This had been preceded by the 1887 Yellow River flood, which killed around one million people, and is the second-worst natural disaster in history.
The extent of floodplain inundation depends in part on the flood magnitude, defined by the return period.
In the United States the Federal Emergency Management Agency (FEMA) manages the National Flood Insurance Program (NFIP). The NFIP offers insurance to properties located within a flood prone area, as defined by the Flood Insurance Rate Map (FIRM), which depicts various flood risks for a community. The FIRM typically focuses on delineation of the 100-year flood inundation area, also known within the NFIP as the Special Flood Hazard Area.
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 channel and any adjacent areas that must be kept free of encroachments that might block flood flows or restrict storage of flood waters. Another commonly encountered term is the Special Flood Hazard Area, which is any area subject to inundation by the 100-year flood. A problem is that any alteration of the watershed upstream of the point in question can potentially affect the ability of the watershed to handle water, and thus potentially affects the levels of the periodic floods. A large shopping center and parking lot, for example, may raise the levels of the 5-year, 100-year, and other floods, but the maps are rarely adjusted, and are frequently rendered obsolete by subsequent development.
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 flood proofed 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. Many State and local governments have, in addition, adopted floodplain construction regulations which are more restrictive than those mandated by the NFIP. The US government also sponsors flood hazard mitigation efforts to reduce flood impacts. California's 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.
In some tropical floodplain areas such as the Inner Niger Delta of Mali, annual flooding events are a natural part of the local ecology and rural economy, allowing for the raising of crops through recessional agriculture. However, 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 the densely-populated region.
Media related to Floodplains at Wikimedia Commons100-year flood
A one-hundred-year flood is a flood event that has a 1 in 100 chance (1% probability) of being equaled or exceeded in any given year.The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%. For coastal or lake flooding, the 100-year flood is generally expressed as a flood elevation or depth, and may include wave effects. For river systems, the 100-year flood is generally expressed as a flowrate. Based on the expected 100-year flood flow rate, the flood water level can be mapped as an area of inundation. The resulting floodplain map is referred to as the 100-year floodplain. Estimates of the 100-year flood flowrate and other streamflow statistics for any stream in the United States are available. In the UK The Environment Agency publishes a comprehensive map of all areas at risk of a 1 in 100 year flood. Areas near the coast of an ocean or large lake also can be flooded by combinations of tide, storm surge, and waves. Maps of the riverine or coastal 100-year floodplain may figure importantly in building permits, environmental regulations, and flood insurance.Alluvial plain
An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time.
As the highlands erode due to weathering and water flow, the sediment from the hills is transported to the lower plain. Various creeks will carry the water further to a river, lake, bay, or ocean. As the sediments are deposited during flood conditions in the floodplain of a creek, the elevation of the floodplain will be raised. As this reduces the channel floodwater capacity, the creek will, over time, seek new, lower paths, forming a meander (a curving sinuous path). The leftover higher locations, typically natural levees at the margins of the flood channel, will themselves be eroded by lateral stream erosion and from local rainfall and possibly wind transport if the climate is arid and does not support soil-holding grasses. These processes, over geologic time, will form the plain, a region with little relief (local changes in elevation), yet with a constant but small slope.
The Glossary of Landform and Geologic Terms, maintained by the United States' National Cooperative Soil Survey, defines an "alluvial plain" as "a large assemblage of fluvial landforms (braided streams, terraces, etc.,) that form low gradient, regional ramps along the flanks of mountains and extend great distances from their sources (e.g., High Plains of North America)" Use of "alluvial plain" as a general, informal term for a broad flood plain or a low-gradient delta is explicitly discouraged. The NCSS glossary instead suggests "flood plain".Backswamp
In geology a backswamp is a type of depositional environment commonly found in a floodplain. It is where deposits of fine silts and clays settle after a flood. Backswamps usually lie behind a stream's natural levees. During a flood, water levels will rise over the height of the levee, filling the floodplain with water and sediments. Once the flooding stops there is no place for the water to drain out, so the sediments it carried remain and settle. A backswamp usually lies where a bow river once flowed. A backswamp lies lower than the rest of the river valley.Big Muddy National Fish and Wildlife Refuge
The Big Muddy National Fish and Wildlife Refuge was established in 1994, and has grown to over 16,700 acres (68 km2). Like pearls on a string, these acres are spread out as individual units along the Missouri River between Kansas City and St. Louis. These pearls of habitat benefit floodplain-dependent fish and wildlife species. The Big Muddy Refuge is planning to grow to 60,000 acres (240 km2) by buying land from willing sellers who want to see their properties set aside for the benefit of wildlife and the enjoyment of all.
The pre-development Missouri River as documented by Lewis and Clark was considerably different from today's river. The historic Missouri was a broad, slow-moving, shallow river with braided channels. These past river conditions created a haven for wildlife, which included vast floodplain forests of giant trees, marshes, and even wet prairies. Today's river is channelized. It is deeper and faster, and controlled by levees, dikes, and other containment structures. These controls make the river more navigable and the surrounding floodplain ideal for agriculture.
The Big Muddy Refuge is allowing the Missouri River to be a river again, to enter its floodplain. This occurs during minor flood events. Management has created side channels, cut down levees, and allowed the floodplain vegetation to return. Currently, in many places the refuge is an impenetrable thicket of young trees and vegetation, but, as the trees grow and the refuge matures, its appearance will change. The process may take decades or even centuries.Black Meadow Creek
Black Meadow Creek is a 9.7-mile-long (15.6 km) tributary of the Otter Kill in Orange County, New York, in the United States. Via the Otter Kill, it is part of the Moodna Creek watershed, flowing onward to the Hudson River, in one of New York State's most biodiverse natural areas. Home to 13 species of salamander as well as to New York's largest population of the Northern Cricket Frog (Acris c. crepitans), the state's only listed "Endangered" frog species, the creek area is considered by biologists to be one of the state's herpetological "hot spots". Black Meadow Creek has several confirmed bald eagle nests along its length.Roughly 1/2 of the creek's length runs through a reservoir preserve owned by Orange County. This preserve status is credited with maintaining the upper creek's floodplain in its natural state for over one century.
Black Meadow Creek begins in the town of Warwick, near Glenmere Lake, and flows north into the town of Chester before converging with the Otter Kill in the village of Chester. Studies of the creek and its watershed are conducted by the nonprofit Sugar Loaf Historical Society and the nonprofit Glenmere Conservation Coalition, which maintain a small launch and study area on the creek.The creek was named for the expansive, dark, forested swamp that settlers found along its floodplain, most of which was transformed into agricultural areas by the mid 19th century. Its floodplain hosts the Black Meadow Hunting Club, the Straub Farm and the Chester Industrial Park at its confluence with the Otter Kill.Chowilla Game Reserve
Chowilla Game Reserve is a protected area covering the floodplain on the north side of the River Murray in South Australia from about 8 kilometres (5.0 miles) north-east of Renmark to the New South Wales border. It was proclaimed 8 April 1993 in conjunction with the Chowilla Regional Reserve. Its creation arises from a community consultation process which recommended that ‘hunting of waterfowl be a permitted activity in selected areas of the Chowilla floodplain’. The game reserve is classified as an IUCN Category VI protected area.Culgoa Floodplain National Park
The Culgoa Floodplain National Park is a protected national park that is located in the South West region of Queensland in eastern Australia. The 42,859-hectare (105,910-acre) national park is situated at the western extent of Hebel in the Shire of Balonne and in the east of Jobs Gate in the Paroo Shire, approximately 630 kilometres (390 mi) west of Brisbane. The park occupies the former pastoral and grazing property of Byra Station. The park's southern boundary is defined by part of the state border between Queensland and New South Wales.Eakin Creek Floodplain Provincial Park
Eakin Creek Floodplain Provincial Park is a provincial park in British Columbia, Canada located on the North Thompson River near the community of Little Fort.Florida swamps
Florida swamps include a variety of wetland habitats. Because of its high water table, substantial rainfall, and often flat geography, the U.S. state of Florida has a proliferation of swamp areas, some of them unique to the state.
Swamp types in Florida include:
Cypress dome - most common swamp habitat in Florida
Tupelo gum swamp
Mangrove swampFluvial terrace
Fluvial terraces are elongated terraces that flank the sides of floodplains and fluvial valleys all over the world. They consist of a relatively level strip of land, called a “tread,” separated from either an adjacent floodplain, other fluvial terraces, or uplands by distinctly steeper strips of land called “risers.” These terraces lie parallel to and above the river channel and its floodplain. Because of the manner in which they form, fluvial terraces are underlain by fluvial sediments of highly variable thickness.Fluvial terraces are the remnants of earlier floodplains that existed at a time when either a stream or river was flowing at a higher elevation before its channel downcut to create a new floodplain at a lower elevation. Changes in elevation can be due to changes in the base level (elevation of the lowest point in the fluvial system, usually the drainage basin) of the fluvial system, which leads to headward erosion along the length of either a stream or river, gradually lowering its elevation. For example, downcutting by a river can lead to increased velocity of a tributary, causing that tributary to erode toward its headwaters. Terraces can also be left behind when the volume of the fluvial flow declines due to changes in climate, typical of areas which were covered by ice during periods of glaciation, and their adjacent drainage basins.Kennet and Lambourn Floodplain
The Kennet and Lambourn Floodplain is a 22.9 hectare biological Site of Special Scientific Interest in Berkshire and Wiltshire, England, notified in 1996. It comprises seven separate small areas, four in the Kennet Valley and three in the Lambourn Valley. The areas included in the site support particularly large populations of Desmoulin's whorl snail.Most of the site was designated as a Special Area of Conservation in 2005.Part of the area is within the Rack Marsh nature reserve.List of freshwater ecoregions (WWF)
This is a list of freshwater ecoregions as compiled by the World Wildlife Fund (WWF). The terrestrial scheme divides the Earth's land surface into 8 terrestrial ecozones, containing 426 smaller ecoregions. Each ecoregion is classified into one of 12 major habitat types, or biomes.Meander
A meander is one of a series of regular sinuous curves, bends, loops, turns, or windings in the channel of a river, stream, or other watercourse. It is produced by a stream or river swinging from side to side as it flows across its floodplain or shifts its channel within a valley. A meander is produced by a stream or river as it erodes the sediments comprising an outer, concave bank (cut bank) and deposits this and other sediment downstream on an inner, convex bank which is typically a point bar. The result of sediments being eroded from the outside concave bank and their deposition on an inside convex bank is the formation of a sinuous course as a channel migrates back and forth across the down-valley axis of a floodplain. The zone within which a meandering stream shifts its channel across either its floodplain or valley floor from time to time is known as a meander belt. It typically ranges from 15 to 18 times the width of the channel. Over time, meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.The degree of meandering of the channel of a river, stream, or other watercourse is measured by its sinuosity. The sinuosity of a watercourse is the ratio of the length of the channel to the straight line down-valley distance. Streams or rivers with a single channel and sinuosities of 1.5 or more are defined as meandering streams or rivers.Northwestern deer mouse
The northwestern deer mouse or Keen's mouse (Peromyscus keeni) is a species of rodent in the family Cricetidae. It is found in British Columbia in Canada and in Alaska and Washington in the United States. It was named after the Rev. John Henry Keen in 1894.According to the study from "Spatial Variation in Population Dynamics of Sitka Mice in Floodplain Forests" which was published by Thomas A. Hanley and Jeffrey C. Barnard, the sitka mouse or Northwestern deer mouse has an interesting perspective regarding the floodplain and habitat. The article has done an experiment testing the relationship between floodplain habitat and has concluded that the floodplain forset does not necessarily makes a favorable habitat for sitka mouse.Ord River Floodplain
The Ord River floodplain is the floodplain of the lower Ord River in the Shire of Wyndham-East Kimberley, in the Kimberley region of northern Western Australia. It lies within the Victoria Bonaparte IBRA bioregion and contains river, seasonal creek, tidal mudflat and floodplain wetlands, with extensive stands of mangroves, that support saltwater crocodiles and many waterbirds. It is recognised as an internationally important wetland area, with 1,384 km2 of it designated on 7 June 1990 as Ramsar Site 477 under the Ramsar Convention on Wetlands.Taieri River
The Taieri River is the fourth-longest river in New Zealand and is in Otago in the South Island. Rising in the Lammerlaw ranges, it initially flows north, then east around the Rock and Pillar range before turning southeast, reaching the sea 30 kilometres (19 mi) south of Dunedin.
The upper reaches meander in a series of convoluted loops across a floodplain near Paerau before running through two small hydroelectric power stations before Patearoa in the Maniototo. The Taieri then arcs through almost 180 degrees, entering a broad glacial valley the Strath-Taieri, surrounded by rugged hill ranges. Immediately downstream the river has cut a steep-sided declivity—the Taieri Gorge. This is known for the Taieri Gorge Railway, which follows a route into Central Otago through it. In the Taieri's lower reaches there is a broad floodplain (the Taieri Plains) containing much of Otago's most fertile farmland. The river then flows through the lower Taieri Gorge to the Pacific Ocean at Taieri Mouth. Taieri Island lies in the Pacific Ocean several hundred metres from the mouth of the river.The Taieri is 288 kilometres (179 mi) long of which the last 20 kilometres (12 mi) are navigable. Towns along the river include Middlemarch, Outram, Allanton, Mosgiel, Henley and Taieri Mouth. Its major tributary is the Waipori River, which meets the Taieri near Henley on the Taieri Plains.
The name "Taieri" is thought to come from the Māori word taiari meaning "spring tide".Várzea Alegre
Várzea Alegre is a municipality in the state of Ceará in the Northeast region of Brazil. Its area is 835.706 km2, including the Calabaça (pumpkin), Canindezinho, Ibicatu, Naraniú and Lagoa Verde (green lagoon) districts.Várzea forest
A várzea forest is a seasonal floodplain forest inundated by whitewater rivers that occurs in the Amazon biome. Until the late 1970s, the definition was less clear and várzea was often used for all periodically flooded Amazonian forests.Although sometimes described as consisting only of forest, várzea also contains more open, seasonally flooded habitats such as grasslands, including floating meadows.Watson Island State Forest
Watson Island State Forest is located in St. Johns County, Florida. The state forest includes 199 acres of Bottomland Forest, 129 acres of Wet Flatwoods, 124 acres of Mesic Flatwoods, 26 acres of floodplain, 23 acres of Baygall forest, 2 acres of Floodplain Marsh, 2 acres of ruderal and 1 acre of dome swamp.