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ENCYCLOPEDIC ENTRY
ENCYCLOPEDIC ENTRY

floodplain

floodplain

A floodplain is a generally flat area of land next to a river or stream.

Grades

6 - 12+

Subjects

Biology, Earth Science, Ecology, Geography, Geology, Physical Geography

A floodplain (or floodplain) is a generally flat area of land next to a river or stream. It stretches from the banks of the river to the outer edges of the valley. A floodplain consists of two parts. The first is the main channel of the river itself, called the floodway. Floodways can sometimes be seasonal, meaning the channel is dry for part of the year. The floodway of the Todd River in Australia’s Northern Territory, for instance, is an ephemeral stream, meaning its channel can be dry for months at a time. Beyond the floodway is the flood fringe. The flood fringe extends from the outer banks of the floodway to the bluff lines of a river valley. Bluff lines, also called valley walls, mark the area where the valley floor begins to rise into bluffs. The flood fringe of the seasonal Todd River extends the floodplain to 445 square kilometers (170 square miles). Some floodplains are extraordinarily wide. The Barotse floodplain of the Zambezi River, for example, is a vast wetland stretching thousands of kilometers through Angola, Zambia, and Botswana. The Barotse floodplain includes the sandy Kalahari basin, which is waterlogged during the rainy season and an extension of the nearby Kalahari Desert during the dry season. Some rivers have very narrow floodplains. In fact, some rivers, or parts of rivers, seem to have no floodplain at all. These rivers usually have a steep stream gradient—a very deep, fast-moving channel. Ngonye Falls, Zambia, marks a remote stretch of the Zambezi River where the floodplain is extremely narrow. As the Zambezi leaves the wide floodplain of the sandy Kalahari, it enters a narrow basalt channel as fast-moving whitewater rapids. Geology of a floodplain There are two major processes involved in the natural development of floodplains: erosion and aggradation. The erosion of a floodplain describes the process in which earth is worn away by the movement of a floodway. Aggradation (or alluviation) of a floodplain describes the process in which earthen material increases as the floodway deposits sediment. A river erodes a floodplain as it meanders, or curves from side to side. The massive lowland floodplain of the Amazon River, for instance, is carved with hundreds of oxbow lakes that document the meandering river and its tributaries over time. Oxbow lakes are formed when a meander, or bend, in the river is cut off from the river’s mainstem. Features such as oxbow lakes and seasonal wetlands are often a part of floodplains created through erosion and deposition. A meandering stream can contribute to a floodplain’s aggradation, or build-up in land elevation, as well as its erosion. A typical aggradation environment is a wide, shallow, braided river. Braided rivers often include river deltas, where the main floodway is separated into discrete channels and tiny islands. The process of subsidence, in which the elevation of a delta may sink due to sea level rise or human activity, often offsets aggradation in the floodplains in these areas. The huge aggradation of sediment around the Nile delta, for instance, is subsiding due to the rising level of the Mediterranean Sea. The alluvium, or sediment, of a floodplain varies. Its coarseness and composition depend on the surrounding landscape and the velocity of the currents that created the floodplain. Some floodplains are mostly fine-grained silt, while others are sandy. The deposit of alluvium created as a river or stream breaks, or breaches, its bank is called a crevasse splay. The formation of a crevasse splay is very similar to the formation of an alluvial fan. The thickest layer of sediment is nearest the breach, while the thinnest and youngest sediments are fanned out. The layered sediments of many floodplains can create important aquifers. Clay, sand, and gravel filter water as it seeps downward. Water purification systems often take advantage of this natural phenomenon in a process called bank filtration. In bank filtration, water is deliberately filtered through the banks or floodplain of a river or lake. Nearby wells then collect the filtered water, which is then ready for more intense purification processes. Fluvial Terraces The sedimentary patterns of floodplains often provide scientists with evidence of past geologic activity. Thick layers of sand may indicate flash flooding, for instance, while thin, evenly spaced layers of silt may indicate more moderate and predictable flood patterns. One of the most important geologic features of a floodplain is its fluvial terraces. Fluvial terraces are step-shaped areas of land that flank the banks of a river or stream. Fluvial terraces mark the older, higher-elevation paths of the stream, before erosion and aggredation created the current mainstem of the stream or river. Fluvial terraces can mark the bluff lines—outer edges—of a floodplain. There are two major types of fluvial terraces: fill terraces and cut terraces. Fill terraces are formed as a valley or gorge is filled with alluvium. Alluvium can aggregate as a river meanders and overflows its banks, or it can be deposited by a glacier. While fill terraces are associated with aggredation, cut terraces are associated with erosion. Cut terraces are often formed below fill terraces, as water erodes sediment. Older floodplains and river valleys can have many fluvial terraces. The Rhine Valley of Central Europe, for instance, has dozens of fluvial terraces created by the meandering Rhine as well as intense glaciation. Fluvial terraces in the Rhine allow geologists to examine more than 100,000 years of Europe’s past. Living on the Plain floodplains have dazzling arrays of biodiversity. These seasonal riparian wetlands boast greater biodiversity than the rivers themselves. The floodplains of Congo River tributaries, for instance, boast one of the most unusual fish on the planet: the West African lungfish. The lungfish is adapted to the two seasons in the Congo floodplain. It uses its gills during the rainy season, and its primitive lung during the dry season. The Murray-Darling floodplain in Southeast Australia has remained remarkably unchanged for thousands of years. This floodplain is home to endemic species such as the hairy-nosed wombat, the wedge-tailed eagle, and several types of orchid. Tugay forests are unusual ecosystems that stretch along the floodplains of Central Asia, including western China, the Stans, and Azerbaijan. Tugay forests are sometimes called riparian forests due to their proximity to winding rivers. Tugay forests often serve as green migration corridors through arid or semi-arid environments. Vegetation in tugay forests, such as willow, poplar, and tamarisk, is largely dependent on the water supplied by the floodplain’s rivers and aquifers—not precipitation. People and floodplains Floods are usually seasonal and can be predicted months ahead of time. This predictability can make floodplains ideal locations to develop urban areas. Rivers provide both a natural transportation network and source of water for irrigation and industry. The relatively level land can be developed either as agricultural fields or sites for habitation or business. The three most ancient civilizations on Earth all developed on fertile floodplains. The floodplains between the Tigris and Euphrates rivers, in what are today Syria and Iraq, are known as Mesopotamia, “the land between the rivers.” Ancient civilizations of Mesopotamia include Sumeria, Akkadia, Assyria, and Babylonia. The floodplains of the Indus River, in what is today Afghanistan, India, and Pakistan, gave rise to the Indus River Civilization, also known as the Harappan civilization. Finally, ancient Egyptian culture developed around the fertile floodplains of the Nile. Agriculturefloodplains are usually very fertile agricultural areas. Floods carry nutrient-rich silt and sediment, and distribute it across a wide area. floodplains are flat and often have relatively few boulders or other large obstacles that may prevent farming. The rich floodplains of the Pampas, for example, are nicknamed the “Breadbasket of Argentina.” These lowlands are susceptible to floods, but are also home to some of South America’s most lucrative grain farms and cattle ranches. TransportationA floodplain’s flat terrain and slow-flowing rivers can provide excellent transportation corridors. Roads, bridges, railways, and even airports can be constructed on the even surface. Ships and barges can often haul cargo faster and more efficiently than roadways. The floodplains of the mighty Mississippi-Missouri river system in the central United States, for example, have served as vital transportation corridors for centuries. Native Americans deftly navigated the floodplains, making trade between the East Coast and West Coast of North America possible. During the 19th century, cities on Mississippi floodplains—St. Louis, Missouri; Memphis, Tennessee; New Orleans, Louisiana—became crucial centers of culture and commerce. Flooding
floodplains are natural flooding outlets for rivers. People, agriculture, and businesses on floodplains are always at some risk. The most devastating floods of the 20th century occurred on the floodplains of the Yellow River in China, for example. The 1931 floods were some of the worst natural disasters ever recorded. The 1931 Yellow River floods followed years of drought that left the topsoil on floodplains brittle and eroded. Heavy rains swelled the river and forced it to break its banks, drowning wide swaths of land as the floodplain was unable to efficiently absorb the river’s excess water or dissipate its energy. Managing development of floodplains is a critical responsibility for regional and urban planners. The benefits of floodplains, including prime agricultural land and desirable housing locations, must be balanced with the personal and economic threats posed by floods. Flood Meadows and Water MeadowsMany flood-plain settlements maintain flood meadows and water meadows to reduce the impact of seasonal flooding. Flood meadows are natural areas of grassland immediately adjacent to a floodway. Flood meadows are often used as pastures for livestock when they are not saturated with water. Water meadows are also grasslands adjacent to floodways. Unlike flood meadows, water meadows are created and maintained by people. Water meadows are continuously irrigated through channels from the river. Water meadows were common features of the agricultural landscape in Western Europe throughout the 19th century. The nutrient-rich, silty soils of water meadows supported rich pastures used for livestock, as well as growing hay and other fodder. Urban PlanningCities built on floodplains, such as St. Louis or New Orleans, must incorporate flood-control infrastructure into their organization and architecture. Evacuation procedures, emergency shelters, and building codes must be in place. Levees or other barriers must be a part of the city design. Urban planners try to keep areas near the floodway, called a Special Flood Hazard Area, as free from development as possible. Sometimes, residents on floodplains must relocate entirely. The small town of English, Indiana, for instance, was established on the floodplain of the Blue River, a tributary of the Ohio. Damage caused by frequent floods encouraged the town’s residents and businesses to relocate the town center to elevated agricultural land several kilometers away. About 75% of English was torn down or relocated in 1990. Floodways and floodplain RestorationIn many floodplains, a mass relocation is impossible for logistical and economic reasons. In such cases, engineers can divert the path the floodway—the river—itself. Artificial floodways are a sort of man-made river channel. The Red River floodway, for example, can divert the path of the Red River around the urban area of Winnipeg, Manitoba, Canada. During flooding episodes, the channel can divert up to 4,000 cubic meters (140,000 cubic feet) of water per second before it reaches the Winnipeg area. The floodway carries this outflow around the city before rejoining the mainstem of the Red River in a less-populated area of the floodplain. Since its construction in 1968, the Red River floodway has saved Manitoba more than $32 billion in flood damage. In other places, conservationists and engineers have engaged in floodplain restoration. floodplain restoration is the process of returning a floodplain to its condition before people modified the landscape for development or agriculture. floodplain restoration may include removing dikes and levees, as well as flooding previously drained marshes and swamps. One of the most ambitious floodplain restoration projects is underway in the Lower Danube floodplains of Bulgaria, Romania, Moldova, and Ukraine. The extensive projects aim to reduce flood damage by restoring flood meadows, which will absorb excess water. floodplain restoration projects will also provide habitats for endangered species and reduce pollution in the Danube. (Flood meadows absorb chemicals from agricultural and industrial runoff.) InsuranceHouses and businesses that are built on floodplains often require more insurance coverage than buildings constructed on higher ground, because flood damage is more likely to occur. A FIRM is a flood insurance rate map. FIRMs display Special Flood Hazard Areas within a floodplain. A Special Flood Hazard Area is simply an area that falls within boundary of a 100-year flood. FIRMs are used to balance the risk of flood against the rate of insurance. FIRMs are divided into different zones based on the zone’s proximity to the floodway. Buildings in the A- or V-zones, for example, are near the banks of the river. All buildings in A zones are required to have flood insurance due to their extremely high risk of flood damage. The floors and service facilities of A-zone buildings (such as air-conditioning units and plumbing) must meet a “base high flood elevation.” Base high flood elevations vary depending on the floodplain and risk posed by a 100-year flood. In the Charlotte, North Carolina, floodplain, for example, the base high flood elevation is one foot above the expected depth of floodwater in a 100-year flood. There are strict rules for constructing or remodeling buildings in the A-zone of a floodplain. Basements in A-zones must not be used as living spaces, for example. Urban planners frequently use FIRMs to establish a city’s land-use policies and development zones. Industrial zones, which can include factories with toxic chemicals, may be located far from the floodway in order to prevent pollution of a community’s source of water. Residential zones, which are more difficult to evacuate than hotel-designated zones, may also be more limited along a floodway. Areas closest to the floodway, in contrast, are often designated as “green spaces” and parks.

Fast Fact

Exner Equation
A mathematical calculation known as the Exner equation helps geologists and hydrologists determine the extent of a floodplain. The Exner equation describes the relationship between the sediment that is transported by a river and the sediment that is deposited by a river. The equation is dominated by the density and distribution of sediment in a river.

is the change in bed elevation. is time. is grain packing density. is sediment flux.

Fast Fact

FloodwayThe term “floodway” is sometimes used for a road built at ground level on a flood plain. Floodways are usually constructed on floodplains with low traffic and rare floods.

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Editors
Jeannie Evers, Emdash Editing, Emdash Editing
Melissa MacPhee, National Geographic Society
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National Geographic Society
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Last Updated

May 20, 2022

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