A flood plain (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 flood plain 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 flood plain to 445 square kilometers (170 square miles).Some flood plains are extraordinarily wide. The Barotse flood plain of the Zambezi River, for example, is a vast wetland stretching thousands of kilometers through Angola, Zambia, and Botswana. The Barotse flood plain 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 flood plains. In fact, some rivers, or parts of rivers, seem to have no flood plain 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 flood plain is extremely narrow. As the Zambezi leaves the wide flood plain of the sandy Kalahari, it enters a narrow basalt channel as fast-moving whitewater rapids.Geology of a Flood PlainThere are two major processes involved in the natural development of flood plains: erosion and aggradation. The erosion of a flood plain describes the process in which earth is worn away by the movement of a floodway. Aggradation (or alluviation) of a flood plain describes the process in which earthen material increases as the floodway deposits sediment.A river erodes a flood plain as it meanders, or curves from side to side. The massive lowland flood plain 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 flood plains created through erosion and deposition.A meandering stream can contribute to a flood plain’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 flood plains 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 flood plain varies. Its coarseness and composition depend on the surrounding landscape and the velocity of the currents that created the flood plain. Some flood plains 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 flood plains 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 flood plain of a river or lake. Nearby wells then collect the filtered water, which is then ready for more intense purification processes.Fluvial TerracesThe sedimentary patterns of flood plains 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 flood plain 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 flood plain.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 flood plains 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 PlainFlood plains have dazzling arrays of biodiversity. These seasonal riparian wetlands boast greater biodiversity than the rivers themselves.The flood plains 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 flood plain. It uses its gills during the rainy season, and its primitive lung during the dry season.The Murray-Darling flood plain in Southeast Australia has remained remarkably unchanged for thousands of years. This flood plain 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 flood plains 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 flood plain’s rivers and aquifers—not precipitation.People and Flood PlainsFloods are usually seasonal and can be predicted months ahead of time. This predictability can make flood plains 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 flood plains. The flood plains 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 flood plains 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 flood plains of the Nile.AgricultureFlood plains are usually very fertile agricultural areas. Floods carry nutrient-rich silt and sediment, and distribute it across a wide area. Flood plains are flat and often have relatively few boulders or other large obstacles that may prevent farming.The rich flood plains 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 flood plain’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 flood plains 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 flood plains, making trade between the East Coast and West Coast of North America possible. During the 19th century, cities on Mississippi flood plains—St. Louis, Missouri; Memphis, Tennessee; New Orleans, Louisiana—became crucial centers of culture and commerce.FloodingFlood plains are natural flooding outlets for rivers. People, agriculture, and businesses on flood plains are always at some risk.The most devastating floods of the 20th century occurred on the flood plains 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 flood plains brittle and eroded. Heavy rains swelled the river and forced it to break its banks, drowning wide swaths of land as the flood plain was unable to efficiently absorb the river’s excess water or dissipate its energy.Managing development of flood plains is a critical responsibility for regional and urban planners. The benefits of flood plains, 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 flood plains, 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 flood plains must relocate entirely. The small town of English, Indiana, for instance, was established on the flood plain 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 Flood Plain RestorationIn many flood plains, 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 flood plain. 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 flood plain restoration. Flood plain restoration is the process of returning a flood plain to its condition before people modified the landscape for development or agriculture. Flood plain restoration may include removing dikes and levees, as well as flooding previously drained marshes and swamps.One of the most ambitious flood plain restoration projects is underway in the Lower Danube flood plains of Bulgaria, Romania, Moldova, and Ukraine. The extensive projects aim to reduce flood damage by restoring flood meadows, which will absorb excess water. Flood plain 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 flood plains 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 flood plain. 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 flood plain and risk posed by a 100-year flood. In the Charlotte, North Carolina, flood plain, 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 flood plain. 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.FloodwayThe term “floodway” is sometimes used for a road built at ground level on a flood plain. Floodways are usually constructed on flood plains with low traffic and rare floods.
A mathematical calculation known as the Exner equation helps geologists and hydrologists determine the extent of a flood plain. 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.
Term Part of Speech Definition Encyclopedic Entry 100-year flood Noun
flood that has a one-percent chance of occurring any year.
to soak up.
process of raising the level (grade) of a stream or river through the deposit of sediments.
the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).
Encyclopedic Entry: agriculture alluvial fan Noun
fan-shaped deposit of eroded material, usually sediment and sand.
Encyclopedic Entry: alluvial fan alluvium Noun
gravel, sand, and smaller materials deposited by flowing water.
eager to achieve wealth, power, status, or a specific goal.
an underground layer of rock or earth which holds groundwater.
Encyclopedic Entry: aquifer architecture Noun
style and design of buildings or open spaces.
a slope of land adjoining a body of water, or a large elevated area of the sea floor.
bank filtration Noun
process in which water is deliberately filtered through the banks or flood plain of a river or lake.
type of dark volcanic rock.
a dip or depression in the surface of the land or ocean floor.
Encyclopedic Entry: basin biodiversity Noun
all the different kinds of living organisms within a given area.
Encyclopedic Entry: biodiversity bluff line Noun
furthest extent of a flood plain, where the valley floor begins to rise. Also called a valley wall.
braided river Noun
flowing body of water separated into channels by tiny islands.
behavior exhibited by whales, when they jump above the surface of the water.
goods carried by a ship, plane, or other vehicle.
cows and oxen.
deepest part of a shallow body of water, often a passageway for ships.
complex way of life that developed as humans began to develop urban settlements.
Encyclopedic Entry: civilization commerce Noun
trade, or the exchange of goods and services.
person who works to preserve natural habitats.
crevasse splay Noun
deposit that forms when a stream breaks its natural or artificial levees and deposits sediment on a flood plain.
cut terrace Noun
type of fluvial (river-formed) earthen terrace in which the flowing stream erodes the material aggraded on its banks.
harm that reduces usefulness or value.
in a skillful manner.
the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.
Encyclopedic Entry: delta desert Noun
area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.
Encyclopedic Entry: desert designate Verb
to name or single out.
very destructive or damaging.
construction or preparation of land for housing, industry, or agriculture.
a barrier, usually a natural or artificial wall used to regulate water levels.
Encyclopedic Entry: dike discrete Adjective
individual or distinct.
to scatter and disappear.
to divide and spread out materials.
to direct away from a familiar path.
period of greatly reduced precipitation.
Encyclopedic Entry: drought drown Verb
to die or suffocate in a liquid.
dry season Noun
time of year with little precipitation.
having to do with money.
community and interactions of living and nonliving things in an area.
Encyclopedic Entry: ecosystem efficient Adjective
performing a task with skill and minimal waste.
emergency shelter Noun
place where victims of natural disasters, refugees, and other people relocated from their homes can stay for short periods of time.
endangered species Noun
organism threatened with extinction.
Encyclopedic Entry: endangered species endemic species Noun
species that naturally occurs in only one area or region.
person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).
ephemeral stream Noun
body of water that flows only after a fall of precipitation.
act in which earth is worn away, often by water, wind, or ice.
Encyclopedic Entry: erosion evacuate Verb
to leave or remove from a dangerous place.
removal of people, organisms, or objects from an endangered area.
additional part of a larger project or organization.
unusual or uncommon.
the art, science, and business of cultivating the land for growing crops.
able to produce crops or sustain agriculture.
fill terrace Noun
type of fluvial (river-formed) earthen terrace formed by the build-up of sediment on a river's banks.
to remove particles from a substance by passing the substance through a screen or other material that catches larger particles and lets the rest of the substance pass through.
flood insurance rate map.
side of something.
flash flood Noun
sudden, short, and heavy flow of water.
to overflow or cover in water or another liquid.
flood fringe Noun
outer area of a flood plain, often waterlogged by a flood but not experiencing currents.
flood meadow Noun
area of grassland next to a river or stream, prone to seasonal flooding.
flood plain Noun
flat area alongside a stream or river that is subject to flooding.
Encyclopedic Entry: flood plain floodway Noun
main channel of a river in a flood plain.
fluvial terrace Noun
tiered, step-shaped feature that flanks the banks of a river or stream.
food for livestock consisting of whole plants.
ecosystem filled with trees and underbrush.
having to do with the physical formations of the Earth.
person who studies the physical formations of the Earth.
gills Plural Noun
respiratory organs that draw oxygen from water and into the bloodstream.
mass of ice that moves slowly over land.
Encyclopedic Entry: glacier gorge Noun
deep, narrow valley with steep sides, usually smaller than a canyon.
Encyclopedic Entry: gorge grain Noun
harvested seed of such grasses as wheat, oats, and rice.
Encyclopedic Entry: grain grassland Noun
ecosystem with large, flat areas of grasses.
green space Noun
area of undeveloped land usually used for recreation.
environment where an organism lives throughout the year or for shorter periods of time.
Encyclopedic Entry: habitat industry Noun
activity that produces goods and services.
structures and facilities necessary for the functioning of a society, such as roads.
money paid in good health to guarantee financial or physical health if injury or damage occurs.
watering land, usually for agriculture, by artificial means.
Encyclopedic Entry: irrigation landscape Noun
the geographic features of a region.
Encyclopedic Entry: landscape levee Noun
bank of a river, raised either naturally or constructed by people.
Encyclopedic Entry: levee livestock noun, plural noun
animals raised for sale and profit.
having to do with the management or movement of goods and services.
profitable or money-making.
organ in an animal that is necessary for breathing.
largest river or channel in a watershed or drainage basin.
wetland area usually covered by a shallow layer of seawater or freshwater.
Encyclopedic Entry: marsh meander Noun
large curve in a lake or stream.
ancient region between the Tigris and Euphrates Rivers, today lying mostly in Iraq.
migration corridor Noun
area connecting wildlife habitats disturbed and interrupted by human activity. Also called a green corridor.
natural disaster Noun
an event occurring naturally that has large-scale effects on the environment and people, such as a volcano, earthquake, or hurricane.
to plan and direct the course of a journey.
substance an organism needs for energy, growth, and life.
Encyclopedic Entry: nutrient obstacle Noun
something that slows or stops progress.
oxbow lake Noun
lake formed from an abandoned bend in a river.
Encyclopedic Entry: oxbow lake pasture Noun
type of agricultural land used for grazing livestock.
an unusual act or occurrence.
system of pipes for transporting liquids to and from a building.
introduction of harmful materials into the environment.
Encyclopedic Entry: pollution precipitation Noun
all forms in which water falls to Earth from the atmosphere.
Encyclopedic Entry: precipitation predict Verb
to know the outcome of a situation in advance.
simple or crude.
rainy season Noun
time of year when most of the rain in a region falls.
large farm on which livestock are raised.
areas of fast-flowing water in a river or stream that is making a slight descent.
Encyclopedic Entry: rapids relocate Verb
to move a residence or business from one place to another.
having to do with people's homes.
having to do with a river or stream.
large stream of flowing fresh water.
Encyclopedic Entry: river river valley Noun
depression in the earth caused by a river eroding the surrounding soil.
overflow of fluid from a farm or industrial factory.
Encyclopedic Entry: runoff saturate Verb
to fill one substance with as much of another substance as it can take.
sea level rise Noun
increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.
likely to change with the seasons.
solid material transported and deposited by water, ice, and wind.
Encyclopedic Entry: sediment seep Verb
to slowly flow through a border.
small sediment particles.
Encyclopedic Entry: silt stream Noun
body of flowing fluid.
stream gradient Noun
measurement of how steep a riverbed is.
sinking or lowering of the Earth's surface, either by natural or man-made processes.
able to be influenced to behave a certain way.
land permanently saturated with water and sometimes covered with it.
Encyclopedic Entry: swamp swath Noun
path or line of material.
topographic features of an area.
the most valuable, upper layer of soil, where most nutrients are found.
buying, selling, or exchanging of goods and services.
movement of people or goods from one place to another.
stream that feeds, or flows, into a larger stream.
Encyclopedic Entry: tributary tugay Noun
woodland area flanking a river on a flood plain. Also called a riparian forest.
urban area Noun
developed, densely populated area where most inhabitants have nonagricultural jobs.
Encyclopedic Entry: urban area urban planner Noun
person who works to create or improve the natural, built, economic, and social environments of urban areas.
depression in the Earth between hills.
huge and spread out.
all the plant life of a specific place.
measurement of the rate and direction of change in the position of an object.
necessary or very important.
flooded or overflowing with water.
water meadow Noun
area of grassland next to a river or stream, intentionally flooded to maintain fertility and dissipate floodwaters.
area of land covered by shallow water or saturated by water.
Encyclopedic Entry: wetland whitewater Noun
fast-moving parts of a river.