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 Plain
 
There 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 Terraces
 
The 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 Plain
 
Flood 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 Plains
 
Floods 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.
 
Agriculture
Flood 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.
 
Transportation
A 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.
 
Flooding

Flood 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 Meadows
Many 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 Planning
Cities 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 Restoration
In 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.)
 
Insurance
Houses 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.
flood plain
Flood plains usually only flood during the rainy season.

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

Floodway
The 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.
100-year flood
Noun

flood that has a one-percent chance of occurring any year. 

absorb
Verb

to soak up.

adjacent
Adjective

next to.

aggradation
Noun

process of raising the level (grade) of a stream or river through the deposit of sediments.

Noun

the art and science of cultivating land for growing crops (farming) or raising livestock (ranching).

Noun

fan-shaped deposit of eroded material, usually sediment and sand.

alluvium
Noun

gravel, sand, and smaller materials deposited by flowing water.

ambitious
Adjective

eager to achieve wealth, power, status, or a specific goal.

Noun

an underground layer of rock or earth which holds groundwater.

architecture
Noun

style and design of buildings or open spaces.

arid
Adjective

dry.

bank
Noun

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.

basalt
Noun

type of dark volcanic rock.

Noun

a dip or depression in the surface of the land or ocean floor.

Noun

all the different kinds of living organisms within a given area.

bluff line
Noun

furthest extent of a flood plain, where the valley floor begins to rise. Also called a valley wall.

boulder
Noun

large rock.

braided river
Noun

flowing body of water separated into channels by tiny islands.

breach
Verb

behavior exhibited by whales, when they jump above the surface of the water.

cargo
Noun

goods carried by a ship, plane, or other vehicle.

cattle
Noun

cows and oxen.

channel
Noun

deepest part of a shallow body of water, often a passageway for ships.

Noun

complex way of life that developed as humans began to develop urban settlements.

commerce
Noun

trade, or the exchange of goods and services.

conservationist
Noun

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.

crucial
Adjective

very important.

cut terrace
Noun

type of fluvial (river-formed) earthen terrace in which the flowing stream erodes the material aggraded on its banks.

damage
Noun

harm that reduces usefulness or value.

deftly
Adverb

in a skillful manner.

Noun

the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.

Noun

area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

designate
Verb

to name or single out.

devastating
Adjective

very destructive or damaging.

development
Noun

construction or preparation of land for housing, industry, or agriculture.

Noun

a barrier, usually a natural or artificial wall used to regulate water levels.

discrete
Adjective

individual or distinct.

dissipate
Verb

to scatter and disappear.

distribute
Verb

to divide and spread out materials.

divert
Verb

to direct away from a familiar path.

Noun

period of greatly reduced precipitation.

drown
Verb

to die or suffocate in a liquid.

dry season
Noun

time of year with little precipitation.

economic
Adjective

having to do with money.

Noun

community and interactions of living and nonliving things in an area.

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.

Noun

organism threatened with extinction.

endemic species
Noun

species that naturally occurs in only one area or region.

engineer
Noun

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.

Noun

act in which earth is worn away, often by water, wind, or ice.

evacuate
Verb

to leave or remove from a dangerous place.

evacuation
Noun

removal of people, organisms, or objects from an endangered area.

extension
Noun

additional part of a larger project or organization.

extraordinary
Adjective

unusual or uncommon.

farming
Noun

the art, science, and business of cultivating the land for growing crops.

fertile
Adjective

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.

filter
Verb

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.

FIRM
Noun

flood insurance rate map.

flank
Noun

side of something.

flash flood
Noun

sudden, short, and heavy flow of water.

flood
Verb

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.

Noun

flat area alongside a stream or river that is subject to flooding.

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.

fodder
Noun

food for livestock consisting of whole plants.

forest
Noun

ecosystem filled with trees and underbrush.

geologic
Adjective

having to do with the physical formations of the Earth.

geologist
Noun

person who studies the physical formations of the Earth.

gills
Plural Noun

respiratory organs that draw oxygen from water and into the bloodstream.

Noun

mass of ice that moves slowly over land.

Noun

deep, narrow valley with steep sides, usually smaller than a canyon.

Noun

harvested seed of such grasses as wheat, oats, and rice.

grassland
Noun

ecosystem with large, flat areas of grasses.

green space
Noun

area of undeveloped land usually used for recreation.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

industry
Noun

activity that produces goods and services.

infrastructure
Noun

structures and facilities necessary for the functioning of a society, such as roads.

insurance
Noun

money paid in good health to guarantee financial or physical health if injury or damage occurs.

Noun

watering land, usually for agriculture, by artificial means.

Noun

the geographic features of a region.

Noun

bank of a river, raised either naturally or constructed by people.

livestock
noun, plural noun

animals raised for sale and profit.

logistical
Adjective

having to do with the management or movement of goods and services.

lucrative
Adjective

profitable or money-making.

lung
Noun

organ in an animal that is necessary for breathing.

mainstem
Noun

largest river or channel in a watershed or drainage basin.

Noun

wetland area usually covered by a shallow layer of seawater or freshwater.

meander
Noun

large curve in a lake or stream.

Mesopotamia
Noun

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.

navigate
Verb

to plan and direct the course of a journey.

Noun

substance an organism needs for energy, growth, and life.

obstacle
Noun

something that slows or stops progress.

Noun

lake formed from an abandoned bend in a river.

pasture
Noun

type of agricultural land used for grazing livestock.

phenomenon
Noun

an unusual act or occurrence.

plumbing
Noun

system of pipes for transporting liquids to and from a building.

Noun

introduction of harmful materials into the environment.

Noun

all forms in which water falls to Earth from the atmosphere.

predict
Verb

to know the outcome of a situation in advance.

primitive
Adjective

simple or crude.

proximity
Noun

nearness.

purification
Noun

cleansing.

rainy season
Noun

time of year when most of the rain in a region falls.

ranch
Noun

large farm on which livestock are raised.

Noun

areas of fast-flowing water in a river or stream that is making a slight descent.

relocate
Verb

to move a residence or business from one place to another.

residential
Adjective

having to do with people's homes.

riparian
Adjective

having to do with a river or stream.

Noun

large stream of flowing fresh water.

river valley
Noun

depression in the earth caused by a river eroding the surrounding soil.

Noun

overflow of fluid from a farm or industrial factory.

saturate
Verb

to fill one substance with as much of another substance as it can take.

Noun

increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.

seasonal
Adjective

likely to change with the seasons.

Noun

solid material transported and deposited by water, ice, and wind.

seep
Verb

to slowly flow through a border.

Noun

small sediment particles.

stream
Noun

body of flowing fluid.

stream gradient
Noun

measurement of how steep a riverbed is.

subsidence
Noun

sinking or lowering of the Earth's surface, either by natural or man-made processes.

susceptible
Adjective

able to be influenced to behave a certain way.

Noun

land permanently saturated with water and sometimes covered with it.

swath
Noun

path or line of material.

terrain
Noun

topographic features of an area.

threat
Noun

danger.

topsoil
Noun

the most valuable, upper layer of soil, where most nutrients are found.

toxic
Adjective

poisonous.

trade
Noun

buying, selling, or exchanging of goods and services.

transportation
Noun

movement of people or goods from one place to another.

Noun

stream that feeds, or flows, into a larger stream.

tugay
Noun

woodland area flanking a river on a flood plain. Also called a riparian forest.

Noun

developed, densely populated area where most inhabitants have nonagricultural jobs.

urban planner
Noun

person who works to create or improve the natural, built, economic, and social environments of urban areas.

valley
Noun

depression in the Earth between hills.

vast
Adjective

huge and spread out.

vegetation
Noun

all the plant life of a specific place.

velocity
Noun

measurement of the rate and direction of change in the position of an object.

vital
Adjective

necessary or very important.

waterlogged
Adjective

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.

Noun

area of land covered by shallow water or saturated by water.

whitewater
Noun

fast-moving parts of a river.