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Hamilton S K. (2009) Flood Plains. In: Gene E. Likens, (Editor) Encyclopedia of Inland Waters. volume 3, pp. 378-386 Oxford: Elsevier.

Author's personal copy

Flood PlainsS K Hamilton, Michigan State University, Hickory Corners, MI, USA

ã 2009 Elsevier Inc. All rights reserved.

The term flood plain is defined by the AmericanGeological Institute as ‘the surface or strip of rela-tively smooth land adjacent to a river channel, con-structed by the present river in its existing regime andcovered with water when the river overflows itbanks’. This definition of flood plains includes onlyseasonally or episodically inundated land, but in factmany flood plains also contain water bodies that arepermanent or semipermanent. These water bodiesinclude floodplain lakes and channels, as well as shal-low wetlands (sometimes called backswamps) thatare separated from the river by levees. From thestandpoint of flood plains as inland waters, thesepermanently wet areas can be distinguished fromland subject to temporary, albeit sometimes pro-longed, inundation resulting directly or indirectlyfrom a rise in river level. Seasonal or episodic inunda-tion strongly affects the more permanently wet areas,often completely replacing their surface water andchanging the environment for aquatic life. Thus thehydrology and ecology of floodplain water bodiescharacteristically show strong seasonal dynamics.The earlier-mentioned definition of a flood plain

often does not correspond with how floodplain eco-systems are delineated for ecological studies, whichtend to include more distal or slightly elevated landsthat originated as flood plains and are contiguouswith an active floodplain, but may now rarely ornever be inundated by the parent river. Yet theseareas tend to share ecological characteristics withactive flood plains for several reasons that are dis-cussed later, and the transition between active andrelict flood plains may not be clear.

Examples of Floodplain Environments

Remotely sensed images of contrasting types of flood-plain environments are depicted in Figures 1–7.Images of four large South American flood plains(Amazon, Madre de Dios, Pantanal, and the Llanosde Moxos), as well as the Cooper Creek system inAustralia (Figures 1–5) depict the diversity of land-forms, water bodies, and vegetation in large floodplains with minimal human disturbance. Exampleswhere floodplain hydrology has been strongly alteredare shown for the Kalamazoo River (Figure 6) and theMississippi River (Figure 7). The ensuing discussionwill refer to these images. In addition, photos of manyof these sites appear in the online Flood Plain PhotoGallery (cite web site here).

378Encyclopedia of Inland Water

Geomorphological Processes onFlood Plains

Flood plains are built of alluvial fill that normallyoriginates as sediments carried by the parent riverand deposited as point bars along the migrating chan-nel, or during overbank flooding. Hydrologists referto bankfull discharge as the river discharge abovewhich the flood plain becomes inundated. The con-cept of bankfull can be difficult to apply, however,since much water enters and exits the flood plain vialow areas or discrete openings in the levees (see latertext). Many rivers reach bankfull at least annually,but inundate their flood plains less frequently.

River valleys commonly have one or more aban-doned flood plains lying on elevated terraces that arenormally above the present-day reach of riverineflooding. Such terraces may still contain wetlandsand permanent water bodies that reflect their fluvialorigin. Wetlands on such terraces may be sustained bygroundwater and surface runoff emanating fromadjacent uplands in addition to direct precipitationinputs. Terraces containing palm swamps that retainsurface water all year are visible to the north of theMadre de Dios River just above its confluence withthe Inambari River (Figure 2).

Movements of water and deposition and erosion ofsediments sculpt the surface geomorphology of floodplains, particularly during larger flood events. Muchattention has been paid to the study of fluvial geo-morphology, and the ways in which different hydro-geomorphological regimes produce a myriad offloodplain landforms are well documented. Fluvialdeposits can be highly heterogeneous and subject toconstant change in active flood plains with high ratesof sediment deposition, as for example, in the floodplains close to theMadre de Dios River and on islandsin the Inambari River in Figure 2. Fluvial landformstend to become smoothed out over time, as may bereflected by increased distance from or, in the case ofterraces, elevation above the parent river. Erosionof elevated features and infilling of depressions con-tribute to the long-term homogenization of flood-plain surfaces. Yet even in humid tropical climates,traces of the geomorphic features produced by fluvialaction may remain visible for tens of thousands ofyears (e.g., Llanos de Moxos in Figure 4).

Elevated strips of land known as levees often bor-der the river channel and reflect the higher rates ofsediment deposition where the river water firstdecreases in velocity as it exits the channel.

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Figure 2 Madre de Dios River (upper) and its tributary the

Inambari River (lower) in the Peruvian Amazon basin. This imageland is about 29 km in width. The river channels contain high

inorganic turbidity while some oxbows contain clearer water and

hence appear darker. Lighter areas are vegetation in early

successional stages following fluvial disturbance. The flood plainalong the Madre de Dios contains oxbow lakes in various stages

of separation and infilling. Much of the Inambari flood plain

occurs as islands and bars along the highly braided channel.

Between the two rivers is an extensive interfluvial backswamp,and in the upper right the darker areas are palm swamps lying on

fluvial terraces. Image coordinates are 69.83 W, 12.70 S. Image

shows Landsat 7 ETMþ data (bands 7, 4, 2) from the Geocover2,000 data set, obtained from NASA World Wind version 1.3.

Figure 1 Amazon River (locally known as the Rio Solimoes in

this reach) and its fringing flood plain above the city of Manaus

and the confluence with the Negro River. This image land is about

240 km in width. The Negro River is visible in the upper right. Themain river channel contains high inorganic turbidity and clearer

waters on the flood plain as well as in the Negro River are dark.

The flood plain here shows diverse geomorphic features,including numerous lakes, most conspicuous of which are the

dendritic blocked-valley lakes. Image coordinates are 61.20 W,

3.61S. Image shows Landsat 7 ETMþ data (bands 7, 4, 2) from

the Geocover 2000 data set, obtained from NASA World Windversion 1.3.

Figure 3 Flood plains of the upper Paraguay River and

tributaries in the central Pantanal (Brazil and Bolivia). This image

land is about 200km in width. The flood plains here include thefringing floodplain of the sinuous Paraguay River (running from

north to south), large turbid lakes with permanent connectivity to

the river, and seasonally flooded savannas of the Taquari River

alluvial fan, which covers the right side of the image. Lower lyingareas of the fan are subject to backwater effects from the

Paraguay River. Image coordinates are 57.14 W, 18.51 S. Image

shows Landsat 7 ETMþ data (bands 7, 4, 2) from the Geocover

2000 data set, obtained from NASA World Wind version 1.3.

Figure 4 Savanna flood plains of the Llanos de Moxos (also

spelled Mojos) in the upper Amazon Basin in Bolivia. This image

land is about 108km in width. The meandering Beni River withnumerous oxbow lakes flows from south to north. The flood plain

here reveals topographic features created by past fluvial activity.

Superimposed on this landscape are strikingly regular

depressions filled with shallow turbid water; these lakes havebeen attributed to subsidence patterns reflecting lineaments in

the basement rock far below the alluvium. Image coordinates are

67.20 W, 13.99 S. Image shows Landsat 7 ETMþ data (bands

7, 4, 2) from the Geocover 2000 data set, obtained from NASAWorld Wind version 1.3.

Rivers and Streams _ Flood Plains 379

Encyclopedia of Inland Waters (2009), vol. 3, pp. 378-386


Figure 5 Flood plains and river channels of Cooper Creek near

Windorah (Queensland, Australia). This image land is about 136km

in width. The flood plains were relatively vegetated lands becausethey recently had been inundated, providing soil moisture for

growth of herbaceous plant cover. Surrounding uplands are

semiarid, sparsely vegetated shrublands andgrasslands. The flood

plain here receives river water through a complex system ofanastomosed channels that are mostly dry in between the

occasional flows. A few deeper channel reaches hold water

between flows and are known aswaterholes; these are too small to

resolve here. Image coordinates are 142.47 E, 25.56 S. Imageshows Landsat 7 ETMþ data (bands 7, 4, 2) from the Geocover

2000 data set, obtained from NASA World Wind version 1.3.

Figure 7 Hydrological alterations to the flood plain along the

Mississippi River (northeastern Mississippi east of Helena,

Arkansas, USA). This image land is about 9 km in width. Much ofthe flood plain here has been disconnected from the river by dikes

and is used for agriculture; other parts are deforested but not

diked. Oxbow lakes are visible in the forested parts; topographic

floodplain features can also be seen in cleared and diked areas.Image coordinates are 90.51 W, 34.59 N. Image source is U.S.

Geological Survey digital aerial photography (DOQQ) taken in

April 2001, obtained from NASA World Wind version 1.3.

Figure 6 Hydrological alterations to the flood plain along the

Kalamazoo River (Michigan USA). This image land is about 11 km

inwidth. The reservoir in the lower part of this viewwas created byimpoundment of the main channel for hydroelectric generation.

Downstream is a diked area where water levels are managed for

wildlife. The remaining flood plain is covered by deciduous forest

and retains an approximately natural flood regime because thisand upstream reservoirs do not change seasonally in volume.

Image coordinates are 85.94 W, 42.57 N. Image source: U.S.

Geological Survey digital aerial photography (DOQQ) taken

in March 1999, obtained from NASA World Wind version 1.3.

380 Rivers and Streams _ Flood Plains


Successively formed, concentric levees and swalesmay form meander scrolls. In some flood plains,these are created by the meandering of smaller chan-nels within the flood plain rather than by the migra-tion of the main channel.

Encyclopedia of Inland Water

Flood plains can be readily classified based on theirgeomorphology, and several schemes have been pro-posed. Fewer classification systems deal specificallywith thewater bodies on flood plains, although varioustypes of floodplain lakes are included in some geomor-phological classifications (see later text). Flood plainscan be broadly classified as fringing flood plains (i.e.,found along the banks of rivers), coastal deltaicflood plains (formed where rivers meet larger rivers,lakes or the ocean), and internal deltaic flood plains(formed internally where a tributary meets a largerriver and is subject to backwater effects). Some riverchannels, particularly those with braidedmorphologysuch as the Inambari River in Figure 2, have much oftheir flood plain on mid-channel bars and islands, andsmaller channels can exhibit lake-like conditions atlower discharge. Wetlands that resemble riverineflood plains but may not be subject to direct inunda-tion by river water include flood plains on terraces,and poorly drained plains adjacent to rivers that tendto become flooded with local runoff during the wetseason. Examples of the latter include much of theland in images of the Pantanal (Figure 3) and Llanosde Moxos (Figure 4).

Hydrology

By definition, flood plains share the characteristic ofbeing subject to inundation, but key hydrologicalfeatures of inundation are highly variable. Thetiming, frequency, and duration of inundation can

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Figure 8 Daily river level measurements over 1984 from the

Amazon River (Rio Solimoes at Manacapuru, Brazil), the OrinocoRiver (Ciudad Bolıvar, Venezuela), and the Paraguay River

(Corumba, Brazil). Dashed lines show approximate bankfull

stages, above which the adjacent flood plains become inundatedwith river water. These flood regimes are essentially natural with

no significant upstream regulation. Data collected by the

Brazilian Agencia Nacional de Energia Eletrica (Amazon River),

the Venezuelan Ministerio del Ambiente y de los RecursosNaturales Renovables (Orinoco River), and the Brazilian Navy

(Paraguay River).



be collectively considered as the hydroperiod. Inmany kinds of flood plains, there is a range of hydro-periods depending on the land surface elevation withrespect to the river. Subtle differences in topographycan result in considerable differences in hydroperiodwith corresponding variation in ecological character-istics, particularly when the range of inundationdepth is relatively small.River discharge regimes dictate timing and predict-

ability of inundation, while depth and routing of flowwithin the flood plain reflect its geomorphology andvegetation. Larger river systems tend to have broaderand more predictable flood peaks because their largedrainage basins integrate the smaller-scale variabilityof individual precipitation events. Inundation of theirflood plains has been termed the flood pulse (seeFloodplain Wetlands of Large River Systems). Thepresence of extensive flood plains or water bodiessuch as lakes and reservoirs along a large river systemalso attenuates the flood pulse downstream.Particularly attenuated and prolonged flood pulses

are observed in large, mostly unregulated river sys-tems of South America, where the flood plain typi-cally is inundated once per year, lasting for monthsand covering much of the flood plain with water todepths up to several meters. Figure 8 shows examplesof daily river stage measurements from three largerivers with extensive flood plains: the Orinoco,Amazon, and Paraguay rivers. In the vicinity ofthese stage measurement sites, the Orinoco has theleast flood plain relative to its discharge, while theParaguay River has the most extensive flood plain(these data come from the southern Pantanal inBrazil: Figure 3). In each case, the inundation lastsfor months, with the most protracted inundation inthe savanna flood plains of the Pantanal where stand-ing water often persists for more than 6months of theyear. These river systems and their flood plains are solarge that the passage of the flood wave through thesystem takes months, resulting in a time lag betweenthe wet season runoff and the inundation of the floodplain. This time lag reaches 4–6months in the case ofthe southern Pantanal.At the other extreme are flood plains that typically

are inundated for only a few days or weeks per year.Examples shown here include the Madre de DiosRiver (Figures 2 and 9), which drains mountain andlowland landscapes in Peru, and the Kalamazoo River(Figures 6 and 10), which drains a glacial landscape insouthern Michigan, USA. In these river systems, thefloodplain inundation is seasonal but its timing is notregular, nor is its duration. In a particular year, inun-dation can occur several times or not at all. Thetropical Madre de Dios responds to rain events inthe Andes as well as the lowland plains. The stage

Encyclopedia of Inland Wat

record for the Madre de Dios River spans only a fewyears, which is insufficient to characterize its averagebehavior. The temperate Kalamazoo River respondsto rain events but also to snowmelt and rain on snow,which produce a higher proportion of overland run-off in a landscape where liquid precipitation infil-trates the soils during most of the year. Most floodsin the Kalamazoo River occur during cooler months(especially March and April) when biological activityis relatively low, and the mean number of daysflooded per year is only eight, with substantial inter-annual variability.

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Figure 9 Daily river level measurements over two annual cycles from the Madre de Dios River above the confluence with the Los

Amigos River in Peru. This river drains Andean mountain and lowland environments, and has a natural flow regime. Dashed line showsapproximate bankfull stage, above which most areas on the adjacent flood plains become inundated with river water. Data collected by

the Centro de Investigaci�on y Capacitacion Rıo de los Amigos (CICRA), operated by the Amazon Conservation Association.

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Figure 10 Daily river level measurements over six annual cycles from the Kalamazoo River (Battle Creek, Michigan, USA). Reservoirs

and urbanization may have a small effect on the flow regime of this river. Dashed line shows approximate bankfull stage, abovewhich most areas on the adjacent flood plains become inundated with river water. Data collected by the U.S. Geological Survey.



Even brief flooding can be important as a geomor-phological force, but from an ecological perspective,brief and unpredictable floods may be viewed as adisturbance that limits the plant and animal life


rather than benefits it. In cooler climates, the timingof the inundation is critical as well. Some largeboreal/arctic rivers that flow in a northerly direction,such as the Mackenzie River in northwestern Canada

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and the Ob and Yenisei rivers in Siberia, exhibit spec-tacular flood pulses as the river breaks through ice inthe spring.The most variable river discharge regimes, and

hence the most unpredictable flood pulses, occur incertain dryland rivers such as Cooper Creek and theDiamantina River in the endorheic Lake Eyre basin ofinterior Australia (Figure 5), which tend to flow onlywhen monsoons bring heavy rainfall far into the con-tinent. In these semiarid environments water limitsbiological activity much of the time, and hence inun-dation of extensive flood plains and anastomosedchannel systems can be important as an ecologicalprocess despite its erratic and ephemeral nature. Per-manent waters in these systems are known as ‘water-holes’, which are particularly deep channel reaches ofrestricted length that hold water long after the riverhas ceased to flow and most of the channel has dried.The routing of flood waters across flood plains is

complex and often changes over the course of theflood event. Commonly, water first enters the floodplain through low breaks in the levees known ascrevasse splays, and this water may follow floodplainchannels for some distance before spreading out intobackswamps or lake basins. Water may also back upthrough downstream openings, for example, throughthe lower end of tributaries or oxbow lakes that areconnected with the channel. Thus floodplain inunda-tion commences before the levees become submerged.At the highest river stages, most or all levees may beunderwater, and sheet flow proceeds generally in thedownriver direction.The water that inundates flood plains does not

necessarily originate from overbank flow of the par-ent river, even though the parent river may controlwater levels by backwater effects. Flood plains typi-cally show both spatial and temporal variation inwater sources. Locally derived water can enter fromlateral tributaries, perhaps becoming impoundedtemporarily by river flood waters, or traveling downthe flood plain as deferred flow before mixing with themainstem (parent) river. In expansive flood plains,the flood waters can be derived from delayed drainageof precipitation falling directly on the flood plain.Groundwater inputs from adjacent uplands also canbe important, especially in smaller flood plains or infloodplain wetlands lying close to the upland bound-ary. Such inputs may maintain a high water table, andthus create wetland conditions for most or all of theyear. These distinct sources of flood waters often differin chemistry and suspended matter, enhancing the bio-geochemical and ecological heterogeneity across floodplains.Freshwater rivers near their confluences with the

sea can have flood plains subject to tidal control of


water levels, superimposing a short-term cycle ofvariability on longer-term, discharge-driven floodpulses. Examples include the Amazon and Orinocodeltas, where freshwater discharge is high enough toprevent seawater intrusion yet tidal cycles can beobserved for considerable distances upriver. In con-trast, rivers with little or no dry-season flow canexperience substantial intrusion of seawater in theirlower reaches. In such river systems, seemingly mod-est changes in relative sea level can alter the zone ofseawater influence, producing dramatic implicationsfor floodplain ecosystems. Seawater intrusion as aresult of erosion of low ridges, possibly instigated byintroduced water buffalo, has been documented innorthern Australia east of Darwin, where the salinitycaused massive changes in vegetation. Sea level riseassociated with climate change increasingly will posea threat to many low-lying floodplain ecosystemsthat contain freshwater in close proximity to thecoastal zone.

Floodplain Lakes

More or less permanently flooded depressions on theflood plain include the backswamps behind the leveesand can also include water bodies that are deep andpermanent enough to be called floodplain lakes.There is no universal definition that distinguishes alake from a wetland and usage of ‘lake’ or compa-rable terms varies regionally. Commonly water bod-ies that are called lakes have an open-water area formost or all of the year, which distinguishes them fromvegetated wetlands. Their open-water areas may notfill with aquatic vegetation even though they can bequite shallow at low water; this may be a result of thechanging water levels and low light penetration.Interannual variation in the flood regime can producestriking changes in the proportions of open water andemergent vegetation. Although they are often small inarea, floodplain lakes are one of the most abundanttypes of lakes, particularly in the tropics and arcticwhere large, unregulated floodplain rivers remain.Very large lakes are associated with some floodplains, such as the Grand Lac on the Mekong River,though these may have a distinct geomorphologicalorigin.

Floodplain lakes are formed by a variety of pro-cesses, including the isolation of main channel mean-ders (oxbows), formation of swales between successivelevees, subsidence of alluvial fill, and permanent flood-ing of incised tributary valleys. Examples of each ofthese are visible in Figures 1–7. Floodplain lakes maybe permanently or seasonally connected to the parentriver, and the connections can be broad or quite

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restricted. Lakes that become seasonally isolated fromthe parent river may become perched at higher eleva-tions than the river level as the river falls, and theymayaccumulate water of local origin during the phase ofisolation from the river, often maintaining a streamthat drains to the river.Lakes typically occupy a minority of flood-

plain area but this depends on the geological historyof the flood plain. Sometimes subsidence or back-flooding exceed rates of new alluvial deposition(accretion), producing lakes that cover much of thebackswamp areas. Neotectonic processes that causesubsidence, tilting, or uplift of basement rock cancreate areas of permanent flooding on flood plains.Most kinds of floodplain lakes are no deeper than

the parent river, and they can become very shallowduring low water. During periods of isolation or atleast minimal through-flow of river water, they can berich in phytoplankton, although in many cases theybecome shallow enough that sediments are resus-pended by wind-induced turbulence and inorganicturbidity greatly restricts underwater light availabil-ity (e.g., the large oval lakes in the Llanos de Moxos:Figure 4). During inundation they receive through-flowing river water and this may drastically reducethe water residence time to the point where planktongrowth is suppressed by flushing.

Remote Sensing of Flood Plains

Delineation of floodplain boundaries by remote sens-ing can be challenging due to temporal variability inthe extent of inundation and the difficulty of detect-ing standing water beneath vegetation canopies. Insome humid tropical flood plains, the nearly perpet-ual cloud cover can also impede optical remote sens-ing systems. The difficulty of observing inundationdynamics by remote sensing has led investigators torely more on vegetation and geomorphological fea-tures to delineate flood plains, and these featuresusually provide a reasonable indication of the overallfloodplain extent and the boundaries between floodplains and upland ecosystems. At coarse spatial scalesand in remote regions of the world, new remotelysensed, elevation data (e.g., the Shuttle Radar Topog-raphy Mission) are proving useful as well becausethey reveal the extent of relatively level terrain alongmajor rivers.Remote sensing of flood regimes can provide infor-

mation for hydrological modeling and ecological andbiogeochemical investigations in flood plains. Tradi-tional optical remote sensing, including aerial pho-tography for limited areas and Landsat satelliteimagery for extensive areas, can provide snapshots


of flood extent, but often lacks temporal resolutionand may be inadequate for the reasons mentionedabove. Relatively new microwave technologies suchas radar can be better for hydrological dynamicsbecause of their all-weather capability (i.e., they areless impeded by cloud cover) and their ability topenetrate vegetation to at least some degree. Severalmicrowave systems are currently deployed on orbit-ing satellites and data from these sensors haveprovided new insights into floodplain hydrology.New image analysis approaches that combine multi-ple types of imagery are especially promising forremote sensing of flood plains.

Functions of Flood plains

The permanent and temporary aquatic environmentsof flood plains are locations for a number of impor-tant ecosystem processes, or functions, which providevalues and services to people.

Flood plains tend to be highly productive eco-systems and have long been utilized for productionof food and fiber and harvest of wild plants andanimals. Perhaps the greatest contrast in productiv-ity between uplands and flood plains occurs in dry-land regions, but even in humid climates the floodplain is often desirable for farming and livestockproduction.

The temporary residence of water on flood plains isan important hydrological function because it delaysthe passage of flood waters through the fluvial system.This delay tends to attenuate the flood peak downriver,reducing peak water levels, which often is advanta-geous to riverside communities and agricultural activ-ities. Passage of river water through flood plains cansignificantly enhance evapotranspirative losses, whichin dry regions may be viewed as negative, but also mayincrease groundwater recharge. Some flood plainsoverlie extensive alluvial aquifers and these can pro-vide a readily accessible water supply that is rechargedby seasonal flood pulses.

Passage of river water through floodplain environ-ments changes its content of dissolved and suspendedmatter, which can affect the composition of riverineexports. Suspended sediments tend to show net lossby deposition, whereas nutrients may show net reten-tion or transformations. Concentrations of certainpollutants, particularly those associated with particu-late material (e.g., trace metals) as well as labilenutrients (e.g., nitrate), are often greatly reduced inwater passing through flood plains. High rates ofprimary production can result in net export oforganic matter back to the river, although it is unclear

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whether flood plains are a net source or sink forriverine organic carbon.Under certain circumstances, water quality can be

diminished by passage through flood plains. Strongoxygen depletion upon initial inundation of sometropical flood plains is known to result in fish kills(e.g., the Pantanal in Brazil). In coastal plains oftropical Australia, initial contact of flood waterswith acid sulfate soils can cause marked decreases inpH and result in metal toxicity for fishes. Such leach-ing effects usually diminish as the flood plain isincreasingly flushed by flood waters.Extensive flood plains can be important as sources

of greenhouse gases to the atmosphere. It remainsuncertain whether flood plains tend to be net sourcesof sinks of carbon dioxide, but like other wetlandsthey certainly tend to be net sources of methane whilethey are wet, and they may be important sources ofnitrous oxide as well, particularly in regions withnitrogen pollution. Tropical flood plains are a glob-ally significant source of methane emission to theatmosphere because of their extensive area and highbiological activity. In general methane emission ratesare proportional to primary productivity in wetlands,and this presumably extends to flood plains as well.In rivers with extensive flood plains that are inun-

dated for relatively long periods, much of the primaryand secondary production in the overall river–floodplain system can occur in the flood plains. TheFlood Pulse Concept articulates how such floodplains serve as the locus of biological production,supporting rich aquatic and terrestrial biodiversityincluding economically and culturally valuable fish-eries (refer to ‘see also’ section).

Human Modification of FloodplainHydrology

The age-old proclivity of humans to control riversystems, combined with our ever-increasing techno-logical ability to do so, has made flood plains one ofthe most altered aquatic ecosystems. River regula-tion, mainly through construction of dams, hasstrongly impacted flood regimes of rivers across allspatial scales, and even many of the largest rivers ofthe world have been regulated to some degree.Impoundments tend to create permanently flooded

reservoirs in place of seasonally flooded lands(Figure 6), and in many cases, they alter the dischargeregime and often the water quality and temperaturewell downstream. They can trap a large fractionof the suspended sediment load, leading to geomor-phological destabilization of the river–flood plainsystem downriver of the dam. Dams operated for


hydroelectric generationmay impose highly unnatural,short-term fluctuations in water levels, while thoseoperated primarily for agricultural irrigation tendto change the seasonality of river flow in addition toremoving water from the system.

Modification of river channels to facilitate naviga-tion usually impacts flood plains by altering the rela-tion between water levels and discharge. Removal ofnatural barriers to navigation can entail dredging,channel straightening, and excavation of rock out-crops. All of these measures tend to enhance flowconveyance and diminish the backwater effect thatproduces overbank flooding. Construction of naviga-tion locks is akin to damming rivers. Low-head navi-gation dams that allow passage of flood waters, suchas those on the upperMississippi River (USA), are lessdamaging but still create extensive permanentlyimpounded areas at low water levels.

Flood plains have often been isolated from theirparent rivers by construction of dikes, commonlywith the goal of farming the land (see MississippiRiver example in Figure 7). Such land can be highlyproductive but may require costly measures toremove or control water, and over time land subsi-dence, loss of fertility, and occasional incursion offlood waters can detract from its sustainability. None-theless, agriculture on converted flood plains hasplayed an important role in many societies and con-tinues to be significant throughout the world. In someregions flood plains have been extensively mined forclay, sand or gravel, gold, or diamonds, a provision-ing service that is sustainable only if subsequentfloods replenish the material that is removed.

Aquatic ecosystems on flood plains are alsoimpacted by urban and agricultural developmentin the upland watershed, which results in alterationsto the flow regime and water quality of the parentrivers. Runoff is intensified by impervious surfacesand stormwater runoff drainage in built areas, and byland clearing and wetland drainage for agriculture.Nutrient loading to rivers and their flood plainsincreases with development, although the floodplainbiota can have a large capacity for nutrient uptake andretention, and the effects of intact flood plains onwater quality can be viewed as a valuable ecosystemservice.

See also: Ecology of Wetlands; Floods; Flow in Wetlandsand Macrophyte Beds; Fluvial Export; Fluvial Transportof Suspended Solids; Geomorphology of Streams andRivers; Global Distribution of Wetlands; Riparian Zones;South America; Streams and Rivers as Ecosystems;Streams; The Surface Mixed Layer in Lakes andReservoirs; Tidal Freshwater Wetlands; Wetlands ofLarge Rivers: Floodplains.

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Further Reading

Alsdorf DE, Rodrıguez E, and Lettenmaier DP (2007) Measuringsurface water from space. Reviews of Geophysics 45, RG2002,

doi:10.1029/2006RG000197.

Brinson MM (1993) A Hydrogeomorphic Classification for Wet-lands. Technical report WRP-DE-4. Vicksburg, MS: U.S. ArmyEngineer Waterways Experiment Station.

Church M (2002) Geomorphic thresholds in riverine landscapes.

Freshwater Biology 47: 541–557.

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