Why Floodplains Need to Flood
Periodic high water is not a failure of a floodplain. It is one of the processes that connects rivers to forests, wetlands, fish habitat and the soils beside them.
Photo: U.S. Fish & Wildlife Service · Public Domain
Walk into a bottomland forest after the river rises and the place can look wrong.
Water covers the ground. Tree trunks disappear into brown water. Trails vanish. Low roads may be closed. Places that were dry enough to walk across a few weeks earlier suddenly look more like shallow lakes.
For the trees, fish, soils, wetlands and other organisms that make up a functioning floodplain, though, that water is not necessarily a problem.
It is part of the system.
Rivers do more than move water downstream between two banks. During high flows, they can also move sideways. Water spreads across low ground, fills old channels and depressions, enters bottomland forests, reconnects backwaters, deposits sediment, moves nutrients and gives aquatic animals temporary access to habitat that does not exist at ordinary river stage.
Ecologists have described this relationship for decades as the flood pulse: the recurring rise and fall of water that connects a river to its floodplain. More recent research has placed even greater emphasis on hydrologic connectivity — when, where, how often and for how long water can move between the river channel and the surrounding floodplain. [1] [2]
That does not mean every flood is beneficial. A flood that kills crops, damages a home, closes a highway or threatens a community is a very real human problem. Extreme or poorly timed flooding can also damage floodplain forests and wildlife habitat.
The distinction is simpler than it sometimes sounds: flood damage is a problem. Flooding itself is a natural river process.
And on land that still functions as floodplain habitat, taking the flooding away can change the ecosystem almost as much as clearing the forest.
Floodplains are low areas beside rivers that are periodically reached by high water. That inundation is part of what makes them floodplains in the ecological sense, not just in the mapping sense.
When a river spreads onto its floodplain, water slows. Sediment can settle. Nutrients and organic material move between river and land. Wetlands refill. Fish may gain access to shallow, low-velocity habitat. Floodplain soils become saturated, and the timing and duration of that saturation help determine which plants and trees can survive there.
The process matters across the Lower Ohio region. Hovey Lake Fish & Wildlife Area sits within the Ohio and Wabash River floodplains in extreme southwestern Indiana. Sloughs Wildlife Management Area across the river in western Kentucky includes seasonally flooded bottomland forest and wetland habitat tied directly to Ohio River levels. Patoka River National Wildlife Refuge protects one of Indiana’s remaining intact floodplain forest systems. [8] [9] [10]
But more water is not automatically better. Depth, duration, season, frequency, temperature, water quality and the species present all matter. A functioning floodplain needs flooding. It does not need every possible flood.
In This Report
- A floodplain is part of the river
- The flood pulse
- What happens when water leaves the channel
- Why fish use flooded land
- Flooding helps write the forest
- Floodplains and water quality
- When rivers lose their floodplains
- Not every flood is a good flood
- What this looks like in the Tri-State
- What restoration actually means
- Common Questions
A floodplain is part of the river
At normal water level, a river looks neatly contained.
There is the channel. Then there is the bank. Beyond that is land.
That boundary works pretty well until the river rises.
Floodplains are the low areas beside rivers and streams that can become inundated during high flows. Some flood frequently. Others are reached only during larger events. Within one floodplain, a few inches of elevation can determine whether a location floods repeatedly or only occasionally.
That variation creates oxbows, sloughs, marshes, wet forests, temporary pools, backwaters, natural levees, low terraces and other habitats that may sit side by side while experiencing very different water conditions.
Floodplain ecology begins with that changing relationship between elevation and water.
A 2025 review by U.S. Geological Survey researchers examined 1,920 studies of hydrologic connectivity and found that river-floodplain connections regulate the movement of water, sediment, nutrients, chemicals and organisms. Dams, levees, water-management practices and other human changes can disrupt those connections. [1]
So simply knowing that land lies inside a mapped floodplain does not tell us how well it functions ecologically.
The better question is: Does river water still reach it, and what happens when it does?
The flood pulse: when the river moves sideways
Water usually moves downstream.
During a flood, some of it moves sideways too.
That lateral movement is central to the flood-pulse concept developed by Wolfgang Junk, Peter Bayley and Richard Sparks. Their work described periodic expansion and contraction of water across the river-floodplain boundary as a major ecological force in large river systems. [2]
Imagine the river as something that expands and contracts rather than as a fixed blue line.
At low water, fish and other aquatic organisms may be concentrated in the main channel, oxbows, sloughs, permanent backwaters and deeper wetlands.
As the river rises, those pieces can begin connecting. Water enters side channels and depressions. Then it reaches low forest. With additional rise, previously dry ground becomes shallow aquatic habitat.
When the river falls, the process reverses. The floodplain drains. Aquatic habitat contracts. Sediment and organic material remain behind. Some fish return to permanent water. Plants begin growing on exposed mud and saturated soils.
That cycle is why a floodplain can be biologically productive without being permanently wet.
Higher flows push water toward side channels, sloughs, wetlands and the lowest parts of the floodplain.
Areas separated at normal river stage can temporarily become one connected aquatic system.
Sediment, nutrients, organic material and aquatic organisms move between channel and floodplain.
Habitat contracts again while deposited sediment, moisture and organic material remain on the floodplain.
What actually happens when floodwater leaves the channel
A flood is easy to describe as simply “more water.”
Ecologically, several processes are happening at the same time.
Water spreads out and slows down
Once water leaves the main channel, it often moves across a much wider area and encounters trees, shrubs, grasses, fallen wood, uneven ground and other resistance.
Velocity drops.
Floodplains can therefore provide floodwater storage and conveyance, reduce velocities and help moderate flood peaks. That does not mean an undeveloped floodplain can make a major flood disappear. It means the floodplain provides somewhere for part of that water to go. [3]
Sediment has a chance to settle
Fast-moving water can keep fine particles suspended. Slow it down and some of those particles fall out.
Over repeated floods, deposition helps build and reshape floodplain soils. It can also move nutrients and contaminants attached to sediment out of the main channel.
Water reaches soil that is normally dry
Saturation changes oxygen conditions, soil chemistry, microbial activity and nutrient cycling. Processes that occur slowly in dry soil can behave very differently after inundation.
Separate habitats become connected
A backwater that appears isolated in August may connect to the river during high spring water. A wooded depression may temporarily become fish habitat. An oxbow can exchange water and organisms with the main channel.
The floodplain gets larger without gaining an acre of land.
It simply becomes accessible.
Why fish use flooded land
A fish swimming through a forest sounds wrong until you remember that the forest is in a floodplain.
When rising water reaches low ground, terrestrial habitat can temporarily become aquatic habitat.
Flooded areas may provide shallow water, slower current, submerged vegetation, insects, worms, seeds, organic material and other food resources that differ from conditions in the main channel. Young fish may also find refuge from stronger currents.
Research following the 1993 Upper Mississippi River flood documented extensive fish use of connected floodplain habitat. Researchers captured 42 species of adult and young-of-the-year fish across the study’s floodplain sites and concluded that some species appeared to use flooded terrestrial habitat for feeding, spawning or both. [4]
Access is the key.
Excellent habitat sitting behind a barrier may still support the organisms already living there, but it cannot function as temporary river habitat if fish cannot reach it.
That is why floodplain management increasingly focuses on connectivity, not simply acreage.
Flooding helps write the forest
Bottomland hardwood forest is not just ordinary forest growing close to a river.
Water helps decide which trees live there.
Flood frequency, depth, timing, duration, groundwater levels, soil drainage and small differences in elevation can all influence forest composition.
U.S. Geological Survey research on bottomland hardwood management describes hydrology, soils, landforms and tree species as tightly connected. Reduced flooding, altered timing and groundwater declines can favor species that are less tolerant of inundation and change the structure of the forest. [5]
That gives us an important distinction.
A floodplain forest can still have trees and still be ecologically altered.
If a levee, channel modification, drainage system or other change keeps water out, the site may gradually stop functioning like the forest that developed under the older flood regime.
The reverse can happen too. Too much water, especially prolonged growing-season inundation, can stress or kill trees.
Different trees tolerate different floods. There is no single water depth or flooding schedule that works for every bottomland forest.
Floodplains can also change water quality
This part of the story becomes especially relevant in the Wabash Basin.
When floodwater spreads across low ground, reduced velocity can allow sediment to settle instead of continuing downstream. Wet soils and organic matter also support microbes capable of transforming nutrients.
A 2026 study at the confluence of the Wabash and Tippecanoe rivers examined denitrification, a microbial process that converts biologically available nitrogen into gaseous forms that can leave the aquatic system.
Researchers measured four floodplains with different restoration and management approaches. Denitrification rates were significantly lower in an agricultural floodplain than in restored floodplains with native vegetation. Higher soil moisture, silt, organic matter and other wetland-like conditions were associated with greater denitrification potential. [6]
The 2026 Indiana denitrification study compared four floodplain sites near the Wabash-Tippecanoe confluence.
The property sits within the Ohio and Wabash River floodplains in extreme southwestern Indiana.
The refuge boundary follows roughly 30 miles of the Patoka River corridor and includes wetlands, floodplain forest and upland buffers.
That is useful regional evidence because it moves the discussion beyond theory. Floodplain restoration can influence what happens to nutrients moving through an Indiana river system.
There is a catch, because ecology rarely cooperates with a clean slogan.
Floodplains do not permanently trap every nutrient that reaches them. Soil chemistry matters. Phosphorus can behave differently from nitrogen. A site may retain material under one set of conditions and release some of it under another.
So “floodplains clean water” is broadly useful shorthand.
It is not a magic trick.
What happens when rivers lose access to their floodplains
Levees are the most obvious example, although roads, berms, drainage ditches, culverts, channel incision and water-control structures can interrupt floodplain connections at smaller scales.
A levee separates high river water from land behind it.
When that land contains houses, towns, roads, utilities, businesses or productive farmland, the purpose is obvious. Flood protection has prevented enormous human and economic losses.
Ecologically, though, the tradeoff is also real.
Research examining levees and wetlands across the Wabash Basin found that levee placement affects potential river-floodplain connectivity and the relationship between wetlands and the river network. The authors also cautioned that wetland distributions around levees can be complex and may require finer-scale study. [7]
Once regular overbank flow disappears, fish may lose access to off-channel habitat. Sediment no longer reaches some low areas. Wetland water levels may depend more heavily on rainfall or artificial water control. Floodplain soils experience different saturation patterns. Forest composition can begin changing.
None of this means “remove the levees.”
That would be a wonderfully simple answer to a question that is not remotely simple.
Some levees protect people and infrastructure that cannot reasonably be exposed to recurring floods.
The more useful conservation question is where room for flooding still exists — on public conservation land, restored wetlands, frequently flooded agricultural ground, voluntary easements and other places where periodic inundation does not create unacceptable human risk.
Not every flood is a good flood
There is a temptation in environmental writing to flip an old assumption completely around.
Floods used to be treated only as disasters, so the correction becomes: floods are good.
That is not accurate either.
Floodplain organisms respond to a flow regime, not simply to the presence or absence of water.
When the water arrives matters. How deep it becomes matters. How long it remains matters.
So do temperature, velocity, dissolved oxygen, sediment load, contamination and whether native or invasive organisms gain access.
A short spring flood may create useful feeding habitat. Weeks of deep growing-season flooding may damage tree seedlings or mature trees. High water can connect native fish to spawning or nursery areas. The same connection can move invasive fish.
The natural-flow-regime concept describes river ecology through characteristics including the magnitude, frequency, duration, timing and rate of change of flows. Those dimensions help explain why two floods reaching similar heights can produce different ecological results. [12]
The management goal is not maximum flooding.
It is functional hydrology.
What this looks like in the Tri-State
The Lower Ohio region still contains places where the relationship between river and floodplain is easy to see.
Hovey Lake Fish & Wildlife Area
Hovey Lake sits within the Ohio and Wabash River floodplains in Indiana’s southwestern corner.
Indiana DNR describes approximately 7,404 acres containing a 1,400-acre oxbow lake, smaller sloughs and marshes, and extensive bottomland hardwood forest. [8]
The property is useful because it shows that a floodplain is not one habitat. It is a collection of elevations, water depths, old channels, forest, marsh, open water and transition zones responding differently as river levels change.
Patoka River National Wildlife Refuge
Farther north, Patoka River National Wildlife Refuge protects one of the few remaining large expanses of bottomland forested wetland in the Midwest and one of Indiana’s two intact floodplain forest systems.
The refuge’s authorized boundary encompasses 23,743 acres of wetlands, floodplain forest and upland buffer along approximately 30 miles of the Patoka River corridor. [9]
The refuge has also restored frequently flooded agricultural fields in the river bottoms to bottomland hardwood forest.
That phrase matters: frequently flooded agricultural fields.
Land that repeatedly creates problems for one use can be well suited to another.
Sloughs Wildlife Management Area
Across the Ohio River in Henderson and Union counties, Kentucky, Sloughs Wildlife Management Area makes the connection between river level and floodplain habitat unusually visible.
Kentucky Fish & Wildlife describes seasonally flooded bottomland forest, natural marsh, moist-soil vegetation, flooded agricultural habitat and sloughs containing flooded stands of bald cypress and buttonbush. [10]
The agency even tells hunters to monitor Ohio River levels because high water can determine whether roads remain accessible or a boat is needed to reach parts of the property.
There is the floodplain-management problem in miniature.
The same water that creates habitat can make human access difficult.
Both statements are true.
What floodplain restoration actually means
Restoring a floodplain does not necessarily mean opening every barrier and waiting for the river to take over.
Sometimes the first job is simply protecting remaining floodplain from further development.
Elsewhere it may mean reconnecting a backwater, modifying a drainage structure, restoring a wetland depression, reforesting frequently flooded cropland or allowing water onto conservation land at river stages that once produced seasonal inundation.
The hydrology has to match the objective.
The U.S. Forest Service’s guide to bottomland hardwood restoration makes that point directly: hydrology is a driving force of wetland ecosystems, and restoration is unlikely to succeed when the site’s hydrologic regime does not fit the tree species being planted. [11]
That is why planting trees alone does not restore a floodplain forest.
And digging a pond does not automatically restore a river wetland.
The water source, timing, depth, duration, soils, vegetation, connection to the river and surrounding land use all matter.
In some places, active reconnection may make sense. In others, controlled water levels may be more realistic. Beside a town or critical road, flood protection may remain the priority. On conservation land or repeatedly flooded property, giving the river more room may make considerably more sense.
There is no single floodplain prescription because there is no single floodplain.
What to remember
- Periodic flooding is part of normal floodplain ecology. It reconnects rivers with wetlands, forests, backwaters and temporary aquatic habitat.
- The flood pulse moves more than water. Sediment, nutrients, organic material and organisms can all move between the river channel and floodplain.
- Hydrology helps determine what grows in a bottomland forest. Change the timing, frequency or duration of flooding and forest composition can change too.
- Indiana research shows restored floodplains can affect nutrient processing. A 2026 Wabash River study found greater denitrification rates in restored floodplains with native vegetation than in the agricultural floodplain included in the study. [6]
- More flooding is not automatically better. Ecological effects depend on timing, depth, duration, water quality and the organisms present.
- Flooding and flood damage are different problems. Protecting people and infrastructure can coexist with allowing periodic inundation where land can safely accommodate it.
Common Questions
Do floodplains have to flood every year?
No. Flood frequency varies enormously within and among floodplains. Some low areas may inundate frequently while slightly higher terraces flood only during larger events. Ecological function depends on the flood regime appropriate to that particular site.
Does flooding a floodplain help reduce flooding downstream?
Floodplains can temporarily store and convey water, slow velocities and reduce flood peaks under some conditions. The effect depends on floodplain size, topography, vegetation, connectivity and the scale of the flood. Floodplains can reduce risk; they do not eliminate it. [3]
Why not just keep rivers inside levees?
In developed areas, levees may be essential for protecting people, property, infrastructure and agriculture. The ecological tradeoff is that they can also separate rivers from wetlands and other floodplain habitat. The management question is therefore where flood protection is necessary and where safe reconnection remains possible.
Can trees really survive underwater?
Some floodplain tree species tolerate inundation much better than others. Species composition often reflects how frequently, how deeply and how long a site floods. Even flood-tolerant species can be damaged when inundation occurs for too long or at the wrong time of year. [5]
Is a floodplain the same thing as a wetland?
No. A floodplain is land beside a river or stream that can be inundated during high water. Wetlands are areas where recurring or persistent water conditions influence soils, vegetation and other ecological characteristics. Many floodplains contain wetlands, but portions of a floodplain may be dry for most of the year.
Where can I see floodplain habitat in the LWR region?
Hovey Lake Fish & Wildlife Area in southwestern Indiana and Sloughs Wildlife Management Area in western Kentucky are strong examples along the Lower Ohio system. Patoka River National Wildlife Refuge protects extensive floodplain forested wetland farther north in southwestern Indiana. Water levels can change access quickly, so check current agency conditions before visiting.
The river is bigger than its channel
We have spent generations making rivers more predictable.
That was not irrational. Towns, farms, roads, industry, utilities and navigation all work better when water stays where we expect it to stay.
But ecological systems do not care much about our preference for tidy boundaries.
The forests, sloughs, oxbows, wetlands and low ground beside the Ohio, Wabash, Patoka, Green and other rivers developed under conditions in which water periodically crossed the bank.
Remove that process everywhere and the floodplain may remain on the map.
It just stops behaving like one.
Where people and infrastructure must be protected, flood control has an obvious job. Where low ground can still safely accept water, allowing a river to occupy part of its floodplain is not a failure to control nature.
Sometimes it is simply giving the river enough room to remain a river.
References
- Ahmad, H., Miranda, L. E., Dunn, C. G., Boudreau, M. R., & Colvin, M. E. (2025). Hydrologic connectivity in floodplain systems: A multiscale review of concepts, metrics and management. Hydrological Processes, 39(9), e70260. U.S. Geological Survey.
- Junk, W. J., Bayley, P. B., & Sparks, R. E. (1989). The flood pulse concept in river-floodplain systems. In D. P. Dodge (Ed.), Proceedings of the International Large River Symposium, Canadian Special Publication of Fisheries and Aquatic Sciences 106, 110–127. USGS Upper Midwest Environmental Sciences Center.
- Federal Emergency Management Agency. Floods and Floodplain Management. FEMA floodplain-management guidance describing the natural and beneficial functions of floodplains, including floodwater storage, reduced velocities and flood peaks, water-quality functions and fish and wildlife habitat. FEMA.
- Barko, V. A., Herzog, D. P., & O’Connell, M. T. (2006). Response of fishes to floodplain connectivity during and following a 500-year flood event in the unimpounded Upper Mississippi River. Wetlands, 26(1), 244–257. U.S. Geological Survey.
- King, S. L., & Keim, R. F. (2019). Hydrologic modifications challenge bottomland hardwood forest management. Journal of Forestry, 117(5), 504–514. U.S. Geological Survey.
- Lay, D. W., McMillan, S. W., Hosen, J. D., Dey, S., & Noe, G. E. (2026). Assessing environmental drivers of denitrification in restored riverine floodplains. Journal of Ecological Engineering Design, 4(1). U.S. Geological Survey.
- Morrison, R. R., Bray, E. N., Nardi, F., Annis, A., & Dong, Q. (2018). Spatial relationships of levees and wetland systems within floodplains of the Wabash Basin, USA. Journal of the American Water Resources Association, 54(4), 934–948. U.S. Geological Survey.
- Indiana Department of Natural Resources. Hovey Lake Fish & Wildlife Area. Indiana DNR.
- U.S. Fish & Wildlife Service. Patoka River National Wildlife Refuge and Management Area: About Us. U.S. Fish & Wildlife Service.
- Kentucky Department of Fish & Wildlife Resources. Sloughs Wildlife Management Area waterfowl habitat and access information. Kentucky Fish & Wildlife.
- Allen, J. A., Keeland, B. D., Stanturf, J. A., Clewell, A. F., & Kennedy, H. E. Jr. (2001). A Guide to Bottomland Hardwood Restoration. USDA Forest Service Southern Research Station. U.S. Forest Service.
- Poff, N. L., Allan, J. D., Bain, M. B., Karr, J. R., Prestegaard, K. L., Richter, B. D., Sparks, R. E., & Stromberg, J. C. (1997). The natural flow regime. BioScience, 47(11), 769–784. BioScience.