How Do Wetlands Help Reduce

Flooding?

The effectiveness of wetlands for flood abatement

may vary, depending on the size of the area, type

and condition of vegetation, slope, location of the

wetland in the flood path and the saturation of wetland soils before flooding. A one-acre wetland can

typically store about three-acre feet of water, or one

million gallons. An acre-foot is one acre of land,

about three-quarters the size of a football field, covered one foot deep in water. Three acre-feet

describes the same area of land covered by three feet

of water. Trees and other wetland vegetation help

slow the speed of flood waters. This action, combined with water storage, can actually lower flood

heights and reduce the water’s destructive potential.

(Source: EPA) 

The Wetlands Initiative completed an 18-month

study, “Flood Damage Reduction in the Upper 

Mississippi River Basin: An Ecological Means.” The

study revealed that restoring the 100-year flood zone

of the Upper Mississippi five-state watershed could

store 39 million acre-feet of floodwater, the volume

that caused the Great Flood of 1993, and save over

$16 billion in projected flood damage costs.

In Minnesota, an additional study by The Wetlands

Initiative noted that flood peaks and damage costs

would be decreased by restoring the natural hydrology of the floodplain. The cost of replacing the

flood control function of the 5,000 acres of wetlands drained each year in Minnesota alone would

be $1.5 million, compared to the potentially millions of dollars lost to flooding. Preserving wetlands

in the first place and restoring some of those that

have been drained could help reduce future flood

losses. (Source: The Wetlands Initiative)

Wetlands: Protecting Life and

Property from Flooding

The Federal Emergency Management Agency (FEMA) states that

floods are the most common and widespread of all natural disasters—except fire. Most communities in the United States

have experienced some kind of flooding. FEMA encourages

the use of wetlands for stormwater detention in lieu of, or in

conjunction with, traditional structural flood control measures.

(Source: FEMA)

Wetland

Hydrology and

Flood Control

Wetlands are

transition zones

between uplands

and deeper

water, unique

ecosystems characterized by their

hydrology, soils

and vegetation.

They function like

natural tubs,

storing flood

waters that overflow riverbanks

and surface

water that collects in depressional areas. In

this way, wetlands can help

protect adjacent

and downstream

property from

flood damage. 

Preserving and protecting

coastal wetlands can help

reduce storm damage. 

St. Stanislaus was a boy’s

Catholic Boarding School

over a hundred years old in

Bay St. Louis, MS. Located

on the beach overlooking

the Gulf, it was destroyed

by Hurricane Katrina. This

picture was taken before

the building disintegrated.
Where Wetland

Charles River, Massachusetts

Along the Charles River in Massachusetts, the 

U.S. Army Corps of Engineers (the Corps) has 

acted to utilize wetlands in preventing flood 

damage. It was calculated that loss of all wetlands 

in the Charles River watershed would have caused 

an average annual flood damage cost of $17 

million. The Corps concluded that conserving 

wetlands was a natural, less expensive solution to 

controlling flooding than the construction of 

dikes and dams alone, and they proceeded to 

acquire 8,103 acres of wetlands in the Charles 

River basin for flood protection. (Source: U.S. 

Army Corps of Engineers – Charles River Natural 

Valley Storage Area)

Horseshoe Park, Colorado

In 1982, an earthen dam on Lawn Lake in Rocky 

Mountain National Park collapsed, suddenly 

releasing almost 700 acre-feet of water into the 

Roaring River. A wall of water 25 to 30 feet high 

moving at 9 miles per hour rushed downstream 

and entered Fall River at Horseshoe Park. The 

Park contained wetlands adjacent to the river, with 

meadow grasses, reed and dense willow stands. 

Here the flood wave spread across the broad, flat 

valley and was slowed by wetland vegetation. The 

height of the wall of water was reduced to about 

10 feet, and the water spread out over the meadow 

to a width of 1,300 feet. The flood was finally 

contained by Olympus Dam on Lake Estes, but it 

had claimed 4 lives and caused $31 million in 

damage. If not for the wetlands and meadows at 

Horseshoe Park, the damage would have been 

much worse. The height and speed of flood waters

were reduced by the wetland vegetation, and the 

damaging flood peak was greatly reduced. (Source: 

Jarrett and Costa 1984)

Grand Kankakee Marsh, Indiana

In 1900, the Kankakee Marsh was one of the 

largest, most ecologically diverse wetlands in the 

United States. During the 20th century much of 

the marsh was drained and converted to

agricultural use. Channelization of the Kankakee 

River, which fed the marsh, reduced its length 

from 250 to 90 miles. As a result, water quality 

was degraded and flooding increased. An 

ambitious project was undertaken to address these 

concerns. The project, featuring diverse partners 

from all levels of government, private conservation 

groups and business, was designed to restore over 

25,000 acres of wetlands. With a grant from the 

North America Wetlands Conservation Act and 

donations of cash and land, 3,000 acres of 

wetlands have already been restored. Waterfowl 

populations have increased, water quality is 

improving and flooding has decreased. (Source: 

National Park Service, “Floods, Floodplains and 

Folks”, 1996, U.S. Fish and Wildlife Service, 

Private Lands Office)

Mayview Wetland Project, Pennsylvania

The Pennsylvania Department of Transportation 

(DOT) completed a wetland restoration project to 

offset impacts to 32 acres of wetlands that were 

filled during the construction of Interstate 279 

through Southwestern Pennsylvania and the 

Southern Expressway. The site of the wetland 

restoration is Mayview, a 65-acre piece of land, 

flanking Chartiers Creek, a major stream. The 

creek was subject to frequent, high velocity 

flooding and constructing wetlands there is 

helping control these floods. The new wetlands 

provide flood storage capacity for 63 million 

gallons of water and serve as an outdoor classroom 

for nearby schools. The Department of 

Transportation is seeking funding to restore 

additional acreage. (Source: National Park Service, 

“Floods, Floodplains and Folks”, 1996)

Prairie Wolf Slough, Illinois

The Middle Fork of the North Branch of the 

Chicago River flows through an abandoned farm 

field in the suburbs. The area was identified as the 

future location for a trail, part of the North 

Where

Wetlands

are Helping

These studies and

others indicate

that wetlands

may play a part

in flood abatement. The following examples

illustrate how

communities

across the country are restoring

wetlands in order

to reduce the

threat and costs

of flood damage.

In 1982 these meadows and wetlands at Horseshoe Park in

Colorado were hit by a 25 to 30 foot wall of water. The height

and speed of the flood waters were reduced by the wetland 

vegetation, and the damaging flood peak was greatly reduced.

Paul McIver
ds are Helping

Branch of the Chicago River Open Space Plan designed

by the Friends of the Chicago River and Lake County 

Stormwater Management Commission, a regional open 

space advocacy organization. By restoring wetlands 

hydrology, clearing non-native vegetation and planting 

wetland, prairie and savanna vegetation, the functions and 

values of the wetlands have been restored. Structures used 

to drain the area for farming were removed, and a new water 

control structure was constructed to decrease sedimentation 

of the river. The result was moderation of stormwater flows 

which provided the area with flood protection, as well as 

permanent open space and new environmental education 

opportunities. (Source: National Park Service, “Floods, 

Floodplains and Folks”, 1996, Friends of the Chicago River)

Vermillion River, South Dakota

The Vermillion River has always flooded. It has a narrow 

channel and flows slowly, making it “flood prone.” 

Thousands of years ago, this part of South Dakota was 

scoured by glaciers that carved out shallow depressions 

which remain today and seasonally fill with water. These 

“prairie potholes” are intermittent, seasonal wetlands which 

dot the landscape. They quickly thaw in spring and provide 

habitat for a multitude of migratory birds and other water 

fowl. 

For hundreds of years, the rain and snow melt in the 

watershed were held in these wetlands, and runoff across the 

prairie was slowed. As South Dakota became populated, 

many prairie potholes were filled to facilitate farming. 

While these wetlands are small, they are numerous and can 

hold a significant amount of flood water. As more wetlands 

were filled, flooding increased. 

The Great Flood of 1993 was devastating to the area. To 

combat future flooding, structural flood controls were put 

in place, but they were not sufficient. In response to this 

problem, the National Park Service and the Federal 

Emergency Management Agency formed a partnership with 

the South Dakota Division of Emergency Management and 

Turner-Lincoln-Clay Counties Water Project District. 

Working together, this coalition assessed the area and 

condition of the remaining network of potholes. They 

developed a plan to protect the remaining wetlands and 

restored some of those that had been filled. 

(Source: National Park Service, “Floods, Floodplains and 

Folks”, 1996, The Vermillion River: Managing the 

Watershed to Reduce Flooding, Federal Emergency 

Management Agency)

The Special Case of Coastal Wetlands

Wetlands in many locations play an important role in flood 

protection. Nowhere is this function more important than along

coastal areas. Coastal areas are vulnerable to hurricanes and

other powerful storms, and the flat coastal terrain means that

land and property can be exposed to the full power of these

storms. Preserving and reconstructing coastal marshes can help

reduce storm damage. Coastal wetlands serve as storm surge

protectors when hurricanes or tropical storms come ashore. in

the Gulf coast area, barrier islands, shoals, marshes, forested

wetlands and other features of the coastal landscape can provide

a significant and potentially sustainable buffer from wind wave

action and storm surge generated by tropical storms and hurricanes. (Source: Working Group for Post-Hurricane Planning for

the Louisiana Coast)

More Wetlands Mean Less Flooding

These examples illustrate how protecting and restoring wetlands

can reduce the destructive potential of flooding. Wetland

restoration and preservation is an important component of a

comprehensive flood protection strategy. EPA, working with

other federal agency partners, is a resource for state and local

decision-makers, providing tools and limited funding for development of state wetland programs. Preserving wetlands, along

with other flood control measures, can offer a degree of protection against flooding that is often more effective and costs less

than a system of traditional dikes and levees. If more communities protect existing wetlands and increase the quantity of wetlands through restoration projects, we will be better protected

against the consequences of floods.

This diagram indicates that wetlands reduce peak

stormwater flows. (Source: Kusler 1983)
On the Internet:

Charles River Natural Valley Storage Area . . . . . . . . . . . . . . . www.nae.usace.army.mil/recreati/crn/crnhome.htm

Federal Emergency Management Agency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.fema.gov

Friends of the Chicago River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.chicagoriver.org

National Park Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.nps.gov

The Wetlands Initiative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.wetlands-initiative.org

U.S. Army Corps of Engineers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.usace.army.mil

In Print:

Floods, Floodplains and Folks. 1996. National Park Service. Rivers, Trails and Conservation Assistance Program.

Flood Damage Reduction in the Upper Mississippi River Basin—An Ecological Alternative. 2004. Donald L. Hey, et al.

The Wetlands Initiative, Chicago, IL. Available at www.wetlands-initiative.org

Jarrett, R.D., and J.E.Costa. 1984. Hydrology, geomorphology, and dam break modeling of the July 15, 1982 Lawn 

Lake Dam and Cascade Lake Dam Failures, Larimer County, Colorado: U.S. Geological Survey Professional Paper 1369.

Johnson, Rex R. 1997. The Vermillion River: Managing the Watershed to Reduce Flooding. Clay County Conservation 

District, Vermillion, SD.

Additional Wetland Resources

For additional information, visit the U.S. EPA’s website (www.epa.gov/owow/wetlands/), call the toll-free 

Wetlands Helpline at 1-800-832-7828 or refer to the sources below.

On the Internet:

“A New Framework for Planning the Future of Coastal Louisiana 

after the Hurricanes of 2005.” January 26, 2006. Working Group 

for Post-Hurricane Planning for the Louisiana Coast . . . . . . . . . . . . . . . . . . . . . . . . . . . www.umces.edu/la-restore

Association of State Floodplain Managers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.floods.org

Association of State Wetland Managers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.aswm.org

“Reinventing a Flood Control Strategy.” 1994. Donald L. Hey and 

Nancy S. Philippi. The Wetlands Initiative, Chicago, IL. . . . . . . . . . . . . . . . . . . . . . . www.wetlands-initiative.org

Society of Wetland Scientists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.sws.org

U.S. National Weather Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . www.nws.noaa.gov

Wetlands Status and Trends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . http://wetlandsfws.er.usgs.gov

In Print:

Bradley, A.A., K.W. Potter, T. Price, P. J. Cooper, J. Steffen and D. Francz. 1994. Dahl, T.E. 1990. “Wetland losses

in the United States: 1780’s to 1980’s.” Washington, DC. U.S. Department of Interior.

“Flood analysis in DuPage County using HSPF,” Proceedings of the Transportation Research Board (TRB) Annual 

Meeting, Washington, DC.

Protecting Floodplain Resources, a Guidebook for Communities. June 1996. The Federal Interagency Floodplain 

Management Task Force.

Shabman, L. 1994. “Responding to the 1993 Flood: The Restoration Option,” Water Resources Update, University 

Council on Water Resources, 95, 26-30.

U.S. National Weather Service, 1993, “Update on Midwestern floods, heat and drought in the East: Special 

Climate Summary,” 93/2, Climate Analysis Center.

EPA843-F-06-001

Office of Water

May 2006