Streamflow in Mercer Creek, an urban stream in western Washington, increases more 

quickly, reaches a higher peak discharge, and has a larger volume during a one-day storm 

on February 1, 2000, than streamflow in Newaukum Creek, a nearby rural stream. Streamflow 

during the following week, however, was greater in Newaukum Creek.

Flooding in Hickory Hills, Illinois, prompted 

the construction of a reservoir to control 

runoff from upstream areas. Source: Loren 

Wobig, Illinois Office of Water Resources.

rainfall and snowmelt. Construction 

of roads and buildings often involves 

removing vegetation, soil, and depressions 

from the land surface. The permeable 

soil is replaced by impermeable surfaces 

such as roads, roofs, parking lots, and 

sidewalks that store little water, reduce 

infiltration of water into the ground, and 

accelerate runoff to ditches and streams. 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS-076-03

November 2003

Even in suburban areas, where lawns 

and other permeable landscaping may 

be common, rainfall and snowmelt can 

saturate thin soils and produce overland 

flow, which runs off quickly. Dense 

networks of ditches and culverts in cities 

reduce the distance that runoff must travel 

overland or through subsurface flow paths 

to reach streams and rivers. Once water 

enters a drainage network, it flows faster 

than either overland or subsurface flow. 

With less storage capacity for water 

in urban basins and more rapid runoff, 

urban streams rise more quickly during 

storms and have higher peak discharge 

rates than do rural streams. In addition, 

the total volume of water discharged 

during a flood tends to be larger for 

urban streams than for rural streams. For 

example, streamflow in Mercer Creek, 

an urban stream in western Washington, 

increases earlier and more rapidly, has 

a higher peak discharge and volume 

during the storm on February 1, 2000, 

Over the past century, the United 

States has become an increasingly 

urban society. The changes in land use 

associated with urban development 

affect flooding in many ways. Removing 

vegetation and soil, grading the land 

surface, and constructing drainage 

networks increase runoff to streams from 

rainfall and snowmelt. As a result, the 

peak discharge, volume, and frequency 

of floods increase in nearby streams. 

Changes to stream channels during urban 

development can limit their capacity to 

convey floodwaters. Roads and buildings 

constructed in flood-prone areas are 

exposed to increased flood hazards, 

including inundation and erosion, as new 

development continues. Information about 

streamflow and how it is affected by land 

use can help communities reduce their 

current and future vulnerability to floods.

Effects of Urban Development on Floods

HYDROLOGIC EFFECTS OF 

URBAN DEVELOPMENT

Streams are fed by runoff from 

rainfall and snowmelt moving as overland 

or subsurface flow. Floods occur when 

large volumes of runoff flow quickly into 

streams and rivers. The peak discharge 

of a flood is influenced by many factors, 

including the intensity and duration of 

storms and snowmelt, the topography and 

geology of stream basins, vegetation, and 

the hydrologic conditions preceding storm 

and snowmelt events.

Land use and other human activities 

also influence the peak discharge of 

floods by modifying how rainfall and 

snowmelt are stored on and run off the 

land surface into streams. In undeveloped 

areas such as forests and grasslands, 

rainfall and snowmelt collect and are 

stored on vegetation, in the soil column, 

or in surface depressions. When this 

storage capacity is filled, runoff flows 

slowly through soil as subsurface flow. 

In contrast, urban areas, where much of 

the land surface is covered by roads and 

buildings, have less capacity to store 

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The hydrologic effects of urban 

development often are greatest in 

small stream basins where, prior to 

development, much of the precipitation 

falling on the basin would have become 

subsurface flow, recharging aquifers 

or discharging to the stream network 

further downstream. Moreover, urban 

development can completely transform 

the landscape in a small stream basin, 

unlike in larger river basins where areas 

with natural vegetation and soil are likely 

to be retained.

Road washed out by a flood in West 

Virginia. Source: A. Rothstein, Farm Security 

Administration.

Annual maximum discharge increased with urban development in Mercer Creek from 1960 to 

2000, but remained essentially the same in nearby rural Newaukum Creek during that period.

and decreases more rapidly than in 

Newaukum Creek, a nearby rural stream. 

As with any comparison between streams, 

the differences in streamflow cannot be 

attributed solely to land use, but may also 

reflect differences in geology, topography, 

basin size and shape, and storm patterns.

HYDRAULIC EFFECTS FROM 

CHANGES TO STREAM CHANNELS AND FLOODPLAINS

Development along stream channels 

and floodplains can alter the capacity 

of a channel to convey water and can 

increase the height of the water surface 

(also known as stage) corresponding to a 

given discharge. In particular, structures 

that encroach on the floodplain, such as 

bridges, can increase upstream flooding 

by narrowing the width of the channel and 

increasing the channel’s resistance to flow. 

As a result, the water is at a higher stage 

as it flows past the obstruction, creating a 

backwater that will inundate a larger area 

upstream.

Sediment and debris carried by 

floodwaters can further constrict a channel 

and increase flooding. This hazard is 

greatest upstream of culverts, bridges, 

or other places where debris collects. 

Small stream channels can be filled 

with sediment or become clogged with 

debris, because of undersized culverts, for 

example. This creates a closed basin with 

no outlet for runoff. Although channels 

can be engineered to convey floodwater 

and debris quickly downstream, the local 

benefits of this approach must be balanced 

against the possibility of increased 

flooding downstream.

Erosion in urban streams 

represents another consequence of 

urban development. Frequent flooding 

in urban streams increases channel and 

bank erosion. Where channels have been 

straightened and vegetation has been 

removed from channel banks, streamflow 

velocities will increase, allowing a stream 

to transport more sediment. In many urban 

areas, stream-bank erosion represents an 

ongoing threat to roads, bridges, and other 

structures that is difficult to control even 

by hardening stream banks.

EFFECTS OF URBAN DEVELOPMENT ON FLOOD DISCHARGE 

AND FREQUENCY

Common consequences of urban 

development are increased peak discharge 

and frequency of floods. Typically, the 

annual maximum discharge in a stream 

will increase as urban development 

occurs, although the increase is sometimes 

masked by substantial year-to-year 

variation in storms, as is apparent in the 

annual maximum discharge for Mercer 

Creek from 1960 to 2000. In comparison, 

the annual maximum discharge for rural 

Newaukum Creek varied during the period 

but showed no clear trend.

The effects of development in urban 

basins are most pronounced for moderate 

storms following dry periods. For larger 

storms during wet periods, the soil in rural 

basins becomes saturated and additional 

rainfall or snowmelt runs off much as it 

does in an urban basin.

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The number of times per year that daily discharge in the Northeast Branch of the Anacostia 

River, Maryland, (USGS gaging station 01649500) was greater than 1,000 ft3/s has increased 

over time.

The relative increase in annual maximum discharge in Salt Creek, Illinois, (USGS gaging 

station 05531500) has been greater for small floods (solid line, less than 95 percent of the 

annual peaks for the period of record) than for large floods (dashed line, more than 95 

percent of the annual peaks for the period of record).

The relative increase in peak discharge 

is greater for frequent, small floods than 

infrequent, large floods.

Flood 

frequency

Chance that 

flood’s peak 

discharge will 

be exceeded 

in any year

Increase in 

flood peak 

discharge 

because 

of urban 

development

2-year 50 percent 100 to 600

 percent

10-year 10 percent 20 to 300

 percent

100-year 1 percent 10 to 250

 percent

The effect of urban development in 

the last half of the 20th century on small 

floods is evident in Salt Creek, Illinois. 

With the exception of an unusually large 

flood in 1987, large floods have increased 

by about 100 percent (from about 

1,000 cubic feet per second to about 

2,000 ft3/s) while small floods have 

increased by about 200 percent (from 

about 400 ft3/s to 1,200 ft3/s). Nonetheless, 

even a small increase in the peak discharge 

of a large flood can increase flood 

damages. 

The frequency of moderate 

flooding can increase substantially after 

development. The annual frequency that 

daily discharge exceeded 1,000 ft3/s on the 

Northeast Branch of the Anacostia River 

in Maryland increased from once or twice 

per year in the 1940s and 1950s to as 

much as six times per year in the 1990s.

REDUCING FLOOD HAZARDS IN 

URBAN AREAS

There are many approaches for 

reducing flood hazards in basins under 

development. Areas identified as floodprone have been used for parks and 

playgrounds that can tolerate occasional 

flooding. Buildings and bridges have been 

elevated, protected with floodwalls and 

levees, or designed to withstand temporary 

inundation. Drainage systems have been 

expanded to increase their capacity for 

detaining and conveying high streamflows; 

for example, by using rooftops and 

parking lots to store water. Techniques that 

promote infiltration and storage of water 

in the soil column, such as infiltration 

trenches, permeable pavements, soil 

amendments, and reducing impermeable 

surfaces have also been incorporated 

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areas. Because flood hazard maps based 

on streamflow data from a few decades 

ago may no longer be accurate today, 

floodplain managers need new peak 

streamflow data to update flood frequency 

analyses and flood maps in areas with 

recent urbanization. Streamflow-gaging 

stations provide a continuous record of 

streamflow that can be used in the design 

of new urban infrastructure including 

roads, bridges, culverts, channels, 

and detention structures. Stormwater 

managers can use streamflow information 

in combination with rainfall records to 

evaluate innovative solutions for reducing 

runoff from urban areas. Real-time 

streamflow-gaging stations, which make 

streamflow and rainfall data available via 

the internet and other communications 

networks as they are recorded, offer 

multiple benefits in urban watersheds. In 

particular, they provide flood managers 

with information that can guide flood 

control operations and emergency actions 

such as evacuations and road closures. 

James, J.D., 1965, Using a digital computer to estimate the effects of urban 

development on flood peaks: Water 

Resources Research, v. 1, no. 2, 

p. 223-234.

Konrad, C.P., and Booth, D.B., 2002, 

Hydrologic trends associated with urban 

development for selected streams in 

western Washington: U.S. Geological 

Survey Water-Resources Investigations 

Report 02-4040, 40 p., accessed on July 

16, 2003, http://water.usgs.gov/pubs/

wri/wri024040.

Leopold, L.B., 1968, Hydrology for urban 

land planning–A guidebook of the 

hydrologic effects of urban land use: 

U.S. Geological Survey Circular 554, 

18 p.

Sauer, V.B., Thomas, W.O., Jr., Stricker, 

V.A., and Wilson, K.V., Flood characteristics of urban watersheds in the 

United States: U.S. Geological Survey 

Water-Supply Paper 2207, 63 p.

U.S. Geological Survey, 2003, USGS 

hydrologic data in and around Mecklenburg County, N.C., accessed October 

27, 2003, at http://nc.water.usgs.gov/

char/.

Zarriello, P.J., 1998, A Precipitation-runoff 

model for part of the Ninemile Creek 

watershed near Camillus, Onondaga 

County, New York: U.S. Geological 

Survey Water-Resources Investigations 

Report 98-4201, 60 p., accessed on July 

16, 2003, at http://ny.water.usgs.gov/

pubs/wri/wri984201.

Floodwalls along Willow Creek in Rosemont, 

Illinois. Source: Kevin D. Richards, U.S. 

Geological Survey.

The U.S. Geological Survey in 

cooperation with the City of Charlotte 

and Mecklenburg County, North Carolina, 

developed a flood information and 

notification system (FINS) to address 

the need for prompt notification of flood 

conditions in urban areas where streams 

rise and fall rapidly. FINS is based on a 

large network of streamflow gaging and 

rainfall stations that broadcast information 

within minutes of being recorded via 

radio telemetry. The system automatically 

notifies the National Weather Service 

and emergency responders in the region 

when rainfall and streamflow indicate 

the likelihood of flooding, giving these 

agencies additional time to issue warnings 

and evacuate areas if necessary.

into new and existing residential and 

commercial developments to reduce runoff 

from these areas. Wet-season runoff from 

a neighborhood in Seattle, Washington, 

was reduced by 98 percent by reducing 

the width of the street and incorporating 

vegetated swales and native plants in the 

street right-of-way.

In response to frequent flooding 

along the Napa River in California, the 

local community integrated many of these 

approaches into a single plan for flood 

protection that is expected to reduce flood 

damage while helping to restore the river 

ecosystem. The plan involves bridge 

reconstruction, levee setbacks, a floodwall, 

moving of vulnerable structures, detention 

basins, larger stormwater conveyances, 

and a high-flow bypass channel.

CONCLUSIONS

Urbanization generally increases 

the size and frequency of floods and may 

expose communities to increasing flood 

hazards. Current streamflow information 

provides a scientific foundation for flood 

planning and management in urban 

ADDITIONAL READING

Anderson, D.G., 1968, Effects of urban 

development on floods in northern 

Virginia: U.S. Geological Survey WaterSupply Paper 2001-C, 22 p.

Bailey, J.F., Thomas, W.O., Wetzel, K.L. 

and Ross, T.J., 1989, Estimation of 

flood-frequency characteristics and the 

effects of urbanization for streams in the 

Philadelphia, Pennsylvania area: U.S. 

Geological Survey Water-Resources 

Investigations Report 87-4194, 71 p.

Carter, R.W., 1961, Magnitude and frequency of floods in suburban areas: U.S. 

Geological Survey Professional Paper 

424-B, p. 9-11.

Hollis, G.E., 1975, The effects of urbanization on floods of different recurrence 

intervals: Water Resources Research, 

v. 11, no. 3, p. 431-435.

Horner, R.R., Lim, H., and Burges, S.J., 

2002, Hydrologic monitoring of the 

Seattle Ultra-urban stormwater management projects: University of Washington, Department of Civil and Environmental Engineering, Water Resources 

Series Technical Report 170, 31 p.

FOR MORE INFORMATION ON...

U.S. Geological Survey stream gaging 

and the National Streamflow Information 

Program: http://water.usgs.gov/nsip

Current streamflow conditions around the 

U.S.: http://water.usgs.gov/waterwatch/

Author:

Christopher P. Konrad

 —Research Hydrologist

U.S. Geological Survey–Water Resources

1201 Pacific Avenue, Suite 600

Tacoma, WA 98402

Phone: 253-428-3600 ex. 2634

E-mail: cpkonrad@usgs.gov