1
COVER PHOTOGRAPH:

Rapid late-spring melting of a deep snowpack in June 1894 resulted in the largest flood on record 

for the Columbia River, which backed up the Willamette River into downtown Portland, Oregon. 

(Photograph courtesy of Oregon Historical Society; negative number ORH; 9906.)
Large Floods in the United States: 

Where They Happen and Why

By JIM E. O’CONNOR and JOHN E. COSTA

U.S. GEOLOGICAL SURVEY CIRCULAR 1245
iii

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

CHARLES G. GROAT, Director

The use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by

the U.S. Government.

___________________________________________________________________________________________

2003

Library of Congress Cataloging-in-Publication Data

O’Connor, Jim E., 1959-

Large floods in the United States : where they happen and why / Jim E., O’Connor and

John E. Costa.

p. cm. -- (U.S. Geological Survey circular ; 1245)

Includes bibliographical references.

ISBN 0-607-89380-X (pbk.)

1. Floods--United States. 2. Flood forecasting--United States. I. Costa, John E. II.

Geological Survey (U.S.) III. Title. IV. Series.

GB 1399.3.033 2003

551.48’9’0973--dc21 2003043891
iv

CONTENTS

Floods—A National Hazard ..................................................................................................................... 1

Largest of the Large................................................................................................................................... 3

Where Are the Largest Flows? .................................................................................................................. 6

Why Are Large Flows Where They Are?.................................................................................................. 8

Summary.................................................................................................................................................. 12

For More Information .............................................................................................................................. 13

FIGURES

1. The U.S. Geological Survey has collected streamflow measurements at more than 23,000 

gaging stations on streams and rivers throughout the United States and Puerto Rico.......................... 2

2. The streamflow gage on the Willamette River near Salem, Oregon, is one of many in the 

Willamette Valley and is typical of the more than 7,000 active USGS gages on streams and 

rivers in the U.S. and Puerto Rico......................................................................................................... 4

3. The largest annual flow from each of 22,063 streamflow stations in the U.S. and Puerto Rico.......... 5

4. Locations of 2,929 stations, color coded by drainage area, for which the largest peak discharge 

exceeds the criteria indicated by the red line of figure 3 ...................................................................... 7

5. The subset of large-stormflow gaging stations, as defined in this report, has a distribution 

similar to that of all U.S. Geological Survey gaging stations relative to the size of the basin

being gaged ........................................................................................................................................... 8

6. Rainfall amounts are relatively smaller in storms that have a greater than 6-hour duration 

and that exceed about 1,000 square miles in area ................................................................................. 8
v
1

Large Floods in the United States: 

Where They Happen and Why

By Jim E. O’Connor and John E. Costa

FLOODS—A NATIONAL HAZARD

Floods are the most chronic and costly natural hazard in the United States, causing an average of 

140 fatalities and $5 billion damage each year (Schildgen, 1999). Despite advances in flood science and 

implementation of Federal hazard-reduction policies, damage from flooding continues to escalate (Pielke 

and Downton, 2000). Damage from floods results from a combination of the great power of flowing 

water and the concentration of people and property along rivers. In the United States, about 3,800 towns 

and cities of more than 2,500 inhabitants are on floodplains (Miller and Miller, 2000).

Persistent, widespread torrential rainfall caused flooding across central Michigan in September 1986, resulting in unprecedented damage. 

(Photograph by Scott Harmesen, Bay City Times, used by permission.)
2

The New York State Thruway 

bridge over Schoharie Creek near 

Amsterdam, New York, collapsed 

on April 5, 1987, claiming 10 lives, 

after the foundation soils under a 

pier footing were washed away 

during a severe flood. (Photograph 

by Sid Brown, Schenectady 

Gazette, used by permission)

Damaging floods result when the volume of 

river flow exceeds levels of flood preparedness, 

either because flow is greater or longer than 

expected or because of incomplete understanding 

of local hazards. Consequently, a primary means 

of reducing flood hazards is by better understanding the magnitude and likelihood of large flows. 

The data underlying most studies of flood magnitude and frequency in the United States are 

records of U.S. Geological Survey (USGS) 

streamflow gages (fig. 1), which for the last 110 

years have been collecting streamflow data at 

more than 23,000 locations (although not all the 

streamflow-gaging stations have operated continuously). This circular summarizes the locations 

and magnitudes of large flows recorded by these 

gaging stations, showing locations in the United 

States where relatively large floods occur, and 

describes, in general terms, some of the climatologic and topographic factors that contribute to 

large floods.

The source of data for this analysis of large 

U.S. floods is the peak flow files maintained as 

part of the USGS National Water Information 

System. These files are maintained for each 

streamflow-gaging station and include values for 

the largest instantaneous discharge (peak flow) 

for each water year (October 1 to September 30) 

(see for example, fig. 2). As of June 2002, there 

were records for 23,216 stations on streams of all 

sizes that together comprise more than a million 

annual peak flow measurements and estimates. 

These measurements do not account for or signify 

the damage caused by floods, but simply indicate 

the maximum volume of water passing the gaging 

station during the peak of the flood.

LARGEST OF THE LARGE

From these peak flow files, we extracted the 

single largest flood discharge (excluding damfailure floods) from all stations that monitor flow 

from drainage areas greater than 0.01 mi2

 (square 

miles) (0.26 km2 [square kilometers]). This 

resulted in records of the maximum flood from
3

Figure 1. The U.S. Geological Survey has collected streamflow measurements at more than 23,000 gaging stations on streams and rivers throughostations have recorded flow since the late 1800s. About 7,000 stations are presently active. This map shows all 22,063 stations, color coded by drathe largest flow for each year that the gage was operated. Typical long-term stations are the Willamette River at Salem, Oregon, which has recordinformation for historic peak discharges back to 1861), and the Platte River near Duncan, Nebraska, which has collected daily flow measurements to 1896.
4
5

Figure 2. The streamflow gage on the Willamette River near Salem, Oregon, is one of many in the Willamette Valley and is typical of the 

more than 7,000 active USGS gages on streams and rivers in the U.S. and Puerto Rico. Most stations monitor river stage (elevation of the 

water surface), which is then converted to discharge (flow volume per unit time) by use of a stage-discharge rating curve. These rating 

curves are derived from and continuously updated by several measurements per year of flow discharge and stage (indicated by the solid 

dots). The largest flow of each water year (October through September), either directly measured or, more commonly, determined from the 

highest recorded stage and the rating curve, is compiled into the station’s peak flow file. The annual peak flows for this station are shown 

on the bottom plot, including notable floods in 1861, 1943, and 1964. The generally smaller magnitude of peak flows since the 1960s owes to 

construction of several flood control reservoirs in the basin.

_____________________________________________________

each of 22,063 stations (fig. 1). Discharge 

values ranged from 1 ft3/s (cubic feet per 

second) (0.03 m3/s [cubic meters per second]) 

to 2.47 million ft3/s (70,000 m3/s) with 

drainage areas up to 1.13 million mi2 (2.93 

million km2) (fig. 3). These records include 

discharge estimates for historic floods at 

gaging stations dating back to the late 18th

century as well as all gaged measurements 

through water year 1997 (September 30, 

1997). The majority of these floods were 

caused by weather and climate conditions, 

such as rainfall and snowmelt, but many were 

also affected by regulation and diversion. 

Most of these flows were not directly measured 

at the time, but were estimated from post-flood 

surveys and calculations or by comparing 

elevations of maximum flood levels to past 

measured flows (fig 2).

In general, larger flows have come from 

larger drainage basins because they accumulate 

more rain and snow (fig. 3). Despite this overall

Figure 3. The largest annual flow from each of 22,063 streamflow stations in the U.S. and Puerto Rico. In general, larger basins produce

larger-peak flows, although there is substantial variation. Points above the red line, such as the 500,000 cubic feet per second peak 

discharge on the Willamette River at Salem, are the largest 13 percent of these flows, relative to drainage area. The stations recording 

these largest flows are shown on figure 4.
6

relation between basin size and peak flow, some 

basins produce larger flows than other, similarly 

sized or even larger basins. To identify the 

basins that produce the largest floods relative 

to drainage area, we determined which stations 

had flows that exceeded the criteria:

Q >1,000 A0.667 for A <1,000 and 

Q >10,000 A0.333 for A >1,000,

where Q is the peak discharge in cubic feet per 

second and A is the drainage area in square miles. 

This set of equations is represented by the red line 

in figure 3. The largest flood for each streamflow 

gaging station meets or exceeds the criteria for 

2,929 stations, about 13 percent of the 22,063 stations with peak flow records. For example, the 

largest flood for the streamflow-gaging station 

“Willamette River at Salem,” for which 500,000 

ft

3

/s was estimated for December 9, 1861, 

plots well above the line, considering the 7,280 

mi2 drainage area of the station. In contrast, the 

largest flow in nearly 100 years of record for the 

“Platte River at Duncan,” which records flow from 

a much larger basin of 60,900 mi2, was the 44,000 

ft3/s flood of June 23, 1905, a value that plots well 

below the line. The locations of stations with the 

largest relative flows—those that fall above the 

line—are shown on figure 4, which forms the 

basis of our conclusions on some of the geographic and meteorologic controls on large flows 

within the United States.

The two criteria for stations with contributing 

basins smaller and larger than 1,000 mi2 (2,690 

km2) result in the slope change of the red line 

shown in figure 3. In developing these criteria for 

defining the large-flow stations, we found that this 

slope change was necessary to ensure that our subset of large-flow stations was not biased relative 

drainage area. In other words, the statistical distribution of drainage areas for stations plotting above 

the line is similar to the distribution of areas contributing to all of the analyzed stations (fig. 5). 

Beyond providing an unbiased selection of 

large-flow stations, the change in slope of the 

large-flow criteria for drainage basins larger that 

1,000 mi2

 (2,560 km2) likely reflects physical processes underlying flood runoff. For example, similar slope decreases appear in envelope curves 

bounding the maximum global and U.S. rainfall-runoff floods (Costa, 1987) as well as some 

exceptional regional floods resulting from individual storms (Miller, 1990). Further, the decrease in 

slope for the large-flow criterion separating the 

largest U.S. gaged flows corresponds with the 

marked drop-off in rainfall depth for U.S. storms 

with areas larger than 1,000 mi2 (2,690 km2) and 

durations greater than 6 hours (fig. 6), and may 

reflect limits on storm size and rainfall rates, perhaps corresponding to the spatial extent of intense 

convective precipitation within tropical storms.

WHERE ARE THE LARGEST FLOWS?

The geographic distribution of stations with 

the largest flow magnitudes relative to drainage 

area (fig. 4) is substantially different from the distribution of all stations (fig. 1), showing that the 

distribution of large flows is not random but must 

reflect climate and landscape properties. It is also 

evident, however, that the distribution of stations 

with large floods depicted in figure 4 is locally 

biased by areas of high concentrations of flow stations, such as in the Dallas-Fort Worth and Houston metropolitan areas in Texas, and in the 

Appalachians of Virginia and Maryland. Nevertheless, there are notable concentrations of exceptional flows, such as in Hawaii and Puerto Rico, 

where 78 percent of the 441 stations have maximum station discharges above the large-flow criteria shown in figure 3.

Within the conterminous United States, certain areas produce larger flows than others. Particularly notable are the concentrations of relatively 

large flows in central Texas and in the central and 

northern Appalachians. Also evident are relatively 
7

Figure 4. Locations of 2,929 stations, color coded by drainage area, for which the largest peak discharge exceeds the criteria indicated by the red distribution of these stations across the U.S. and Puerto Rico owes to specific climate and landscape characteristics that produce large floods.
8

Figure 5. The subset of large-stormflow gaging stations, as defined 

in this report, has a distribution similar to that of all U.S. Geological 

Survey gaging stations relative to the size of the basin being gaged.

high flow magnitudes along the Pacific coast. 

Another area susceptible to relatively large 

flows is the southern midwestern United 

States.

Broad areas are also depicted by figure 4 that 

have very few stations with discharges exceeding 

the large-flow criteria. In the interior of the western United States, only isolated stations with small 

drainage basins have had exceptionally large 

flows. Likewise, very few stations in the northern 

tier States of the eastern and midwestern parts of 

the country have had flows exceeding the 

large-flow criteria. Similarly, very few relatively 

large flows have been measured in the southeastern coastal plain and Florida.

Figure 6. Rainfall amounts are relatively smaller in storms that have 

a greater than 6-hour duration and that exceed about 1,000 square 

miles in area. (Data from World Meteorological Organization.)

WHY ARE LARGE FLOWS WHERE THEY ARE?

The overall higher density of large-flow stations in the southern States reflects the greater 

moisture-holding capacity of warmer air. However, most aspects of the distribution of relatively 

large flows shown in figure 4 are attributable to 

regional climatic and physiographic conditions. 

Additionally, basin size influences the nature of 

climatologic and topographic influences on flow 

magnitude. 

Puerto Rico and Hawaii both have many relatively large flows because they are mountainous 

areas in the midst of the tradewind belt, where 

moisture-laden tropical storms and hurricanes are 

intercepted and forced upward by the island 

masses, triggering orographic and convective 

uplift and enhanced precipitation. It follows that a 

greater proportion of stations with the largest 
9

flows are on the windward (eastern) side of these 

island complexes.

Within the eastern conterminous United 

States, the pattern of large flows closely corresponds to the proximity of subtropical moisture 

derived from the Gulf of Mexico and the Atlantic 

Ocean. Finer-scale patterns within this region, 

however, are clearly linked to topography. Orographic lifting of moisture from the Gulf of Mexico triggers convective instability and has caused 

concentrations of large floods at topographic features such as the Balcones Escarpment in south 

central Texas, the Ozark Mountains of western 

Missouri, and along the eastern edge of the Appalachians on the eastern seaboard. The important 

role of topographic relief is also illustrated by the 

lack of exceptional discharges in Florida, and 

along the coastal plain of the southern and eastern 

tier States. Despite being closest to the moisture 

sources and subject to frequent landings of major 

hurricanes, these relatively flat areas have few stations that have recorded flows exceeding the 

large-flow criteria of figure 3. 

The high incidence of large flows in the central and southern Midwest and upper Mississippi 

Valley is partly due to mesoscale convective complexes—large, multiple-celled, persistent thunderstorm systems fed largely by moisture from the 

Gulf of Mexico (Hayden, 1988; Hirschboeck, 

1991). The Rapid City, South Dakota, flood of 

1972, the Big Thompson River, Colorado, flood of 

1976, and the catastrophic 1993 Mississippi River 

System flood in the Midwest were caused by such 

systems. 

Within the area of influence of moisture from 

the Gulf of Mexico, a variety of moisture transport 

and precipitation processes have caused large

Topography enhances precipitation by: 

(a) forcing air to ascend (orographic lifting), causing cooling and, consequently, condensation and precipitation. This is a major factor 

enhancing rainfall on the western flanks of West Coast mountain ranges.

(b) triggering convective instability either by forcing air masses to rise as they impinge upon upward slopes or by causing differential 

heating of the ground surface because of the variety of ground surfaces associated with varied topography. In either case, the 

atmospheric instabilities that result can result in towering cloud growth and intense precipitation. This is a major factor along the 

flanks of the Appalahians, in the Midwest, and the interior West, wherever mountains and escarpments are in the path of 

subtropical moisture.
10

Melting snow and heavy spring rain combined to cause severe flooding on the upper Mississippi River in April 2001. Davenport, Iowa, 

was particularly hard-hit. In this photograph, all but the infield of John O'Donnell Stadium, home of the Quad Cities River Bandits, lies 

underwater. The record Mississippi River flood of 1993, caused by a persistent mesoscale convective complex, along with other cities 

in the Mississippi Basin. (Photograph by John Schultz, Quad Cities Times, used by permission.)

flows, including individual thunderstorm cells, 

mesoscale convective complexes, Atlantic 

hurricanes, and quasistationary upper 

atmosphere circulation patterns producing 

several weeks or months of persistent precipitation. These processes are of such diverse 

temporal and spatial scales that there does not 

seem to be any systematic pattern of drainage 

basin size associated with exceptionally large 

flows in this region. The only exception is 

along the southeastern coastal plain, where 

only larger rivers draining upland areas have 

produced large flows, primarily from 

hurricanes.

West of about longitude 105o west, approximately corresponding to the eastern limit of the 

Rocky Mountains, moisture from the PacificOcean 

chiefly influences flooding within the conterminous United States and Alaska. Consequently, the 

stations with largest flows in this region are concentrated along the States contiguous to the Pacific 

Ocean. The interior western States of Idaho, 

Nevada, Utah, Wyoming, and Colorado have very 

few stations with large flows, and, furthermore, 

the interior West stations that have had relatively 

large flows generally represent small basins (less 

than 10 mi2

) influenced by isolated thunderstorms. 

The distribution of stations with the largest flows 

within the interior West reflects local orographic 

lifting and convective instability associated with 

physiographic features such as the eastern limits of 

the Rocky Mountains in eastern Wyoming, central 

Colorado, and eastern New Mexico; the Wasatch 

Mountains in Utah; and the Mogollon Rim in 

Arizona. 
11

Orographic effects are even more pronounced, however, along the western continental 

margin of North America, where the mountains of 

southeastern Alaska; Cascade Range of Oregon 

and Washington; Coast Ranges of Washington, 

Oregon, and California; and Sierra Nevada and 

San Gabriel Mountains of California all strongly 

influence the distribution of stations with large 

flows. In Oregon, Washington, and northern California, north-south trending mountain ranges intercept the westerly flow of extratropical cyclones 

from the Pacific Ocean. The largest floods for 

most basins are winter floods caused by intense 

cyclonic storms or the combined effects of 

cyclonic storms and coincident snowmelt. The 

extensive 1964 and 1996 floods throughout this 

region were produced by substantial rain-on-snow 

events. Because these types of storms persist for 

several days, it is the larger basins—generally 

from 100 to 100,000 mi2—that are preferentially 

affected by these types of events (fig. 4). 

Large flows in southern California and Arizona are from various sources (Hirschboeck,

1991), including extratropical cyclonic 

systems, dissipating tropical storms moving 

northward from the Pacific, mesoscale 

convective complexes, and local convective 

thunderstorms. Consequently, there is 

substantial variation in the size of basins and 

seasonality of the largest flows. The 

Transverse Ranges of southern California and 

Mogollon Rim of central Arizona are oriented 

in particularly favorable directions to intercept 

moisture from northward incursions of tropical 

storms; consequently there are distinct 

concentrations of stations with large floods 

aligned along these features. 

The big river systems of the Western United 

States that drain the extensive and snowy mountainous regions of the western North America, 

such as the Yukon and Columbia Rivers, also produce large flows. These flows are invariably 

caused by snowmelt in late spring and summer. 

The large northern basins are the only ones for 

which snowmelt alone appears to be capable of 

producing flows greater than the large-flow criterion shown on figure 3.

A rain-on-snow flood in February 1996 resulted in near-record to record flooding in the Willamette River Basin and other river basins in 

Oregon. In the photograph on the left, Willamette Falls (near Oregon City), shown during more typical flow at right, is almost completely 

submerged. (Photographs by Jim E. O'Connor [left] and John S. Williams [right], U.S. Geological Survey)
12

The "State Street River" (floodwater from City Creek) rushes through downtown Salt Lake City, Utah, in May 1983. Rapid melting of a

record-water-content snowpack caused the most widespread flooding in the State's history, resulting in damages totaling $621 million. 

(Photograph by Rulon Christensen, U.S. Geological Survey.)

SUMMARY

The spatial distribution of large gaged

floods throughout the United States shows that 

the locations of most of the largest flows are 

related to specific combinations of regional

climatology, topography, and basin size. Key 

factors include the general northward trend of 

decreasing atmospheric moisture, proximity to 

oceanic moisture sources such as the Pacific 

Ocean and the Gulf of Mexico, and orientation 

of topographic features relative to directions of 

moisture flow, with the largest flows being at 

locations where topographically high areas are 

oriented perpendicular to directions of moisture 

flow. Additionally, the largest flows in large river 

basins are primarily caused by persistent climatologic conditions such as seasonal snowfall. In 

contrast, the largest flows in smaller basins are 

most commonly the result of intense precipitation 

due to convective storms.

Understanding hydrologic extremes, 

such as floods and droughts, requires records that 

are both long and of wide spatial extent, especially for a land of such diverse landscapes and 

climatology as the United States. As one of the 

few sources of long-term flow measurements, 

USGS streamflow records are a rich source of 

data for analyzing the hydrologic conditions of 

the Nation. As the USGS streamflow measurement program progresses into its second century, 

it is committed to improving the Nation’s level of 

streamflow information. Components of this commitment include increasing the completeness and 

efficiency of the streamgaging network, improving methods of data storage and delivery, and new 

emphasis on analysis of the wealth of information 

already collected, such as the spatial distribution 

of large flows described here.
13

FOR MORE INFORMATION 

Floods and Climate

Barry, R.G., and Chorley, R.J., 1998, Atmosphere, weather 

and climate: London, Routledge, 464 p.

Costa, J.E., 1987, A comparison of the largest rainfall-runoff 

floods in the Unites States with those of the People’s 

Republic of China and the world: Journal of Hydrology, 

v. 96, p. 101–115.

Hayden, B.P., 1988, Flood climates, in Baker, V.R., Kochel, 

R.C., and Patton, P.C., eds., Flood Geomorphology: 

New York, John Wiley and Sons, p. 13–26.

Hirschboeck, K.K., 1991, Climate and floods, in Paulsen, 

R.W., Chase, E.B,. Roberts, R.S., and Moody, D.W., 

compilers, National Water Summary 1988–89, Floods 

and Droughts: U.S. Geological Survey Water-Supply 

Paper 2375, p. 67–88.

Hoyt, W.G., and Langbein, W.B., 1955, Floods: Princeton 

University Press, 469 p.

Miller, J.J., 1990, Flood hydrology and geomorphic 

effectives in the central Appalachians: Earth Surface 

Processes and Landforms, v. 15, p. 119–134.

Miller, E.W., and Miller, R.M., 2000, Natural disasters—

Floods—A reference handbook: ABC-CLIO, Santa 

Barbara, California, 286 p.

O’Connor, J.E., Grant, G.E., and Costa, J.E., 2002, The 

geology and geography of floods, in House, P.K., 

Webb, R.H., Baker, V.R., and Levish, D.R., eds., 

Ancient floods, modern hazards—Principles and 

application of paleoflood hydrology: American 

Geophysical Union Water Science and Application 

Series, no. 5, p. 359–385.

Perry, C.A., 2000, Significant floods in the United States 

during the 20th century—USGS measures a century of 

floods: U.S. Geological Survey Fact Sheet 024–00, 4 p.

Pielke, R.A., Jr., and Downton, M.W., 2000, Precipitation 

and damaging floods—Trends in the United States, 

1932–97: Journal of Climate, v. 13, p. 3625–3637.

Schildgen, R.,1999, Unnatural disasters: Sierra, v. 84, 

p. 48–57.

The U.S. Geological Survey streamflow measurement 

program

Hirsch, R.M., and Norris, J.M., 2001, National Streamflow 

Information Program—Implementation plan and 

progress report: U.S. Geological Survey Fact Sheet 

048–01, 6 p.

United States Geological Survey, 1999, Streamflow 

information for the next century—A plan for the 

National Streamflow Information Program of the U.S. 

Geological Survey: U.S. Geological Survey Open-File 

Report 99–456, 13 p. 

Related Web sites:

USGS Water Watch

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

USGS Fact Sheet: Significant Floods in the United States 

During the 20th Century

http://ks.water.usgs.gov/Kansas/pubs/fact-sheets/

fs.024-00.html

USGS Flood Information

http://water.usgs.gov/osw/programs/floods.html

EarthSat(TM) flood threat map 

http://www.earthsat.com/flood/maps/ffgusa.gif

Federal Emergency Management Agency (FEMA) Flood 

Hazard Mapping

http://www.fema.gov/mit/tsd/fq_genhm.htm

Dartmouth University Flood Observatory

http://www.dartmouth.edu/artsci/geog/floods/

CBS News Disaster Links

http://www.disasterlinks.net/

Societal Aspects of Weather: Floods

http://sciencepolicy.colorado.edu/socasp/floods.html

National Weather Service: Significant River Flood Outlook

http://www.hpc.ncep.noaa.gov/nationalfloodoutlook/