“I’m stuck, I’m right in the middle of it, I can’t get out . . . 

about a half mile east of Drake on the highway. Get the cars

out of the low area down below . . .”

Last words received from Sergeant Willis Hugh Purdy, Colorado 

State Patrol. Sergeant Purdy was a victim and a hero of the Big 

Thompson Flood of 1976. Purdy had finished his shift when Greeley dispatchers informed him of severe weather problems in the Big Thompson 

Canyon. As Purdy proceeded into the canyon, he ordered the evacuation 

of the lower areas below the canyon, a decision that saved hundreds

of lives. Purdy was posthumously awarded the Colorado State 

Patrol Medal of Valor. Two years later, a memorial was 

dedicated in the Big Thompson Canyon, which is

located about 1.3 miles downstream from Drake, 

honoring Sergeant Purdy and Estes Park 

Officer Michael Conley. 

(Courtesy of The Colorado Law

Enforcement Memorial 

Book Online; http://www.csp.

state.co.us/academy/

acfallen.htm)

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3095

July 2006

Introduction

In the early evening of Saturday, July 31, 1976, a large stationary 

thunderstorm released as much as 7.5 inches of rainfall in about an 

hour (about 12 inches in a few hours) in the middle reaches of the Big 

Thompson River Basin and to a lesser extent in parts of the Cache la 

Poudre River Basin (U.S. Geological Survey, 1979). In steep mountain 

terrain with thin or no soil, this large amount of rainfall in such a short 

period of time produced a flash flood that caught residents and tourists by surprise. The sudden flood that churned down the narrow Big 

Thompson Canyon scoured the river channel that night, caused over $35 

million in damages (1977 dollars) to 418 homes and businesses, many 

mobile homes, 438 automobiles, numerous bridges, paved and unpaved 

roads, power and telephone lines, and many other structures. The tragedy 

claimed the lives of 144 people, including two law enforcement officers 

trying to evacuate people in danger, and there were 250 reported injuries 

(U.S. Geological Survey, 1979). Scores of other people narrowly escaped 

with their lives. More than 800 people were evacuated by helicopter the 

following morning.

July 2006 revisits the 30th anniversary of the Big Thompson flood—

one of the most deadly flash floods in Colorado’s recorded history (Jarrett and Vandas, 2006). Many residents and visitors who were present in 

the Big Thompson Canyon on July 31, 1976, recall the flood with vivid 

memories. This fact sheet presents a summary of the hydrologic conditions of the 1976 flood, describes some of the advances in U.S. Geological Survey (USGS) flood science as a consequence of this disaster, and 

provides a reminder that extreme floods like the 1976 Big Thompson 

flood have occurred in other locations in Colorado in the past and will 

occur again. The USGS conducts research and operates a Nationwide 

streamgage network to help understand and predict the magnitude and 

likelihood of large streamflow events such as the Big Thompson flood. 

The Flood

A complex system of thunderstorms produced intense rainfall from 

about 6 to 7 pm (MDT) on July 31, 1976, in the Front Range foothills of Colorado’s Big Thompson River (fig. 1) and Cache la Poudre 

River Basins in Larimer County. This Saturday night marked the eve 

of Colorado’s 100th anniversary of Statehood, and at the height of the 

tourist season an estimated 3,500 people were enjoying the cool beauty 

and recreation of the mountain canyons, unaware of the unusual and 

unique atmospheric conditions that were occurring.

The topography of the affected area is characterized by narrow 

canyons bordered by steep, rocky, mountain slopes (fig. 1). On July 31, 

1976, a moist airmass began pushing westward from the Great Plains 

on the east side of the Rockies. During the afternoon, the moist air rose 

up the mountain slopes and the unstable air began to build into thunderstorms; a schematic illustration showing the cause of the storm and 

flood is provided in figure 2. Large thunderstorms formed along the 

Front Range and began to dump heavy rain on the region about 6:30 p.m. 

This event turned deadly when high-altitude westerly winds, which are 

usually strong enough to push thunderstorms eastward and out of the 

area, were unusually weak. The thunderstorms stalled for more than 3 

hours over the Big Thompson Canyon, and built into a gigantic thunderstorm system. The thunderstorms produced as much as 12–14 inches of 

Figure 1. Digital Elevation Model (DEM)

perspective view with draped digital orthophotography of the middle Big Thompson Canyon

and surrounding area. Modified from Jarrett 

and Vandas (2006). Image created by Paco Van 

Sistine, USGS.

Printed on recycled paper

Glen Haven

Estes Park

Big

Glen Comfort

North Fork

Big Thompson River

Thompson

Drake

River

Loveland

Lake Estes

Mouth of canyon

Estes Park

Loveland

Big

Thompson

River

Glen Comfort

Glen Haven

Drake North Fork

Big Thompson River

EAST

Lake Estes

Mouth of canyon

FOOTHILLS

FRONT RANGE

GREAT PLAINS

Big Thompson Flood, Colorado —
rain from July 31 to August 1 (fig. 3), although most rain 

fell in the first 4–6 hours (U.S. Geological Survey, 1979). The total rainfall from this event is nearly equivalent to a year’s average annual precipitation in this area. To place the infrequent nature of the 1976 rainstorm amounts in perspective, the 100-year, 6-hour duration rainfall is about 3 inches in the area of heaviest rainfall (http://www.noaa.gov). The storm was limited to a narrow band 5–10 miles wide extending from just southeast of Estes Park (fig. 3) northward to the Wyoming border. The storm was located just east and downstream from Olympus Dam in Estes Park, which is at an altitude of about 7,500 feet. Rainfall estimated from radar and observation data indicated that about 7.5 inches of rain fell in about 70 minutes near Glen Comfort in the heart of the canyon below Estes Park, turning the normally placid Big Thompson River and tributaries into a raging torrent of water. People had only minutes to escape to higher ground before cars, roads, trees, and buildings were damaged or washed away. 

Peak discharges were extremely large on many streams 

in and downstream from the storm’s rainfall center. The peak discharge estimated at the streamflow-gaging station located on the Big Thompson River at the mouth of the canyon near Drake was 31,200 ft3/s (cubic feet per second); 

this was almost 4 times the previous maximum discharge of 8,000 ft3/s since record keeping began in 1887 (table 1). 

River depths went from a few feet to nearly 20 feet, and peak discharges occurred in less than an hour along many reaches of the Big Thompson Canyon (fig. 4). The peak discharges for three tributaries near the area of heaviest rainfall exceeded previously recorded maximum discharges for basins less than 4 square miles in Colorado (U.S. Geological 

Survey, 1979). Flood recurrence intervals were far in excess of the estimated 100-year flood, and in some areas this was the largest flood since the retreat of the glaciers about 10,000 years ago (Jarrett and Costa, 1988). Spatial variability of runoff was extreme; on the Big Thompson River at the Estes Park gage, the peak discharge of 457 ft3/s (table 

1) was less than a 2-year flood. Once the floodwaters flowed into the wide floodplain downstream from the mouth of the Big Thompson Canyon, attenuation reduced the peak discharge to 2,440 ft3/s (recurrence interval of about 10 years) in the Big Thompson 

River at the confluence with the South Platte River (table 1). The flood in the Cache la Poudre River at the mouth near the LaSalle gage originated over sparsely populated areas northwest of Fort Collins; flood damage was less extensive and, fortunately, no deaths occurred there (U.S. Geological Survey, 1979).

Probably the most destructive c

the combination of the sudden rise floodwater velocities, and the maxivelocities as much as 20–25 feet peconstricted tributaries and canyonssevere channel erosion and transponearly everything in their path (figswas 12×12×23 feet and weighed apical Survey, 1979). The sudden nigsurprise, thus they were unaware oconditions, and some people were umade after the flood showed that pin the canyon had the greatest chanthose who attempted to escape in aat the highest risk (Gruntfest, 1977

What did the Big Thompson flo

about flash floods?

If you tried to escape the flood 

located in the floodplain, the cMany roads followed the narrand evacuating in a car often lraging water, which washed oflood cautions people to climb 

People underestimate the pow

to misjudge the depth of watedepth of only 1–2 feet is dangfloat automobiles into the rivein deeper water.

••

Figure 2. Schematic diagram showing what caused the Big Thompson flood. Modified from the 

University Corporation for Atmospheric Research.

1 Easterly winds 

pushed very humid air upthe mountains.

2 The humid air continued rising, forming 

large thunderstormsthat produced heavy rain.

3 Weak westerly winds above 

10,000 feet were unable to move the thunderstorms, thus the rain was concentrated in a small area.

4 Hardly any of the rain soaked 

into the steep-sided canyon, thus the river rose quickly over its banks.

What caused theBig Thompson flood?

The causes of the July 31, 1976, Big Thompson flood—about 50 miles northwest of Denver—were similar to those of other flash floods, especially a flood in the South Dakota Black Hills on June 9, 1972, which killed 238 people.

5 The debris-choked floodwater 

destroyed most everything in its path.

6 As the floodwater 

squeezed through the narrow canyon, its velocity and destructiveness increased.

Site identifier 

(see. fig. 3) Station name Station number Drainage area (mi2)b Peak flows before July 1976 Flood of July 31, 1976 Peak flows after July 1976 Period of record Year Streamflow (ft3/s) Streamflow (ft3/s) Date Streamflow (ft3/s)

A Big Thompson River below Moraine Park near Estes Park 402114105350101 39.8 1996-97 2001-06 -- -- -- 05-31-2003 828B Big Thompson River at Estes Park (a) 06733000 137.0 1946-76 1949 1,660 457 06-18-1995 1,870 (c)

C Big Thompson River tributary below Loveland Heights none 1.4 -- -- -- 8,700 -- --

D Big Thompson River above Drake none 189 (34) -- -- -- 28,200 -- --

E North Fork Big Thompson River tributary near Glen Haven none 1.4 -- -- -- 9,670 -- --

F North Fork Big Thompson River at Drake (a) 06736000 85.1 1947-76 1965 1,290 8,710 05-30-1995 572

G Big Thompson River at mouth of Canyon, near Drake (a) 06738000 305 (150) 1887-76 1919 8,000 31,200 04-30-1980 6,150

H Big Thompson River at mouth, near LaSalle (a) 06744000 828 (673) 1914-15 1927-76 1951 6,100 2,440 05-31-1995 6,710

Table 1. Comparison of 1976 peak discharges with other peak discharges for selected current (2006) U.S. Geological Survey and Colorado Department of Water Resources streamflow-gaging stations and miscellaneous 

sites in the Big Thompson River basin, Colorado. Modified from U.S. Geological Survey (1979).

[--, not applicable or no data.]

aOperated by Colorado Department of Water Resources.bWhere known, 1976 estimated contributing drainage areas are shown in parentheses.cPeak discharge was 5,500 ft3/s from the 1982 Lawn Lake dam failure.

2pgspread.indd 4-6 pgspread.indd 4-6 
10

9

8

7 6 5

4

3

2

1

The Big Thompson flood caused geologists to look at the stratigraphy 

of sediment deposits in the canyon, and from them to try to interpret the 

occurrence and frequency of paleofloods, particularly in ungaged basins, 

and to use such information to better understand rare floods such as the Big 

Thompson flood. This work helped foster and improve the methods of the 

emerging science of paleoflood hydrology to improve the understanding of 

flood frequency and flood-hazard mitigation (Jarrett and Costa, 1988).

Colorado Flood History 

Flash flooding is just one of three main types of floods in Colorado 

(Jarrett and Costa, 1988; Collins and others, 1991). Another type is 

snowmelt flooding, which predominantly occurs in May and June of 

each year but usually does not produce substantial flooding. A hazard 

of such flooding is the possibility of being swept away in cold, fast 

moving water. The third type is long-duration low-intensity rainfall 

associated with frontal activity. Such rainfall occurs over large areas 

and can produce widespread flooding in Colorado, particularly in large 

drainage basins (Collins and others, 1991).

Although rainstorms and flash floods the magnitude of the Big 

Thompson flood are infrequent in any one location, they are common 

from May to October in much of Colorado. Other notable catastrophic 

flash floods in Colorado occurred in July 1896 and September 1938 in 

the Bear Creek Basin (foothills west of Denver), October 1911 in the 

San Juan River Basin, June 1921 in the Arkansas River Basin, May 

1935 in the Republican and South Platte River Basins, June 1965 in the 

South Platte and Arkansas River Basins, and July 1997 in Fort Collins 

(Collins and others, 1991; http://ccc.atmos.colostate.edu/~odie/rain.

html). Many smaller thunderstorms produce locally hazardous flashflood conditions each summer. It has been estimated that each year, 

on average, Colorado experiences at least one-hundred-fifty 100-year 

or larger rainstorms, many of which are very localized (N.J. Doesken, 

Colorado Climate Center, written communication, 2006). Flooding is 

particularly common in urbanized areas (even in areas with little topography) where impervious areas can produce hazardous flash flooding 

as suddenly as the 1976 Big Thompson flood, but with as little as a few 

inches of rain in an hour. Over 100 dam-failure floods have occurred in 

Colorado, such as the1982 Lawn Lake Dam failure (Jarrett and Costa, 

1986). Extremely hazardous flash floods can also occur in recently 

burned wildland fire areas, which if unburned would not produce substantial flooding during intense rainstorms. 

•

Figure 4.  Hydrograph showing the discharge of the Big Thompson River at the

mouth of the canyon near Drake (site G in table 1 and on fig. 3); the hydrograph

is estimated because the gage was destroyed by the record flood. Modified from 

U.S. Geological Survey (1979).

Figure 5 (Above).  Damaged and destroyed homes and extensive flood-deposited sediment along the Big Thompson River.

This photograph was taken from the hillside looking downstream at Drake. Riverflow is from bottom right to top center.

Figure 6 (Left).  Remains of an automobile flattened and

wrapped around a large boulder in the North Fork Big Thompson River about 4.5 miles upstream from Drake. River flow is

from right to left. The U.S. Army Corps of Engineers removed

197 flood-damaged vehicles in the North Fork Big Thompson

and Big Thompson Rivers.

12 11

10

9

8

7 6 5

4

3

2

1

11

10

9

8

7 6 5

4

3

2

1 12

35,000

30,000

25,000

20,000

15,000

Peak discharge

31,200 cubic feet per second

Observer readings

10,000

5,000

0

12:00

Midnight

12:00

Midnight

12:00

Noon

JULY 31, 1976

DISCHARGE, IN CUBIC FEET PER SECOND

6:00 am 6:00 pm

Recorded

8:50 pm

9:40 pm

Estimated
Flash floods can develop with almost no warning. Therefore, it is important to know what to do before and during flash floods. 

BEFORE a flash flood:

Identify safe areas in your home or wherever you are located; bring outdoor possessions indoors or secure them if time permits.

Have a portable 3-day emergency supply kit (such as food, water, medical supplies, clothing, rain gear, portable radio, 

flashlights, blankets).

Monitor local radio, television, and National Weather Service (NWS) weather radio for potential adverse weather conditions.

Be familiar with the NWS’s three types of flash flood watches, warnings, and advisories for specific areas: Flash Flood 

Watch (heavy rains are occurring or expected to occur that could produce flash flooding), Flash Flood Warning (flash 

flooding is occurring or imminent), and Small Stream and Urban Flood Advisory (minor flooding is occurring or expected 

that could produce flash flooding).

Be aware of nature’s environmental warning signs (such as dark clouds, heavy rain, wind, and sounds—trees breaking and 

loud roars).

DURING flash-flood conditions:

Stay away from streambeds and other low areas.

Climb to higher ground and a safer location.

Don’t walk or drive in flooded areas; avoid downed power lines, gas leaks, wild animals, and snakes in and near flooded 

areas.

Notify local authorities of flash flooding, if you can safely do so.

•

•

•

•

•

•

•

•

•

Advances in Flood Protection and Warning

In the 30 years since the Big Thompson flood, there have been 

important advances in streamflow monitoring and flood warning. The 

National Weather Service’s (NWS) NEXRAD radar allows real-time 

monitoring of precipitation in most places in the United States (http://

www.roc.noaa.gov/). The USGS operates 270 real-time streamflowgaging stations on Colorado streams and rivers (http://co.water.usgs.

gov/), and the Colorado Department of Water Resources also operates more than 300 real-time gages (http://water.state.co.us/) that are 

monitored by the USGS, the NWS, and emergency managers. When 

substantial flooding occurs, the USGS mobilizes personnel to collect 

streamflow data in affected areas. Streamflow data improve flood forecasting and provide data for flood-frequency analysis for floodplain 

management, design of structures located in floodplains, and related 

water studies. Currently (2006), there are nine real-time streamflowgaging stations in the upper Big Thompson River Basin operated by the 

USGS and Colorado Department of Water Resources.

As a result of the June 1965 South Platte River flood that severely 

affected the Denver area, the Urban Drainage and Flood Control 

District (UDFCD) was established for the purpose of assisting local 

governments in the Denver metropolitan area with multi-jurisdictional 

drainage and flood-control problems. The 

District covers an area of 1,608 square 

miles that includes about 1,600 miles of 

major drainageways and serves a population of approximately 2.3 million people.

Soon after the 1976 Big Thompson flood, 

the District set up an early flood detection 

and local flood warning program. Currently 

(2006), UDFCD operates an automated 

flood detection network comprised of 175 

ALERT gaging stations (including 152 rain, 

87 stream stage, 18 weather). Other large 

cities such as Fort Collins and Colorado 

Springs have similar flood warning and 

floodplain management programs. However, 

most Colorado streams are ungaged with 

their flood potential being monitored by the 

NWS using rainfall-runoff models and flash 

flood guidance (http://www.noaa.gov). 

Figure 8 (Right).  This house was built on

an alluvial fan that was produced by sediment discharged from a side canyon to the

Big Thompson River. Floodwaters from the

side canyon damaged part of the house and

pushed it about 4 feet off its foundation.

Figure 7 (Above).  Upstream view of the

Big Thompson River showing extreme flood

damage to U.S. Highway 34 upstream from

Drake. The U.S. Geological Survey estimated

the flood’s peak discharge to be 28,200 cubic

feet per second in this area.
Today, flood specialists and emergency responders recognize that 

awareness of flooding is a combination of weather preparedness and 

personal responsibility. Before flooding occurs, residents or visitors 

need to know if buildings are in flood-prone areas, the dangers along 

stream channels, and if roads may become impassible. In addition, 

during floods, law-enforcement officials might not be able to drive 

through flooded areas to provide warnings that people may expect. 

Flood warnings, such as NWS watches and warnings, UDFCD’s 

ALERT data, and Colorado Department of Water Resources and USGS 

real-time rainfall and streamflow data, may not be available particularly in remote mountainous areas. Communications may be disrupted 

during extreme weather events such as floods, tornadoes, and hailstorms.

Nature often provides early warning signs of impending hazardous 

flooding. Knowledge and observation of developing thunderstorms 

(fig. 9) can provide important early warning in areas subject to lightning, hail, tornadoes, and flood-producing rains. In some cases, such as 

during the July 1976 flood in the Big Thompson Canyon downstream 

from Drake near the canyon mouth, there is little or no rainfall that 

might alert people that floodwaters are advancing. Once a flood (meteorologic or dam failure) or debris flow begins, many eyewitnesses 

often mention hearing rumbling sounds (described as trees breaking, 

jet airplanes, locomotives, and other loud roars) moving closer to 

their location. A combination of personal responsibility, planning and 

awareness, National Weather Service and other local flood agency programs and resources, and personal observations of developing weather 

conditions are vital to prepare people to help protect themselves when 

hazardous flooding is imminent.

Acknowledgments

The authors appreciate thorough technical reviews by J.P. Capesius, 

N.J. Doesken, W.J. Graham, C.A. Perry, K.G. Stewart, and S.J. Vandas, 

which improved the fact sheet. 

References

Collins, D.L., Doesken, N.J., and Stanton, W.P., 1991, Colorado floods and 

droughts: U.S. Geological Survey Water Supply Paper 2375, p. 207–214.

Costa, J.E., 1978, Colorado Big Thompson flood—Geologic evidence of a rare 

hydrologic event: Geology, v. 6, no. 10, p. 617–620.

Costa, J.E., and Jarrett, R.D., 1981, Debris flows in small mountain stream channels of Colorado and their hydrologic implications: Association of Engineering 

Geologists Bulletin, v. 18, p. 309–322.

Gruntfest, E.C., 1977, What people did during the Big Thompson Flood: Natural 

Hazards Research and Applications Information Center, Working Paper, 62 p.

Jarrett, R.D., 1987, Peak discharge errors in slope-area computation in mountain 

streams: Journal of Hydrology, v. 96, no. 1–4, p. 53–67.

Jarrett, R.D., and Costa, J.E., 1986, 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, 79 p.

Jarrett, R.D., and Costa, J.E., 1988, Evaluation of the flood hydrology in the Colorado Front Range using streamflow records and paleohydrologic data for the 

Big Thompson River Basin: U.S. Geological Survey Water-Resources Investigations Report 87–4117, 37 p.

Jarrett, R.D., and Vandas, S.J. (compilers), 2006, 1976 Big Thompson Flood, Colorado: U.S. Geological Survey General Information Product 35, 1 sheet (poster).

Simons, D.B., Nelson, J.D., Reiter, E.R., and Barkau, R.L., 1978, Flood of 31 July 

1976 in Big Thompson Canyon, Colorado: National Academy of Sciences, 

Washington, D.C., 85 p.

U.S. Geological Survey, 1979, Storm and flood of July 31–August 1, 1976, in the 

Big Thompson River and Cache la Poudre River Basins, Larimer and Weld 

Counties, Colorado: U.S. Geological Survey Professional Paper 1115, 152 p.

Figure 9. Growing thunderstorm clouds

and rain forewarn of potentially severe

weather conditions including lightning,

intense rainfall, hail, flooding, and tornadoes.

Monitoring the development and movement

of such clouds, as well as awareness of

weather forecasts, can help people prepare

for potentially hazardous conditions. Photograph courtesy of Lester Zinser, University 

Corporation for Atmospheric Research.

For more information on USGS science: http://www.usgs.gov; http://water.usgs.gov/osw/; http://ks.water.usgs.gov/Kansas/floodsummary; http://water.usgs.gov/usgs/osw/FS-List-2003.html

For more information on floods and Colorado flood hydrology: http://www.fema.gov; http://www.usace.army.mil; http://www.usbr.gov/pmts/flood/; http://www.udfcd.org/; http://cwcb.state.co.us; 

http://www.casfm.org; http://floodsafety.org/colorado/index.htm

This fact sheet (FS-2006–3095) and accompanying poster (General Information Product 35) and other publications can be ordered from the USGS Store at http://store.usgs.gov or call 1-888-ASK-USGS 

(1-888-275-8747).

Authors: Robert D. Jarrett and John E. Costa

Photographs by R. D. Jarrett, except as noted otherwise

Edited by F. Craig Brunstein, Denver Publishing Service Center, Graphics and layout by Carol A. Quesenberry, Denver Publishing Service Center