FLOOD02855

• County: Larimer
• Community: Larimer County
• Stream Name: Big Thompson River
• Basin: South Platte
• Title: Big Thompson River Flood Report
• Date: 10/1/1976
• Prepared For: Larimer County
• Prepared By: UDFCD
• Floodplain - Doc Type: Flood Documentation Report

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ABSTRACT

As much as 12 inches (305 millimeters) of rain
fell on the Big Thompson River basin, a favorite
summer-home and vacation area in Colorado, during
the evening of July 31, 1976, causing a devastating flood
on the Big Thompson River and its tributaries between
Estes Park and Loveland, Colo. At the latest count
(October, 1976), Larimer County officials reported 139
persons lost their livcs, with 5 still reported mlssmg,
and property damage of $16.5 million. .
Dcscriptions of the storm and flood, peak dIS-
charges, flood elevations, photographs of flooded areas,
and aerial photographs of the Big Thompson and the
North Fork Big Thompson Rivers, outlining inundated
areas, are included in this report to assist public offi-
cials and private citizens in planning for reconstruc-
tion of the roads, homes, and vacation areas in the Big
Thompson River basin.

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INTRODUCTION

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Throughout Colorado, August 1, 1976, was planned
as a day of celebration to commemorate 100 years of
statehood. In spite of the elaborate plans for "Cen-
tennial Sunday" in Big Thompson Canyon, the cele-
bration was not to be, for tbe dawn of that day re-
vealed a scene of tragedy never before experienced
during the State's bistory. During the evening hours
of July 31, a series of violent thunderstorms released
large volumes of rain along a path sevcral miles wide
from Estes Park to the Wyommg border. The BIg
Thompson River basin (fig. 1) west of Drake was es-
pecially hard hit by the storm; devastating flooding
occurred along the Big Thompson River between Estes
Park and Loveland, and along tbe North Fork Big
Thompson River from Glen Havcn to its mouth at
Drake. Tbe flood lasted only a few hours but during
this time an appalling amount of death and destruction
occurred along the Big Thompson River and its tribu-
taries. The area is a favorite summer-home and vaca-
tion area in Colorado and many people had permanent
homes along the streams. Based on the latest figures
available from Larimer County officials (October, 1976),
139 deaths have been confirmed, 5 names remain on
the list of missing persons, and about $16.5 million of
property damage has been reported.
This report, a joint effort of the U. S. Geological
Survey and the Colorado Water Conservation Board,
was prepared to assist public officials and private citi-
zens in planning for reconstruction of the flooded area
in the Big Thompson River basin. The report con-
tains the following:

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THE BIG THOMPSON RIVER FLOOD OF
JULY 31-AUGUST 1, 1976,
LARIMER COUNTY, COLORADO

By Richard U. Grozier and Jerald F. McCain
U. S. Geological Survey

and

Larry F. Lang and Danny C. Merriman
Colorado Water Conservation Board

1. A map showing precipitation volumes and a
discussion of the conditions associated with the
storm.
2. Photographs of pre- and post-flood conditions at
two locations that vividly portray the vast
amount of destruction that occurred.
3. Photographs that illustrate the enormous dam-
age caused by the flood.
4. A table of peak discharges and drainage areas
at selected locations in the flood area.
5. A table of high-water clcvations and cross-sec-
tion data at selected locations in the Big Thomp-
son River basin.
6. Aerial photographs showing flood boundaries
along the Big Thompson River and the North
Fork Big Thompson River.
The information contained in this report is limited
to the Big Thompson River basin except for precipita-
tion amounts and peak discharges at several other lo~
cations in Larimer County.

ACKNOWLEDGMENTS

The aid of many individuals, corporations, State
and governmental agencies who furnishcd data and
assistance for the preparatiun of this report is grate-
fully acknowledged.
The Colorado Dcpartment of Natural Resources,
State Engincer's Office furnished funds and assistance
to the U. S. Geological Survey and the Colorado Water
Conservation Board in the collection of data and out-
lining the flooded areas.
The National vVeather Service made precipitation
surveys to supplement their regular rain-gage network.
The U. S. Bureau of Reclamation furnished assist-
ance to the National Weather Service in the collection
of rainfall data and furnished funds to the Geological
Survey for thc collection of streamflow data.
The U. S. Army, Corps of Engineers developed a
flood profile in parts of the canyon and summarized
flood damages. They also furnished funds to assist
the Geological Survey in collecting the .field data and
preparation of this rcport.
Collection of field data, necessary for thc compu-
tation of peak discharges by indirect methods, was
greatly aided by personnel detailed from the South
Dakota, Arkansas. and Wyoming offices of the Geo-
logical Survey.

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GENERAL METEOROLOGICAL CONDITIONS

The National Weather Service prepared an iso-
hyetal map (fig. 1) of the total precipitation from July
31 to August 2, 1976. They also provided the follow-
ing statement on the general meteorological conditions
existing in the area that L'Ontrihuted to the extreme
rainfall and runoff in the Big Thompson River basin.

"Eastern Colorado was under conditions favorable
for heavy rain on July 31, 1976 for a number of
reasons. The surface map of that morning showed
a slowly moving cold front in the State. Such
fronts are lines of convergence that lift air to form
thunderstorms. Also favorable was the east wind
just north of the front, moving air upslope and
aiding the frontal lifting.
"The low-level air was very moist, well above the
seasonal normals, and the moisture aloft was also
unusually high. These factors combined to give
stability conditions unusually favorable for thun-
derstorms.
"That these are valid conditions for thunderstorms
was soon borne out by radar and satellite data
when a line of thunderstorms developed in ex-
treme east-central Colorado and quickly extended
eastward paralleling and just north of the cold
front. The thunderstorms extended westward less
rapidly until early in the evening when a sudden
explosive thunderstorm developed on or just west
of the front range of the mountains southeast of
Estes Park. The cause of such a strong develop-
ment at this place and this time is not yet fully
understood.

"Thunderstorms move with the speed and direction
of the winds aloft, and the 500-mb (millibar) level
is usually auequate for judging such movement.
The 500-mb wind was only about 5 knots and
was not expected to change much during the day.
This was the case with the thunderstorms near
Estes Park. They moved very slowly while putting
out large amounts of water over a period of
several hours."

Rainfall began about 6:30 p.m. on Jnly ,31, 1976,
and ended abollt ] 1:.30 p.m. that evening. Additional
rainfall fell on August 1 and 2. Precipitation totals
were as much as 10 inches (254 mm) bctween Estes
Park and Drake and more than 12 inches (305 mm) in
tIle Clen Havcn area. Very little rainfall contributing
to the flood occurred east of Drake and west of Estes
Park.