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STORM AND FLOOD OF JULY 31-AUGUST I, 1976, IN THE BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS, LARIMER AND WELD COUNTIES, COLORADO METEOROLOGY AND HYDROLOGY IN THE BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS ByJERALD F. McCAIN oj the U.S. GEOLOGICAL SURVEY, and LEE R. HOXIT, ROBERT A. MADDOX, CHARLES F. CHAPPELL, and FERNANDO CARACENA oj the NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION ABSTRACT Devastating flash floods swept through the canyon aectIon of Larimer County in north-central Colorado during the night of July 31-August I, 1976, cauaing 139 deaths, 6 missing persona, and more than $35 million in total damages. The brunt of the storms 0c- curred over ..the Big Thompson River baain between Droke and Estes Park with rainfall amounts as much as 12 inches being reported during the storm period. In the Ceche Is Poudre River basin to the north, a rainfall amount of 10 inches was reported for one loealIty while 6 inches fell over a widespread area near the cen- tral part of the beain. The storms developed when strong low-level easterly winda to the rear of a polar front pushecj a moist, conditionally unstable airmass upslope into the Front Range of the Rocky Mountains. Orogrsphic nplift released the convective instability, and light south- southeasterly winds at middle and upper levels allowed the storm complex to remain nearly stationary over the foothiDa for several hours. Minimal entrainment of relatively moist air at middle and up- per levela, very low cloud bases, and a slightly tilted updraft struc- ture contrihuted to a hlgb precipitation efficiency. Intense rainfall hegan soon after 1900 MDT (Mountain Daylight Tlmel in the Big Thompson River and the North Fork Cache Is Pouclre River hssins. A cumulative rainfall curve developed for Glen Comfort from radar data indicates tbat 7 .5lnehes of rain fell during the period 1930-2040 MDT on July 31. In the central part of the storm area west of Fort Collins, the heaviest rainfall began about 2200 MDT on July 31 and continued until 0100 MDT on August 1. Peak discharges were extremely large on many streams in the atorm area-exeeeding previously recorded maximum diseharges at severalloeatioDS. The peak discharge of the Big Thompson River at the geging station at the canyon mouth, near Drake was 31,200 cubic feet per second or more than four times the previous max. imum discharge of 7,600 cubic feet per second at the site during 68 years of flood history. At the geging station on the North Fork Big Thompson River at Drake, the peak diseharge on July 31 was 8,710 cubic' feet per second as compared to the previous maximum discharge during 29 years of record of 1,290 cubic feet per second. Peak diseharges for three small tributaries near the area of heaviest rainfall northeast of Estes Park exceeded previously recorded max- imum diseharges for baains of less than 4 square miles in Colorado. Stream velocities were rapid along the tnbutaries near the storm center and on the Big Thompson River in the canyon section, with average velocities of 20-25 feet per second heing common. The flood crest on the Big Thompson River moved through the 7.7-mlIe resch between Drake and the canyon mouth in about 30 minutes for an average travel rate of 15 mlIes per hour, or .bout 23 feet per second. The peak discharge of the flood on the Big Thompson River at the canyon mouth exceeded the l00-year flood discherge for the site by a ratio of 1.8. Upstream in the Big Thompson River basin, the flood was even more rare being 3.8 times the eetimeted 10o-year flood diseharge at the site on the Big Thompson River just upstream from Drake. In the Cache Is Poudre River basin, recurrence inter. vals were computed to he 100 years for the flood on Deadman Creek and 16 years for Rist Canyon and the Cache la Poudre River at the canyon mouth near Fort Collins. Althongh the rainfall and flood diseharges were unusually large, they are not unprecedented for some areas along the eastern foothills and plains of Colorado. The May 1935 and June 1965 floods on some streama along the eastern plains greatly exceeded the 1976 flood peaks in the storm area. Prior floods on several other streams in the foothiDa bave approximately equaled the 1976 peak discharges. INTRODUCTION During the night of July 81-August I, 1976, a com- plex system of thunderstorms produced intense rain- falls along the eastern foothills of the Rocky Moun- tains in northern Colorado (fig_ 1). Devastating flash floods quickly swept through several streams in the area causing an appalling amount of death and destruction. The purpose of this report is to present an analysis of the genesis, growth, and culmination of the severe storm system and the disastrous floods which followed. Coming on the eve of Colorado's 100th anniversary of statehood, the storm and flood quickly occupied the centerstage of attention. Centennial Sunday was still observed throughout most of the State but in a much subdued manner as the tragic news slowly filtered from a large area almost stripped bare of normal chan- nels of communication. During the ensuing days of 1 I~