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

Abstract

PART A: Devastating flash floods swept through the canyon section of Larimer County in north-central Colorado during the night of July 31-August I, 1976, causing 139 deaths, 5 missing persons, and more than $35 million in total damages. The brunt of the storms occurred over the Big Thompson River basin between Drake and Estes Park with rainfall amounts as much as 12 inches being reported during the storm period. In the Cache la Poudre River basin to the north, a rainfall amount of 10 inches was reported for one locality while 6 inches fell over a widespread area near the central part of the basin. The storms developed when strong low-level easterly winds to the rear of a polar front pushed a moist, conditionally unstable airmass upslope into the Front Range of the Rocky Mountains. Orographic uplift released the convective instability, and light south-southeasterly winds at middle and upper levels allowed the storm complex to remain nearly stationary over the foothills for several hours. Minimal entrainment of relatively moist air at middle and upper levels, very low cloud bases, and a slightly tilted updraft structure contributed to a high precipitation efficiency. Intense rainfall began soon after 1900 MDT (Mountain Daylight Time) in the Big Thompson River and the North Fork Cache la Poudre River basins. A cumulative rainfall curve developed for Glen Comfort from radar data indicates that 7.5 inches 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 storm area-exceeding previously recorded maximum discharges at several locations. The peak discharge of the Big Thompson River at the gaging station at the canyon mouth, near Drake was 31,200 cubic feet per second or more than four times the previous maximum discharge of 7,600 cubic feet per second at the site during 88 years of flood history. At the gaging station on the North Fork Big Thompson River at Drake, the peak discharge 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 discharges for three small tributaries near the area of heaviest rainfall northeast of Estes Park exceeded previously recorded maximum discharges for basins of less than 4 square miles in Colorado. Stream velocities were rapid along the tributaries near the storm center and on the Big Thompson River in the canyon section, with average velocities of 20-25 feet per second being common. The flood crest on the Big Thompson River moved through the 7.7-mile reach between Drake and the canyon mouth in about 30 minutes for an average travel rate of 15 miles per hour, or about 23 feet per second. The peak discharge of the flood on the Big Thompson River at the canyon mouth exceeded the 100-year flood discharge 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 estimated 100-year flood discharge at the site on the Big Thompson River just upstream from Drake. In the Cache la Poudre River basin, recurrence intervals were computed to be 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. Although the rainfall and flood discharges 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 streams along the eastern plains greatly exceeded the 1976 flood peaks in the storm area. Prior floods on several other streams in the foothills have approximately equaled the 1976 peak discharges. PART B: Intense rainfall from the Big Thompson thunderstorm complex on the evening of July 31,1976, and the ensuing floods that evening and the fol