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METEOROLOGY. HYDROLOGY. BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 3 search, rescue, and readjustment, as the death toll and damage estimates continued to rise, it became obvious that the flood would be classified as Colorado's worst natural disaster. The official tabulation by Larimer County officials lists 139 deaths, 5 persons reported as missing, and about $35.5 million in property damage. ACKNOWLEDGMENTS The data and interpretations contained in this report were obtained through the combined efforts of many individuals and local, State, and Federal agencies. This assistance, including factual information and financial support, is gratefully acknowledged. Because of the sparsity and destruction of recording instruments, much of the hydrologic and meteorologic data were provided by residents of the area. Also, ac- cess to private property was readily granted to the many field personnel working in the flooded area. Sincere appreciation also is extended to Larimer Coun- ty officials and to local municipal officials for their aid and patience during the data-collection period. The Colorado Department of Natural Resources, Division of Water Resources, Office of the State Engineer furnished funds to support data-collection ac- tivities and furnished streamflow data for gaging sta- tions operated by that agency. The Colorado Water Conservation Board of the same department developed flood and streambed profiles for the affected streams, assisted in the collection of other hydrologic data, and participated in the preparation of a basic flood-data report. Appreciation also is extended to the Colorado National Guard and private firms for providing helicopter transportation during the fieldwork. The Department of Atmospheric Science, Colorado State University, provided meteorological data for Fort Collins. Dr. C. Glenn Cobb supplied meteorological data for Greeley, measured at Ross Hall, University of Northern Colorado. John M. West of Rockwell International Corp. obtained the meteorological data for the Rocky Flats plant near Boulder. The National Center for Atmospheric Research Field Observing Facility obtained and sup- plied the invaluable sounding, radar, and surface data from the National Hail Research Experiment site in northeastern Colorado. The U.S. Army Corps of Engineers collected hydrologic data for parts of the area and provided estimates of flood damage. The Corps also provided funds for collection and reporting of hydrologic data. The U.S. Bureau of Reclamation assisted the Na- tional Weather Service in the collection of rainfall data and provided funds for the collection and analysis of hydrologic data. The Bureau also provided hydrologic data, including an independent estimate of the peak discharge at the mouth of Big Thompson Canyon. THE SETTING LOCATION The area from which the flood of July 31-August 1 originated lies in central Larimer County along the Front Range of the Rocky Mountains (fig. 2). The southern limit of flooding was near Estes Park about 50 miles northwest of Denver, while the northern limit was approximately 50 miles farther north near the Wyoming border. In the southern part of the storm area, the band of intense rainfall was only about 6 miles wide with the western edge located near Estes Park and the eastern edge located about 2 miles west of Drake. The western edge of the storm near the Wyoming border was located just west of Virginia Dale, and the eastern edge was about 10 miles east of Virginia Dale. According to information from the U.S. Army, Corps of Engineers (1976), rainfall of 5 inches or greater covered an area of more than 1,000 square miles during the storm period. The southern part of the flooded area is drained by the Big Thompson River and the northern part, by the Cache la Poudre River. Both streams flow in a general- ly eastward direction through Larimer and western Weld Counties to join the South Platte River in the vicinity of Greeley. Principal tributaries affected by flooding in the Big Thompson River basin were the North Fork Big Thompson River which enters at Drake, Buckhorn Creek which drains the northern part of the basin, and the Little Thompson River, which heads just southeast of Estes Park. The Little Thomp- son River flows eastward and generally parallels the main stem to the confluence near LaSalle. The North Fork Cache la Poudre River is the principal tributary of the Cache la Poudre River in the flooded area. Several small tributaries in the Bellvue area northwest of Fort Collins, including Rist and Soldier Canyons, also received severe flooding. TERRAIN FEATURES RELATED TO FLOODING The intense rains of July 31 fell on a part of the Col- orado Front Range, commonly referred to as the foothills area. This area is underlain by a complex assortment of metamorphic rocks of Precambrian age with numerous intrusives of igneous origin. Many faults and shear zones also complicate the bedrock geology of the area and appear to exert considerable