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3 thoroughly explored. Phase II was considered most important because the current criteria available for developing probable maximum precipitation in the study area are limited to watersheds with drainage areas less than 5,000 square miles. The total cost of determining the PMF, including all five parts of the meteorologic investigation and the accompanying hydrologic studies, would be approximately $1,250,000 and would require about 3.5 years to complete. Because of the high estimated study cost, an alternative method for determining the snowpack and snowmelt runoff was developed. Phase II and the initial project specifications were the only portions of the pas undertaken. The Hoover Dam and Glen Canyon Dam PMF study combines the design rainstorm (ULDRS) and the 100-yr snowmelt flood to obtain an estimate of the PMF. The hydrology study began in January 1987 and involved active participation by Reclamation's Lower Colorado and Upper Colorado Regions and Denver Office and by Morrison-Knudsen Engineers. Study leadership was provided by the Flood Section at the Denver Office. A field reconnaissance of the subbasins around Lake Mead and upstream of Lake Powell was completed in June 1987. The regional hydrology staffs were responsible for subbasin delineation and map preparation, and assisted in subbasin hydrologic parameter estimation. The Upper Colorado regional staff prepared the preliminary data for the 100-yr snowmelt flood analysis. All of the offices played a role in preparing the necessary computer files and data for testing and final use of the meteorological analysis. 1.3 BASIN DESCRIPTION The Colorado River above Hoover Dam drains an area of 167,000 square miles. The drainage basin includes parts of Wyoming, Colorado (western half), Utah (eastern half), New Mexico, Arizona, and Nevada, as shown on figure 1.1. Approximately 108,000 sq. mi. of the drainage basin are above Glen Canyon. The river originates in the Rocky Mountains of Colorado and Wyoming and flows south-southwest about 800 river miles to Hoover Dam. The basin is about 600 miles in length from its northern boundary in the Wind River Range in Wyoming to its southern boundary at Hoover Dam and varies in width from 300 miles in the upper section to 500 miles in the lower section. The basin boundary consists of mountains that are 13,000 to 14,000 feet high in Wyoming, Colorado, and Utah; the boundary drops to elevations of less than 1,000 feet at Hoover Dam. The northern portion of the basin, in Colorado and Wyoming, is a mountainous plateau that ranges 5,000 to 8,000 feet in elevation. This plateau encompasses deep canyons, rolling valleys, and intersecting mountain ranges. The central and southern portions of the basin, in eastern Utah, northwestern New Mexico, and northern Arizona, consist of rugged mountain ranges interspersed with rolling plateaus and broad valleys. In general, the mountains in the southern part of the basin are much lower than those in the northern part. Much of