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. Alternative flash flood detection systems are formulated for the mDuntain portion of the Cache la Poudre River basin. DESCRIPTION OF THE STUDY AREA GEOGRAPHY The Cache la PDudre River basin is located in north-central Colorado. A major left-bank tributary of the SDuth Platte River, the Cache la Poudre River originates within Rocky Mountain National Park. It drains an area of 1,890 square miles, and flows about 121 miles from its origin to its confluence with the South Platte River about 5 miles east of Greeley. Except for a small area in Wyoming, almost all of the basin area is in Larimer and Weld Counties in Colorado. The basin is shown on plate 1. The western portion of the basin lies within the southern Rocky Mountain Province of the Rocky Mountain system. Precambrian age rDcks were uplifted and expDsed throughout the Rockies. The eastern part of the basin lies within the Great Plains Province of the Interior Plains. Underlying the plains are sedimentary sandstones, shales, and limestone of cretaceous to tertiary age. During recent geologic time, stream erosion, glaciation, slope movement, and wind action produced flood plains, terraces, and the overlying mantle. . Most of the basin west Df Fort Collins is mountainous. The drain- age area at the point where the Cache la Poudre River emerges from the mountains is 1,055 square miles. This area is very rugged, featuring steep stream slopes and narrow valleys. Elevations in the mountains range from about 6000 feet to as high as 13,500 feet mean sea level (m.s.l.) Local relief often ranges from 500 to 1,500 feet. The plains east of the mountains are characterized by rolling hills interspersed with extensive areas of gently sloped plains. Elevations in the plains range from about 6000 feet m.s.l. at the foothills to about 4600 feet m.s.l. where the Cache la Poudre River joins the South Platte River. 3