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m C\l 'l:t' Watershed Characteristics TopoQraphy and Geology West Rifle, Middle Rifle and East Rifle Creeks make up the triangular shaped watershed that drains into the reservoir proper. Elevation ranges from about 5,800 feet at the confluence of the creeks, to about 9,200 feet on the northwest and northeast summit lands. A large, gently rolling table land characterizes the upper part. Stream canyons dissect the area, be- coming steep and broken at lower elevations. The remainder of the water- shed comprises those lands lying south of the Grand Hogback to the Cactus Valley Canal north of Highway 6-24 and the west portion of Dry Elk Valley north of Harvey Gap Reservoir. Thick beds of Pennsylvanian Age limest0ne characterize the upper watershed. Sandstones and shales (mostly Permian format ions), dominate the eastern and southern portion. Mancos shale outcrops follow West Rifle Creek to the head and make up a sizable area in the southeast comer of the water- shed. Dakota and Jurasic sandstone and shale lies above the Maacos and occurs in the same area at slightly higher el evati ons. The Grand Hegback (Mesa Verde Age), flanks the west side of the watershed and divides the project land between Dry Elk Valley and Harvey Mesa. Valley floors are generally narrow. Near the reservoir pool area and for Short distances up West Rifle and East Rifle Creeks, alluvial bott01lls are wide enough to cultivate. The alluvial soils range from loam to clay loam and are derived from reworked sandstone and shale. The Harvey Mesa area consists of wind-lain and alluvial soils overlying Wasatch shales and sand- stone. Dry Elk Valley is a small upland valley of reworked loam and clay soil which runs eastward from Harvey Gap Reservoir. Precipitation and Runoff Annual precipitation varies from about eleven inches on the proJectis irri- gated lands to nearly thirty inches in the higher Dlountain areas. Approxi- mately fifty percent of the precipitation is received as snow in the winter. June is the driest month. Some high-intensi~ rainstorms occur in JulY$ August and September. Peak streamflows can be expected from sno_el t during Apr 11 and May, and from high intensity rains in July, August and September. VeQetative Cover Vegetative cover for the watershed, by principal type, is shown in table 23. _ <:'n _