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BOOK CLIFFS -~,:VSTAR P~~:~YS:S , l~ GARLEY 70N SS PRICE CITY , FARNHAM ANTICLINE ~ERB ~. ~_. PRICE RIVER (North and San Rafael Swell) ~-fiLUE GATE =~=~.?-1~1 \\\ ~--- (FERRON 55 Formations. These clastics grade eastward into the shales. As the Cretaceous Period f\jdrew to a close, central Utah emerged from ,-..the sea, and the later formations Bre all ..........nonma r i ne. CJ1 CJl The Price River headwaters in the Green River Formation. Most of the river flow, approximately 85 percent, originates in the Wasatch Plateau and from the Book and Roan Cliffs (Utah Division of Water Resources 1975). The river traverses the newer non- marine formations until reaching the Mancos Shales at Castle Gate. From there the river traverses the Mancos formations to Woodside. The three major formations of the Mancos Shales (Masuk, Blue Gate, and Tununk) are separated in places by the sandstone tongues (Figure 2.2). The marine shales are de- scribed as drab and slightly bluish-gray and contain Borne thick lenses of calcareous sandstone, limestone, and concretionary beds. The shales characteristically vary greatly in salt content and are relatively impermeable and erodable. Burge (1974) attributes the impermeability of the shales to the fineness of the contained clays and the rapid weather- ing to cyclic dehydration-hydration of the entrained salts, particularly mirabilite (Na2S04 . 10H20) and thenardite (Na2S04). At elevations above 7,000 feet, average annual precipitation varies between 30 inches and 12 inches and mostly occurs during the winter (Mundorff 1972). Precipitation on the river valley averages less than 10 inches annually, and most rainfall is during the late aummer. These summer and fall storms produce almost all of the surface runoff and erosion on the valley floor. Average pre- cipitation and temperature data for selected stations are given in Table 2.1. Summer storms are typically short duration thunderstorms while most winter precipitation comes from relatively low intensity frontal storms. During the winter, frontal storms from the Gulf of Alaska produce snowpacks in the surrounding uplands. Thunderstorms during the late summer months 2000' 0' 5 -M:l.les I o I develop as warm moist air from the Gulf of Mexico moves into the valley. Monthly distributions of precipitation at selected stations are given in Table 2.1. On the highest 30 percent of the area, about 65 percent of the precipitation falls from October through April, and most of it is snow, The spring melt provides irrigation water for agriculture. Streamflows Moat of the outflow from the Price River Basin originates as snowmelt. The summer thunderstorms are usually of short duration, localized, and intense. Surge flows can develop in the valley channels, eroding and transporting large masses of sediment. Most tributary streams become completely dry during low flow periods. Average annual yield for the Price River Basin ranges from less than 1 inch in the valley to Over 12 inches in the mountains (Figure 2.3). Although about 50 percent of the total basin is below 6,400 feet, only 10 percent of the total water yield originates from these lower elevations. Annual runoff from the Price River valley is estimated to be 1.08 inches or about 9 percent of the average annual precipitation of 11.7 inches. Streamflow in the principal streams is highly regulated. Moat summer flows are diverted for use within the basin. Scofield Reservoir (capacity 45,000 acre-feet), located near the headwaters of the Price River, stores runoff for release during the irrigation sesson. Jeppson et a1. (1968), using the Thorn- thwaite formula, estimated the evapotrans- piration for the valley to generally exceed 24 inches annually. This is about 2.5 times the precipitation, and thus irrigation is used to make up for the moisture deficient in agricultural aress. Water enters the valley floor from the river and tributaries and as imports. Approximately 28,000 acre-feet per year are imported from Huntington Creek Figure 2,2, Mancos Shale cross-section (taken from Williams 1975), 12