Ground-water hydrology of the San Pitch River drainage basin, Sanpete County, Utah

The San Pitch River drainage basin in central Utah comprises an area of about 850 square miles; however, the investigation was concerned primarily with the Sanpete and Arapien Valleys, which comprise about 250 square miles and contain the principal ground-water reservoirs in the basin. Sanpete Valley is about 40 miles long and has a maximum width of 13 miles, and Arapien Valley is about 8 miles long and 1 mile wide. The valleys are bordered by mountains and plateaus that range in altitude from 5,200 to 11,000 feet above mean sea level.

The average annual precipitation on the valleys is about 12 inches, but precipitation on the surrounding mountains reaches a maximum of about 40 inches per year. Most of the precipitation on the mountains falls as snow, and runoff from snowmelt during the spring and summer is conveyed to the valleys by numerous tributaries of the San Pitch River. Seepage from the tributary channels and underflow beneath the channels are the major sources of recharge to the ground-water reservoir in the valleys.

Unconsolidated valley fill constitutes the main ground-water reservoir in Sanpete and Arapien Valleys. The fill, which consists mostly of coalescing alluvial fans and flood deposits of the San Pitch River, ranges in particle size from clay to boulders. Where they are well sorted, these deposits yield large quantities of water to wells.

Numerous springs discharge from consolidated rocks in the mountains adjacent to the valleys and along the west margin of Sanpete Valley, which is marked by the Sevier fault. The Green River Formation of Tertiary age and several other consolidated formations yield small to large quantities of water to wells in many parts of Sanpete Valley. Most water in the bedrock underlying the valley is under artesian pressure, and some of this water discharges upward into the overlying valley fill.

The water in the valley fill in Sanpete Valley moves toward the center of the valley and thence downstream. The depth to water along parts of the sides of the valley is more than 100 feet, but in much of the central part of the valley, the water level is at or above the land surface. The valley fill pinches out in the southern part of the valley, and most of the ground water moves to the surface, where it discharges into the San Pitch River or is consumed by evapotranspiration.

Ground water is discharged principally by wells, springs, and evapotranspiration. The discharge from wells varies considerably from year to year because most of the water is used for irrigation, and the wells are used only as necessary to supplement the available surface-water supply. Thus, in 1965, a year of above-normal precipitation, the discharge from wells was 12,000 acre-feet, whereas in 1966, a year of below-normal precipitation, the wells discharged 21,000 acre-feet. The discharge from springs during 1966 was estimated to be 36,000 acre-feet, and an additional 113,000 acre-feet of water was discharged by phreatophytes.

Water levels in the valleys, for the most part, fluctuate in direct response to variations in precipitation, and the discharge from wells has had little long-term effect on water levels. Approximately 3 million acre-feet of water available to wells is stored in the upper 200 feet of saturated valley fill.

The ground water in most parts of the valleys is fresh and suitable for public supply and irrigation. The Green River and Crazy Hollow Formations may, in some places, yield slightly or moderately saline water.