The upper Duchesne River valley was studied during 1971-74 as part of an investigation of the northern Uinta Basin area, Utah and Colorado. The purpose of the study was to determine the relation of ground water to surface water, to estimate the quantity of ground water that moves to the Duchesne River, and to assess the probable effect of increased ground-water withdrawal on streamflow.

The primary source of water for the study area is precipitation on the highlands adjacent to and north of the area and on the valley itself. Discharge from the area is mainly by flow in the Duchesne River. Adjacent to and within the valley, ground water and surface water are intimately related, and they can interchange in several ways due to both natural and manmade conditions.

Aquifers in the upper Duchesne River valley range from Paleozoic to Quaternary in age. The consolidated aquifers receive recharge from highland precipitation and streamflow, and probably from interformational transfer of water. The consolidated rocks discharge water through springs and by interformational transfer of water to the valley fill of Quaternary age.

The valley fill, which is composed mainly of outwash and related glacial debris, constitutes the main ground-water reservoir in the valley. The fill is, in general, highly permeable and transmits water rapidly. It is recharged by a small amount of underflow beneath the Duchesne River and its tributaries, by intermittent precipitation directly on the fill, by interformational movement of ground water from the adjacent consolidated rocks, and by seepage of surface water from streams, canals, and irrigated fields. The ground water in the fill is unconfined. The volume of ground water stored in the fill and theoretically available by gravity drainage is a minimum of 40,000 acre-feet (50 cubic hectometres); this volume fluctuates by a maximum of 10 percent annually.

Ground water is discharged from the valley fill by wells and springs, by evapotranspiration, and by seepage into the Duchesne River. The discharge from wells and springs used for domestic, stock, public, and irrigation purposes in 1974 was about 2 cubic feet per second (0.06 cubic metres per second). The discharge by evapotranspiration was about 4 cubic feet per second (0.1 cubic metre per second). Discharge of ground water by seepage to the Duchesne River was about 39 cubic feet per second (1.1 cubic metres per second).

Most ground water, except in parts of the Uinta Formation, and all the surface water sampled in the study area, was fresh.

Because of the high permeability of the valley fill and because unconsumed ground water discharges to the Duchesne River, it can be concluded that lowering ground-water levels by large withdrawals of ground water in the upper Duchesne River valley ultimately would diminish the baseflow of the Duchesne River by about the amount of ground water with-drawn minus the amount salvaged from evapotranspiration.