The upper Star Valley covers about 55 square miles of lowland in the westernmost part of Wyoming. The altitude of the floor of the valley is 6,000-6,700 feet. The climate is cool; the growing season, short. Annual precipitation averages about 18 inches, and total precipitation in July and August averages 2.2 inches. Additional supplies of water are needed for irrigation of pasture and hay. The principal water-bearing formation is a thick body of gravel of Pleistocene age. Consolidated to semiconsolidated sedimentary formations of Paleozoic to Tertiary age form the surrounding mountains and underlie the gravel. These bedrock formations yield small amounts of water to wells on the margins of the valley.
Most of the recharge to the gravel aquifer is received at the heads of alluvial fans by infiltration from tributaries that drain the surrounding mountains. Snow upon the valley floor provides a significant amount of recharge. Water moves toward the Salt River, which flows northward through the valley and which has large gains due to ground-water inflow.
On the east side of the valley, the water table is 100-200 feet below land surface at a distance of half a mile from the mountain front. On the west side of the valley, the depth to water is rarely more than 30 feet. Depth to water decreases toward the center of the valley.
The gravel aquifer can provide sufficient water for supplemental irrigation. Irrigation supplies of several hundreds of gallons per minute have been developed at two localities on the west side of the valley. Two pumping tests showed values for transmissibility of 82,500 and 370,000 gallons per day per foot in the vicinity of a well on the east side of the valley and a well on the west side, respectively.
The ground water is of good quality for irrigation usage through most of the valley. Hardness of the water exceeds 200 parts per million, however, and this characteristic makes the water somewhat undesirable for domestic and industrial use. Water beneath the northwestern part of the valley has relatively high content of sodium and chloride.