Average annual runoff (inches per year) ranged from 0.90 to 22 in the Mountainous Region of Fremont County and from 0.06 to 0.72 in the Plains Region. The Wind River Formation of Tertiary age has the most well development. Quaternary alluvium and colluvium is the second-most developed. Some wells and springs discharge more than 300 gallons per minute from older selected geologic units. Geologic units are recharged by infiltration of precipi- tation, surface water, or irrigation water, or by leakage from another unit. The general direction of ground-water movement in the two major basins in the county is toward the Sweetwater and Wind Rivers. Ground water is discharged by wells, springs and seeps, evapotranspiration, and discharge to streams, lakes, drains, and other geologic units. Ground- water levels near Riverton's municipal supply were typically deepest in August when ground water was the sole supply. After 1981, ground water only supplemented the supply and water levels appeared to recover. Surface water supplies about 99 percent of total offstream use in Fremont County; irrigation is the largest use. The largest use of ground water is public supply. Twenty-five samples were collected from the Sweetwater River and tributaries in September 1991. Dissolved-solids concentrations ranged from 39 to 271 milligrams per liter, increasing downstream. All six water samples collected from the Cody Shale (Cretaceous age) exceeded 500 milligrams per liter dissolved solids-all samples from Miocene rocks and the White River Formation (Oligocene age) had dissolved solids less than 500 milligrams per liter.