The Ozark aquifer is the principal source of ground water in the Gasconade River Basin upstream from Jerome, Missouri (herein referred to as the upper Gasconade River Basin), including the Fort Leonard Wood Military Reservation (FLWMR). The Ozark aquifer is composed of, in order of increasing age, the Cotter Dolomite, Jefferson City Dolomite, Roubidoux Formation, Gasconade Dolomite, Eminence Dolomite, and Potosi Dolomite. Sedimentary strata are nearly horizontal, except along folds and collapse zones where dips can be steep. The basin is cut by numerous faults, most of which trend generally northwest-southeast. The Jefferson City Dolomite and the Cotter Dolomite generally yield little water to wells. Wells completed in the Roubidoux Formation and Gasconade Dolomite commonly yield from several tens to several hundred gallons per minute of water. The Eminence Dolomite may form a weak hydrologic barrier to vertical ground-water flow between the overlying Gasconade Dolomite and the underlying Potosi Dolomite. The Potosi Dolomite is the most permeable formation in the Ozark aquifer. Wells completed in the Potosi Dolomite may yield from several hundred to 1,000 gallons per minute of water. Water-table contours indicate several areas of high permeability karst terrain in the upper Gasconade River Basin. Ground-water levels may be as deep as 300 feet below the land surface beneath upland areas where karst features are prevalent. Although the Jefferson City Dolomite and the Roubidoux Formation are the uppermost bedrock formations in the upland areas of the FLWMR, the water table generally is deep enough to occur in the underlying Gasconade Dolomite throughout most of the FLWMR. Discharge from springs [311 ft3/s (cubic feet per second)] represented 56 percent of the August 1999 discharge of the Gasconade River at Jerome, Missouri (552 ft3/s). From 1993 through 1997, annual pumpage from all public water-supply wells in the upper Gasconade River Basin ranged from 1,820 Mgal [million gallons; an average daily rate of 4.99 Mgal/d (million gallons per day)] in 1993 to 2,030 Mgal (an average daily rate of 5.56 Mgal/d) in 1997. Including an estimated 4 Mgal/d from domestic wells, the average daily pumping rate for all wells is estimated to range from 8.99 Mgal/d in 1993 to 9.56 Mgal/d in 1997. During the same period, annual pumpage from the Big Piney River, which supplies most of the water used at the FLWMR, ranged from 1,136 Mgal (an average of 3.11 Mgal/d) in 1997 to 1,334 Mgal (an average of 3.65 Mgal/d) in 1995, and as a percentage of total water use in the upper Gasconade River Basin, ranged from about 24.5 percent in 1997 to about 28.8 percent in 1993.