Hydrologic and chemical data were collected during base-flow conditions on three streams in southeastern Montana from October 26 to November 5, 1977, to characterize groundwater discharge. Maximum measured flow of Otter Creek, an interrupted stream, was 1.2 cubic feet per second. The water chemistry was dominated by sodium, magnesium, and sulfate ions, with a maximum dissolved-solids concentration of 3,540 milligrams per liter. Chemistry of groundwater inflow is dominated by sodium, magnesium, and sulfate. A discharge of 156 cubic feet per second from the Tongue River Reservoir into the Tongue River complicated interpretation of groundwater discharge to the reach downstream from the reservoir. Many reaches of gain and loss were observed along the Tongue River; the flow near the mouth was 233 cubic feet per second. Minor changes in quality of the base flow indicate groundwater discharges dominated by sodium, sulfate, and bicarbonate plus carbonate. Dissolved-solids concentration increased from 506 milligrams per liter near the dam to 630 milligrams per liter near the mouth. Maximum measured flow of Rosebud Creek was 19.0 cubic feet per second. Water quality in the upper reaches was dominated by magnesium, calcium , and bicarbonate, with dissolved-solids concentrations ranging from 560 to 703 milligrams per liter. Sodium, sulfate, and dissolved solids increase downstream.
