The hydrochemical and stable isotope composition of fresh surface water in rivers (Sieber and Oder) of a karst area in the southwestern foreland of the Harz Mountains, Germany, was investigated at several occasions between years 1986 and 1992. The campaigns covered different seasonal and hydrological (discharge) conditions, including a snow-melt induced high water at the rivers. Aim of the study was the investigation of the impact of discharge conditions at the river water loosing water to underground passage in the Basin of Pöhlde, dissolving carbonate and sulfate minerals in the karst aquifers, and reappearing in the karst springs of the Rhume river. Besides physical characterization, hydrochemical major and minor elements were measaured, as weill as the carbon isotope composition of DIC, the sulfur and oxygen isotope composition of sulfate, and the oxygen isotope composition of water. Results reflect the impact of hydrological conditions on the subterrestrial carbon and sulfur cycles.