An investigation was made by the U.S. Geological Survey, under a cooperative agreement with the Texas Water Development Board, to determine the changes in quantity and chemical quality of the base flow of the Nueces River in a 52.2-mile reach beginning at U.S. Highway 90 and extending to the crossing of Farm Road 1025 north of Crystal City. Knowledge of the changes in quantity and chemical quality of base flow is necessary for any evaluation of the effectiveness of the river channel in conveying water released from potential upstream reservoirs. The investigation was made November 23-25, 1964, during a period when evaporation and transpiration were small and the discharge past the stream-gaging station, Nueces River below Uvalde, was constant. Floods of September 1964 contributed considerably to the recharge of the alluvium in the reach investigated, resulting in a higher than average base flow.