Groundwater studies throughout parts of the "Big Bend District" were completed in connection with projected developments along the Rio Grande. Cretaceous limestones are predominant and the major source of ground water. The Edwards limestone, principal aquifer, is overlain by the impervious Kiamichi formation; also favorable structures result in significant production from the younger Georgetown limestone.The "District" consists of: asymmetrically folded and faulted ranges separated by valleys in the Mexican Highlands; and an ovaloid syncline of low-dipping beds in the Edwards Plateau, which is interrupted by asymmetrical, northwestward-trending flexures. Major springs issue along northwestward-trending faults in the "Highlands" and along northeastward-striking breaks in the "Plateau."Solutioning, primarily structurally controlled, is a major factor in groundwater movement. Cored borings confirmed this hypothesis and indicated that faults acted both as "barriers" and "avenues." Initial development of solutioning apparently began in middle to late Tertiary time.Sinkholes as recharge "avenues," particularly in the "Plateau," are associated with crests and troughs of flexures and the intersection of faults, shears, and joints.Wells located on "highs" and "lows" of flexures yield water from the shallower Georgetown limestone; interconnected openings have apparently developed due to solutioning along fractures, thereby facilitating recharge. In addition, deeper wells yield from the underlying Edwards limestone.Where the Del Rio clay is absent, the solutioned and altered upper part of the Georgetown limestone is a potential water producer.Combining calculated permeability factors and geologic data, four generalized reservoir conditions were formulated for the "District."