The objective of this investigation was to determine the effect on runoff of the many stock reservoirs in the Cheyenne River basin above Angostura Dam. As a first step it was necessary to determine, within reasonable limits of accuracy, the number of reservoirs in the basin, the storage capacity, the drainage area, and the water loss from each. A sampling method was adopted because the size of the basin, 9,100 square miles, prohibited examination of all reservoirs within the drainage area. Forty-nine sample areas of 9 square miles each were selected as a 5-percent sample of the 955 complete quarter townships within the basin above Angostura Dam. All reservoirs located within the sample quarter townships were surveyed.