Effective improvement of economic and social conditions of Indians living on the Standing Rock Indian Reservation has been hampered by lack of adequate and reliable information about the quantity and quality of water supplies available for development. Compounding the problem is the recent filling of Oahe Reservoir (now Lake Oahe), which flooded about 55,000 acres of Indian land, and the consequent relocation of many residents. This report summarizes the results of a water-resources study made at the request of the U.S. Bureau of Indian Affairs. It is intended to provide some of the needed information on water supplies that could be used to alleviate the problems mentioned.