Water supply and water pollution in the Souris-Devils Lake area were studied in a cooperative effort by the Bureau of Reclamation and the Public Health Service. Preliminary data are given in this interim report, which also indicates the additional data to be collected during the investigation. The Souris-Devils Lake project proposes to provide a stream where no river previously existed, and to make available an adequate supply of good water where there has been only small, intermittent flows. The project affects areas of Montana, North Dakota, South Dakota, and Minnesota. Conclusions, which are subject to revision, are outlined regarding: (1) 14 public water supplies in the region; (2) augmentation and stabilization of of ground water supply for individual homes and communities; (3) flow for dilution of treated sewage effluent; (4) increased population probability; and (5) control of mosquitoes.