Lake Oahe Lake Oahe Lake Oahe (oh-WAH'-hee) is a large reservoir behind Oahe Dam on the Missouri River; it begins in central South Dakota and continues north into North Dakota in the United States. The lake has an area of and a maximum depth of . By volume, it is the fourth-largest reservoir in the US. Lake Oahe has a length of approximately and has a shoreline of . 51 recreation areas are located along Lake Oahe, and 1.5 million people visit the reservoir every year. The lake is named for the 1874 Oahe Indian Mission. Lake Oahe begins just north of
Folsom Dam project that provides flood control, hydroelectricity, and irrigation and municipal water supply. In order to increase Sacramento's flood protection to 200 year flood protection (meaning that the area is protected from a flood that has a 0.5% chance of occurring in any given year) the US Army Corps of Engineers is constructing an auxiliary spillway. The spillway was completed by October 2017, and enables Folsom Dam to release water before it reaches the height of the gates already in place. Another Central Valley Project dam, Nimbus Dam, is located further down river. Folsom Dam is located just north of the