The Oahe Dam is a large earthen dam on the Missouri River, just north of Pierre, South Dakota, United States. Begun in 1948 and opened in 1962, the dam creates Lake Oahe, the fourth-largest man-made reservoir in the United States. The reservoir stretches 231 miles (372 km) up the course of the Missouri to Bismarck, North Dakota. The dam's power plant provides electricity for much of the north-central United States. It is named for the Oahe Indian Mission established among the Lakota Sioux in 1874.