Oahe Reservoir, South Dakota August, 1997 The fourth largest in the United States, the Oahe Reservoir can be seen in this southwest-looking low-oblique view. Located on the Missouri River 5 miles (8 km) north of Pierre, South Dakota, the Oahe Reservoir stretches 231 miles (371 km) northward to just south of Bismarck, North Dakota (not visible on the image). The Oahe Dam is the second largest earthfill structure of its kind in the world. The lake has nearly 2250 miles (3523 km) of shoreline and is a very popular recreational area known for its excellent fishing. The dam also provides flood control, hydroelectric power, and irrigation. Numerous agricultural field patterns can be seen to the east of the reservoir. The Cheyenne River enters the scene near the upper right of the image and it joins Oahe Reservoir near the Fort Sully Game Refuge (peninsula jutting out into lake)