This photograph of Lake Oahe, South Dakota, was taken by an astronaut looking down from the International Space Station (ISS). The lake stands behind the Oahe Dam, and the Missouri River flows downstream to the southeast. The extensive rectangular fields around the river and lake are a visual reminder that agriculture is the main economic engine of the state.

Lake Oahe is the fourth largest man-made reservoir in North America, extending from Pierre, South Dakota, to Bismarck, North Dakota (a distance of approximately 230 miles/370 kilometers). Since the lake is a storage reservoir, the water volume changes frequently, especially during droughts. Lake Oahe is used for many purposes, including irrigation, hydroelectric power generation, and recreation.

The dam and lake is one of several reservoir systems built on the Missouri River. The main stem Missouri is the longest river in North America, flowing from the Rocky Mountains in Montana to its confluence with the Mississippi River near St. Louis. Along its length, the river feeds reservoirs in Montana, North Dakota, South Dakota, and along the Nebraska-South Dakota border.