Big Stone Lake is a long, narrow freshwater lake and reservoir forming the border between western Minnesota and northeastern South Dakota in the United States. The lake covers 12,610 acres (5,100 ha) of surface area, stretching 26 miles (42 km) from end to end and averaging around 1 mile (1.6 km) wide, and at an elevation of 965 feet (294 m) is the lowest point in South Dakota. Big Stone Lake is the source of the Minnesota River, which flows 332 miles (534 km) to the Mississippi River.