Big Stone Lake (Dakota: Íŋyaŋ Tháŋka Bdé) is a long, narrow freshwater lake and reservoir on the border between western Minnesota and northeastern South Dakota in the United States.
Flow from the lake to the Minnesota River is regulated by the Big Stone Lake Dam, built in 1937 at the lake’s southern end.
Big Stone Lake is the source of the Minnesota River, which flows 332 miles (534 km) to the Mississippi River.