The Lake of the Ozarks, Pomme de Terre Lake, and Truman Lake in the northern Ozarks were formed by impounding the Osage River and its tributary the Pomme de Terre River in 1931, 1961 and 1979 respectively. Grand Lake in northeast Oklahoma was created in 1940. Stockton Lake was formed by damming the Sac River near the city of Stockton, Missouri, in 1969 and supplements the water supply of Springfield in nearby Greene County. Most of the dams were built for the dual purpose of flood control and hydropower generation.