Lake Francis Case is a large reservoir behind Fort Randall Dam on the Missouri River in south-central South Dakota, USA. The lake has an area of 102,000 acres (410 km²) and a maximum depth of 140 ft (43 m). Lake Francis Case has a length of approximately 107 mi (172 km) and has a shoreline of 540 mi (870 km). The reservoir is named for former South Dakota Senator Francis H.