Mississippi River. Mississippi, USA. The Mississippi River is the second-longest river in the United States, with a length of 2,320 miles (3,730 km)from its source in Lake Itasca in Minnesota to its mouth in the Gulf of Mexico. The Mississippi River is part of the Missouri-Mississippi river system, which is the largest river system in North America and among the largest in the world: by length (3,900 miles (6,300 km)), it is the fourth longest, and by its average discharge of 572,000 cu ft/s (16,200 m³/s), it is the tenth largest. The name Mississippi is derived from the Ojibwe word misi-ziibi ("Great River") or gichi-ziibi ("Big River").