Missouri River, second-longest river in the United States (about 2,500 mi/4,023 km) and chief tributary of the Mississippi, with which it forms the world's third-largest river system. Rising in southeastern Montana in the Rocky Mountains, the Missouri river flows north and then east through Montana; it then crosses North Dakota, and continues generally southeast until emptying into the Mississippi north of St. Louis. Its main tributaries include the Cheyenne, Kansas, Osage, Platte, Yellowstone, James, and Milk rivers. The Missouri was explored by Joliet and Marquette in 1673 and by the Lewis and Clark expedition in 1804–05. Like the Mississippi, it is subject to serious flooding, which is under control since three decades.