What do the Mississippi and Missouri Rivers do?

The Mississippi and Missouri Rivers flow into the Gulf of Mexico at the Mississippi River Delta in southeastern Louisiana, with the Missouri joining the Mississippi just north of St. Louis at about river mile 1,257 above the Gulf, and the combined Mississippi continuing about 1,000 miles south to its mouth in the Bird's Foot Delta below New Orleans. The combined Mississippi-Missouri system drains about 1.15 million square miles, the third largest river basin in the world after the Amazon and Congo, including 41 percent of the contiguous United States.

The Missouri rises in the Rocky Mountains of southwestern Montana at the confluence of the Jefferson, Madison, and Gallatin rivers near Three Forks, Montana, then flows about 2,341 miles east and south through Montana, North Dakota, South Dakota, Nebraska, Iowa, and Missouri. It joins the Mississippi at the so-called Mound City confluence about 17 river miles north of downtown St. Louis. Above the confluence the Mississippi is a clear water river of moderate size; below the confluence the muddy Missouri's sediment doubles the river's volume and turns the water characteristically brown.

The Mississippi rises at Lake Itasca in northern Minnesota and flows about 2,320 miles south, picking up the Wisconsin, Illinois, Missouri, Ohio, Arkansas, and Red rivers along the way. Below the Old River Control Structure in Louisiana, the river continues to New Orleans and then divides into several distributaries crossing the delta into the Gulf. The main outlet, the Bird's Foot Delta, has three principal passes (South Pass, Southwest Pass, and Pass a Loutre) extending into the Gulf. Sediment deposited at these mouths has built about 6,000 square miles of delta over the past 7,000 years.

The Atchafalaya River, a distributary that joins the lower Mississippi near Old River, takes about 30 percent of the combined Mississippi flow into Atchafalaya Bay. Without the Old River Control Structure (built 1959 to 1963), the Atchafalaya would have captured the entire Mississippi by now, leaving New Orleans on a stagnant channel. The combined river system drained the central interior of North America since glacial meltwaters carved the channels at the end of the last ice age. The river system carries about 145 million tons of sediment per year to the Gulf, although that has been reduced by upstream dams.

The Gulf of Mexico itself is a small ocean basin of about 615,000 square miles bordered by the United States, Mexico, and Cuba. Five U.S. states (Texas, Louisiana, Mississippi, Alabama, and Florida) face the Gulf. The river-Gulf system supports major shipping (the Mississippi handles about 500 million tons of cargo per year), oil and gas industry, fisheries, and recreation.