Missouri (river) - geography.

Missouri (river) - geography. Missouri (river) (Illinois Emissourita, "dwellers of the big muddy"), river in central United States. The Missouri is formed by the confluence of the Jefferson, Gallatin, and Madison rivers at Three Forks in southwestern Montana. The longest river system in the United States, the Missouri is one of the primary tributaries of the Mississippi River. From its source, it flows 4,090 km (2,540 mi) and drains an area of about 1,370,000 sq km (about 529,000 sq mi). The Missouri initially flows north, skirting the main range of the Rocky Mountains. Then it passes through a 366-m (1200-ft) gorge called the Gates of the Mountains, turns northeast and reaches Fort Benton, Montana, the head of navigation. From Fort Benton th...