A major river of central North America, and is a tributary of the Mississippi River. It is the longest river on the continent at over 2,340 miles (3,770 km) and the second largest tributary of the Mississippi by discharge, after the Ohio River. The watershed of the Missouri River drains nearly 530,000 square miles (1,400,000 km2) of the eastern Rocky Mountains and the Great Plains, spanning parts of ten U. S. States and two Canadian provinces.