The longest river in the United States is the Missouri River. Stretching approximately 2,341 miles (3,767 kilometers), the Missouri runs through several states, including Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas, and Missouri, before merging with the Mississippi River in St. Louis. The Missouri River played a crucial role in the exploration and settlement of the American West, serving as a vital waterway for trade and transportation. Its significant drainage basin covers multiple states, and the river is essential for agriculture, recreation, and wildlife in the region.