Longest and Second-Longest Rivers in the United States

1. Missouri River – 2,341 mi (3,768 km)
Source: Brower’s Spring, Montana (Rocky Mtns). Mouth: Confluence with Mississippi near St. Louis.

2. Mississippi River – 2,340 mi (3,766 km)
Source: Lake Itasca, Minnesota. Mouth: Gulf of Mexico (Louisiana delta).

Note: The Missouri is slightly longer; the Mississippi carries more water. Together they form the 4th-longest river system globally (Mississippi–Missouri–Jefferson ~6,275 km).