A big river is fed by smaller rivers, called tributaries. The Mississippi has the largest family of tributaries in North America.
Key idea
The Missouri, Ohio and Arkansas rivers flow into the Mississippi, and the Tennessee flows into the Ohio. The Mississippi carries all of their water south to the Gulf of Mexico (since 2025 the U.S. federal government calls it the Gulf of America). All the land drained by this system is the river's basin.
Why it works
Why does one river collect so much? The middle of the country is a huge, shallow bowl between the Rockies and the Appalachians. Rain falling anywhere in the bowl runs downhill toward its lowest line, and that line is the Mississippi. The Continental Divide is the western rim of the bowl.
Worked example: A raindrop's trip
A raindrop falls in western Pennsylvania. How does it reach the Gulf?
- It runs into a stream, then into a river that flows to Pittsburgh, where the Ohio River begins.
- The Ohio flows west and joins the Mississippi at Cairo, Illinois.
- The Mississippi carries it south, past New Orleans, to the Gulf.
Answer: Ohio River, then the Mississippi, then the Gulf.
Common mistake
People think a river's basin is just the river. The Mississippi's basin is all the land its tributaries drain: all or parts of 31 states and two Canadian provinces, about 40% of the lower 48 states, according to the National Park Service. A farm field in Montana and a parking lot in Pittsburgh are both in it.
The Mississippi starts at Lake Itasca in northern Minnesota. The Park Service says water leaving the lake takes about three months to reach the Gulf.