Rivers carried explorers, settlers and goods across the country long before railroads and highways. Many cities grew up on their banks.
Key idea
The big rivers to know: the Mississippi (Minnesota to the Gulf), the Missouri (Montana to the Mississippi), the Ohio (Pittsburgh to the Mississippi), the Colorado (the Rockies through the Grand Canyon), the Columbia (the Washington-Oregon border to the Pacific) and the Rio Grande (Colorado to the Gulf, along the Texas-Mexico border).
Why it works
Why do rivers make good borders? A river is easy to see and hard to argue about, so when states were drawn, lawmakers often used them. The Ohio River is Kentucky's whole northern border; the Columbia separates much of Washington from Oregon; the Rio Grande separates Texas from Mexico; and the Mississippi forms the edges of ten states.
| River | Length (USGS) | Ends at |
|---|---|---|
| Missouri | about 2,540 miles | the Mississippi, near St. Louis |
| Mississippi | about 2,340 miles | the Gulf, south of New Orleans |
| Rio Grande | about 1,900 miles | the Gulf, at the Texas-Mexico border |
| Colorado | about 1,450 miles | the Gulf of California (Pacific) |
| Ohio | about 1,310 miles | the Mississippi, at Cairo, Illinois |
| Columbia | about 1,250 miles | the Pacific Ocean |
Worked example: A border river
Which river forms much of the border between Washington and Oregon?
- Find the two states in the far northwest.
- A big river runs west along the line between them to the Pacific.
- It is the Columbia.
Answer: The Columbia River.
Common mistake
Most people think the Mississippi is the longest U.S. river. By the U.S. Geological Survey's measure, the Missouri is a little longer, about 2,540 miles to the Mississippi's 2,340. The Missouri flows into the Mississippi, and together they make one of the longest river systems in the world, about 3,710 miles from the Missouri's source to the Gulf. (Different measurements give slightly different lengths.)