In A River's Journey you saw that a river begins high up and flows downhill to the sea. So which sea? High in the Rockies runs a line that decides.
Key idea
The Continental Divide runs along the high crests of the Rocky Mountains. Rain or snow that falls on its west side flows toward the Pacific Ocean. Water on its east side flows toward the Gulf of Mexico (since 2025 the U.S. federal government calls it the Gulf of America) and the Atlantic.
Why it works
Picture the roof of a house. Rain that lands on one side runs off that side; rain on the other side runs the other way. The ridge of the roof is the divide. The Continental Divide is the ridge of the continent: once water lands on one side, it can only run downhill, and downhill leads to one ocean or the other.
Worked example: Two rivers from one state
The Colorado River and the Rio Grande both start in the Colorado Rockies. Where does each one go?
- The Colorado River starts on the west side of the Divide.
- It flows southwest through the Grand Canyon toward the Gulf of California, an arm of the Pacific.
- The Rio Grande starts on the east side.
- It flows south through New Mexico and along the Texas-Mexico border to the Gulf.
Answer: The Colorado goes toward the Pacific; the Rio Grande goes to the Gulf.
Common mistake
People think rivers flow to the nearest ocean. They don't: they flow downhill. Water that falls just east of the Divide in Colorado is much closer to the Pacific than to the Gulf, but it still travels more than 1,000 miles east and south, because that is the way downhill goes. (A few places are exceptions, like the Great Divide Basin in Wyoming, where water collects and never reaches the sea.)