What is the continental divide?

The Continental Divide is a ridge of high ground in North America along the crest of the Rocky Mountains that separates the watersheds of the Pacific Ocean (rivers flowing west) from the watersheds of the Atlantic Ocean and Gulf of Mexico (rivers flowing east). The divide runs about 3,100 miles from the Bering Strait through Alaska, Canada, the United States, and into South America, with continental divides existing on every continent. In the United States, the Continental Divide enters from Canada in northern Montana, runs south through the Rocky Mountains in Montana, Idaho, Wyoming, Colorado, and New Mexico, then continues into Mexico. The U.S. portion measures about 1,750 miles.

The divide passes through nine of the eleven Mountain States and crosses many famous mountain peaks and passes including Lewis and Clark Pass in Montana, Marias Pass (the lowest crossing of the Northern Rockies at 5,213 feet, used by U.S. Highway 2 and the BNSF Railway), Yellowstone National Park (where the Snake River flows west to the Pacific and the Yellowstone River flows east to the Mississippi), South Pass in Wyoming (the famous gentle crossing used by the Oregon, California, and Mormon trails), the Continental Divide in Rocky Mountain National Park, Colorado, and many others.

East of the Continental Divide, water flows through the Great Plains and the Mississippi-Missouri river system to the Gulf of Mexico, or through the Great Lakes and the St. Lawrence River to the Atlantic, or directly through eastern rivers to the Atlantic. West of the Continental Divide, water flows through the Columbia River, the Snake River, the Colorado River, or the Rio Grande (which flows southeast to the Gulf of Mexico but is technically east of the Divide for most of its length and crosses through it) to the Pacific.

The Rio Grande is a special case: it rises west of the Continental Divide in southwestern Colorado, crosses the Divide near Wolf Creek Pass, and flows southeast to the Gulf of Mexico. North America also has several minor continental divides. The Eastern Continental Divide runs along the Appalachian Mountains, separating Atlantic-bound rivers from Gulf of Mexico-bound rivers. The Saint Lawrence Divide separates Great Lakes water from Hudson Bay water. The Laurentian Divide separates Hudson Bay water from Mississippi water. Triple Divide Peak in Glacier National Park, Montana is the only point on the continent where three watersheds meet (Pacific, Atlantic via Hudson Bay, and Gulf of Mexico).

The Continental Divide Trail, designated 1978, runs about 3,100 miles from Mexico to Canada following the Divide as closely as practical. About 150 to 200 hikers complete the entire trail each year. The divide shapes drainage, weather, and human transportation across the western United States.