The term “Great Divide” is the common name for the Continental Divide of the Americas, a geological feature spanning the length of two continents. This continuous ridge serves as the primary hydrological boundary for North and South America. It dictates the destination of precipitation, channeling water into the two largest ocean basins on the planet. The divide is a major physical feature that shapes the landscape and ecology of the Western Hemisphere.
The Fundamental Concept of a Continental Divide
A continental divide is a geological and hydrological boundary that separates a continent’s major drainage basins. It is characterized by a crest or ridge of high ground, typically a mountain range. This elevated feature acts as a line of separation for surface water runoff. Precipitation that falls on one side of the divide flows to one major body of water, while precipitation on the opposite side flows toward a different one.
The line is defined by the subtle slope of the terrain, determining which side of the crest a raindrop lands on. This separation directs water toward entirely different oceans, making it a continental-scale version of local drainage divides.
The Geographic Course of the Divide
The Great Divide traces a rugged, continuous path along the spine of the Americas, beginning in the far north of Alaska. It runs south through western Canada, where it follows the crest of the Canadian Rockies. The divide forms a boundary for much of the mountainous terrain in the provinces of British Columbia and Alberta.
Upon entering the United States, the divide continues its journey through the main Rocky Mountain chain, traversing the states of Montana, Wyoming, Colorado, and New Mexico. In the United States, the divide is marked by high-elevation passes and peaks, including the highest point on the North American divide at Grays Peak in Colorado. The path continues southward, following the Sierra Madre Occidental mountain range into Mexico.
Determining Water Flow and Drainage
The functional consequence of the divide is to split the continent’s water resources into two distinct drainage systems. Precipitation that lands on the western slopes of the divide flows toward the Pacific Ocean. This water is collected by river systems like the Columbia and Colorado Rivers, which travel thousands of miles to reach the Pacific basin.
Water falling on the eastern side is directed toward the Atlantic Ocean basin, which includes the Gulf of Mexico and the Arctic Ocean. The vast majority of this eastward drainage feeds into the massive Mississippi-Missouri River system, emptying into the Gulf of Mexico. Further north in Canada, water flows through river systems like the Saskatchewan and Nelson Rivers, reaching Hudson Bay.
The difference in drainage affects the climate and ecology on either side of the divide. The western side often receives greater precipitation due to the rain shadow effect created by the high mountains, influencing vegetation and habitat distribution. This flow separation dictates the path of major river headwaters and influences water management across the continent.
Hydrological Exceptions and Triple Points
While the divide generally separates water flow into two major directions, some locations feature more complex hydrology. A triple divide point is a rare geological location where water flows into three distinct drainage basins. These intersections are important junctures in the continent’s water distribution.
The most famous example is Triple Divide Peak in Glacier National Park, Montana. From this single peak, runoff can enter river systems that lead to the Pacific Ocean, the Atlantic Ocean (via the Gulf of Mexico), and the Arctic Ocean (via Hudson Bay). North America also contains secondary continental divides, such as the Eastern Continental Divide, which separates drainage to the Atlantic Seaboard from the Gulf of Mexico.