How much do you know about the Great Divide? Some of us know that it generally tracks the Rocky Mountains in the United States. But did you know it runs north all the way to the coast of the Bering Sea in Alaska? Or that it extends south into Mexico, across the Isthmus of Panama, to the southern tip of South America? Because of its reach, it’s known in the Western Hemisphere as the Continental Divide of the Americas. It’s truly the Great Divide, but it’s not the only divide in North America.
Along its course, a continental divide forces water to flow to one side of the continent or the other. Hydrologists call these areas of drainage “watersheds.” The Great Divide splits Central and South America into eastern and western watersheds, and in those regions it’s the only continental divide. But the situation in North America is messier. Depending on what criteria you cite, the continent has at least three and as many as six continental divides.
To illustrate why agreement on the number of continental divides is difficult, consider the map below from the U.S. Department of the Interior. Three Oceans, five seas, three gulfs, and a bay surround North America, and rivers flow into all of them. Now, if the definition of a continental divide is that it separates river systems and forces each to flow into a different ocean, then at least three continental divides exist: between Pacific and Arctic watersheds, Pacific and Atlantic watersheds, and Arctic and Atlantic watersheds. If scientists count any large body of water, the number of divides increases. The situation is further confused because the smaller bodies of water all belong to one of the oceans: Chukchi and Beaufort Seas, and Hudson Bay flow into the Arctic; the Labrador Sea, Gulf of St. Lawrence, and Gulf of Mexico mix with the Atlantic; and the Sea of Cortez, Gulf of Alaska, and Bering Sea join the Pacific.
In the case of Canada, some observers place a second continental divide above Hudson Bay. The map below shows the second divide demarcating the Arctic and Hudson Bay watersheds. While the divide undoubtedly exists and the runoff on either side drains in opposite directions to different bodies of water, some scientists discount the divide because Hudson Bay forms part of the Arctic Ocean.
A similar consideration leads some hydrologists to reject the Eastern Divide in the United States as a true continental divide. It spans almost the length of the nation from south-central Florida to northern Pennsylvania, sending water either east into the Atlantic Ocean or west into the Gulf of Mexico. The problem is, the Gulf forms part of the Atlantic ocean, so the objection is that the different watersheds drain into the same source.
At least one popular map labels the mountain ranges around Great Basin as a continental divide. (The outline of Great Basin is visible on the map of North America above.) The massive Great Basin in the western United States, which covers portions of six states and the base of Mexico’s Baja Peninsula, is a collection of endorheic drainage basins. An endorheic watershed is one that drains internally. In the case of Great Basin, the drainage evaporates, sinks, or flows into the area’s lakes. Great Salt Lake, which lends it name to Salt Lake City, Utah, is one of the basins. The mountains ringing the Great Basin push water either toward the interior or outward into various rivers that flow into the Pacific Ocean.
I hope you have a better understanding of North America’s continental divides. I leave you with an interesting image that highlights the rivers and streams flowing down either side of the Great Divide. How many can you name?
“Continental Divide.” Resource Library. National Geographic.
“Continental Divide of the Americas.” Wikipedia.
“Eastern Continental Divide.” Wikipedia.
Gonzalez, Mark A. “Continental Divides in North Dakota and North America.” Bureau of Land Management. July 2003.
“Great Basin.” Wikipedia.
“Great Basin Divide.” Wikipedia.
“Laurentian (Northern) Divide.” Wikipedia.
“Saint Lawrence River Divide.” Wikipedia.