Whenever I drive across New Mexico, I’m on the lookout for a little sign by the side of the road unceremoniously announcing "Continental Divide"–a sign like the one on the left (only that one’s actually in North Dakota rather than New Mexico).
Growing up, I had the idea that the continental divide was a the highest mountain ridge on the continent so that it divided the continent into two sides that sloped away to the ocean.
Well, that and it was a John Belushi movie.
But if you go and see the continental divide, you may find that you’re not at a mountain ridge at all. There may not even be a mountain ridge in sight. Where you are may be flat.
That’s the way it looks at the continental divide in New Mexico, and it looks pretty similar at the location in North Dakota.
Even more confusingly, when you’re at the continental divide there can be higher points of land on either side of it, within sight.
What’s the explanation?
The explanation is that the continental divide isn’t a ridge made out of the highest land on the continent. It’s an elevation of land that separates two watersheds, so that rain falling on one side of it will tend to find its way toward one ocean and rain falling on the other side will tend to find its way to another.
That means that the continental divide will tend to be high ground, but it doesn’t all have to be mountainous (it can be in relatiely flat countryside) and it doesn’t have to be the highest thing in the area. There can be higher points on both sides, there just has to be a depression between the higher point and the continental divide so that the flow of water toward the ocean isn’t thwarted.
Now here’s a new twist: There isn’t just one continental divide in North America. There are four (click to enlarge).
The one everybody thinks of as "the Continental Divide" is actually the Great Continental Divide, but there are three others, depending on what body of water a watershed is sloping toward–whether it’s the Pacific Ocean, the Arctic Ocean, the Labrador Sea, the Atlantic Ocean, or the Gulf of Mexico.
I was surprised at how much of the United States is in the watershed leading to the Gulf of Mexico. I’d kind of assumed that water on the east sid of the Great Divide would flow toward the Atlantic, but oh yeah, there’s another bunch of mountains in the way, so it’s going to go down into the Gulf.
Other continents also have continental divides, though they’re not always as clearly demarkable as they are in North America, so GO NORTH AMERICA! We’ve got FOUR, CLEARLY DEMARKABLE divides!
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