Several times going toward Seattle and driving back from Seattle through the Rockies, we came to signs proclaiming ‘The Great Divide’. I knew it had to do with rivers running either to the Pacific or to the Atlantic, but if I ever knew it, I’ve forgotten the specifics. Fortunately, Wikipedia to the rescue:
The Continental Divide of the Americas, or merely the Continental Divide or Great Divide, is the name given to the principal, and largely mountainous, hydrological divide of the Americas that separates the watersheds that drain into the Pacific Ocean from (1) those river systems that drain into the Atlantic Ocean (including those that drain via the Gulf of Mexico or the Caribbean Sea), and (2) along the northernmost reaches of the Divide, those river systems that drain into the Arctic Ocean.
There are other continental divides on the North American continent, however the Great Divide is by far the most prominent of these because it tends to follow a line of high peaks along the main ranges of both the American and Canadian Rocky Mountains, at a generally much higher elevation than the other divides.
They even have a diagram on Wikipedia: