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A continental divide is an imaginary line down a continent that demarcates whether water in the area will flow towards a given body of water (usually an ocean) or another. The most famous continental divide is the north-south Continental Divide of the Americas, sometimes known as the Great Divide or just the Continental Divide. This divide is more than 6,000 miles (9,600 km) long, stretching from the western tip of Alaska all the way to Tierra del Fuego at the southernmost tip of South America. This continental divide mostly demarcates whether water flows into the Pacific or Atlantic Ocean. Some areas close to the divide, like the Great Basin, are endorheic basins, meaning that they retain water in salt lakes or salt pans, and it never flows to the ocean.

In the United States, the Continental Divide does not go clear down the middle of the country, but follows the peaks of the Rocky Mountains, which are somewhat offset to the west. Immediately east of the Rocky Mountains and throughout most of the central United States, water flows to the Gulf of Mexico. In eastern North America there are three more continental divides: the Northern Divide, Eastern Divide, and the St. Lawrence Seaway Divide. These divides indicate whether water flows to the eastern seaboard, Hudson Bay and the Arctic Ocean, or the Gulf of St. Lawrence. For much of its length, the Northern Divide is located roughly at the border between the United States and Canada.