Divided We Stand
Big Basins as the Law of the Land
5861 Image modified from Pfly/Wikipedia
WATERSHEDS ARE PERHAPS THE MOST geographically fundamental divisions of the landscape. While tectonics gives rise to mountains, it is erosion by water that shapes the land. Everything that grows on the ground, from trees to farms, and houses to cities, does so in response to this shedding of water. 
Every river and stream has a watershed – the area where a drop of water falling on the ground finds its way into its channel. These basins are distinct, by definition, and where one ends, another begins, immediately next to it. And so it goes, and flows, from small stream basins to larger river basins, to the ocean. How you enumerate or describe these individual basins is just a matter of scale.
At the largest scale, in the contiguous 48 United States, there are five primary watersheds, separated by five dividing lines, located where the edges of the basins meet. While they have been modified by humans in major ways, like everything else on the earth, these basins remain in essence the most basic architecture of the Continent.  They also suggest an alternate way to divide the landscape into governing units, according to the physical laws of the land.
The Great Divide
Generally referred to as the Continental Divide, this is the line dividing the watersheds of the Pacific Ocean and the Atlantic Ocean, which follows the Rockies from Canada to Mexico.
The Laurentian Divide
North of the Laurentian Divide, water falls into the Hudson Bay, which flows out the Labrador Sea. South of the divide, it drains more directly into the Atlantic Ocean. A small portion of northwest Montana drains on the north side of the line, from Triple Divide Peak, as well as much of northeast North Dakota, and northern Minnesota. The Labrador Sea is technically part of the Atlantic Ocean, so this divide is of lesser order than the Great Divide, as is the case with the other divides too. 
The St. Lawrence Divide
Water on the north side of the St. Lawrence Divide drains into the St. Lawrence River, either directly or through the Great Lakes. South of the divide the water drains more directly into the Atlantic at the Gulf of Mexico, or off the eastern seaboard. 
The Eastern Divide
A high point in the woods near Gold, Pennsylvania, is the intersection of the St. Lawrence Divide, and the Eastern Divide. Northward water flows into the St. Lawrence. East of the Eastern Divide, water flows into the Atlantic seaboard. West of it water flows into the Gulf of Mexico. 
The Great Basin
Located in an arid portion of the western United States, the Great Basin doesn’t drain anywhere, except into itself. It covers much of Nevada, nearly half of Utah, and some of Oregon, Idaho, Wyoming, and California. The shape of the basin has, however, also been altered by the hands of man, through the Owens Valley Aqueduct, which moves water out of the basin, into the Pacific Ocean, via Los Angeles.
The Arctic Divide
Located entirely in Canada, the Arctic Divide is the boundary between basins that drain into Hudson Bay (and thus into the northern Atlantic), and the Arctic Ocean, which is one of the five major oceans of the world (along with the Pacific, Atlantic, Indian Ocean, and Antarctic Ocean). While the Lower 48 is unaffected by this divide, the northeastern part of Alaska drains into the Arctic Ocean, and not into the Atlantic or the Pacific – the only part of the USA to do so.