The Laurentian Divide

The Continental Divide of North America stretches from Alaska to Mexico, generally following the Brooks Range and Rocky Mountain corridor; this divide is commonly known to separate the streams flowing toward the Pacific Ocean from those draining toward the Atlantic Ocean and Gulf of Mexico.  Actually, the northern section of the Continental Divide, from Alaska through the Yukon Territory of Canada, separates streams flowing toward the Pacific from those flowing into the Arctic Ocean.  As far south as the U.S.-Canada border, it divides streams heading for the Pacific from those flowing toward Hudson Bay.

Geographers and hydrologists thus recognize the Laurentian Divide (also known as the Northern Divide), which stretches west to east between the Continental Divide and the Labrador Sea, dividing streams that flow northward to the Arctic or to Hudson Bay from those that flow southward toward the Gulf of Mexico or eastward to the Atlantic Ocean.  From Triple Divide Peak in Glacier National Park, this divide straddles the U.S.-Canada border through Montana, separating the watersheds of the Saskatchewan and Missouri Rivers.  Farther east, it angles southeastward through North Dakota and then northeastward through Minnesota, dividing the watershed of the Red River (which flows northward to Lake Winnipeg and thence northeast to Hudson Bay via the Nelson River) from those of the Missouri, Minnesota and Mississippi Rivers.  Running north of the Great Lakes, the Laurentian Divide follows the northern edge of the St. Lawrence watershed, eventually running atop the crest of the Laurentian Mountains as they curve through Quebec and Labrador.

The Laurentian Divide ends at the mouth of the Hudson Strait on the Labrador Sea.  While some argue that Hudson Bay and its Strait are hydrologically part of the Atlantic Basin (which they are), the ecology of the Hudson Bay region is closer to that of the Arctic than to Temperate latitudes to the south.  For purists, the Arctic Divide runs north of the Saskatchewan watershed and north of Hudson Bay, demarcating the southern edge of various watersheds (including that of the MacKenzie River) that empty into the Arctic Ocean.