Continental divide

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A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not connected to the open sea. The endpoints where a continental divide meets the coast are not always definite, because the exact border between adjacent bodies of water is usually not clearly defined. The International Hydrographic Organization's publication Limits of Oceans and Seas defines exact boundaries of oceans, but it is not universally recognized. Where a continental divide meets an endorheic basic, such as the Great Divide Basin of Wyoming, the continental divide splits and encircles the basin.

[edit] Examples

Continental divides in North America

Major continental divides, showing drainage into the major oceans and seas of the world. Grey areas are endorheic basins that do not drain to the ocean.^{[citation needed]}

• North America can claim 3 or five continental divides, depending on how drainage basins are categorized:
  • The Continental Divide of the Americas, also called the Great Divide, separates the watersheds of the Pacific Ocean from those of the Atlantic or Arctic Oceans. It runs from the Seward Peninsula in Alaska, through western Canada along the crest of the Rocky Mountains to New Mexico. From there, it follows the crest of Mexico's Sierra Madre Occidental and extends to the tip of South America. It is crossed by the Panama Canal.
  • The Northern Divide, or Laurentian Divide, separates the watershed of the Atlantic Ocean from that of Hudson Bay. The western part of it from the Rocky Mountains to the Great Lakes watershed marked the northern boundary of the Louisiana Purchase and was the border between the United States and British North America until it was superseded by the 49th parallel in the treaty of 1818. In Canada, it historically marked the southern boundary of the fur trading monopoly area of the Hudson Bay Company, and the easternmost portion still marks part of the boundary between Quebec and Labrador. The divide traverses very flat terrain, especially in North Dakota, causing many travelers to believe the sign marking the divide is a joke. ^[1]
  • The St. Lawrence River Divide separates the Great Lakes Basin from the rest of the Atlantic Ocean watershed. Two canals cross the divide: The Chicago Sanitary and Ship Canal crosses the Chicago Portage and connects Lake Michigan to the Mississippi River watershed. The Erie Canal connects Lake Erie to the Hudson River watershed. Historically there were additional canals, e.g., the Ohio and Erie Canal, but most of these are no longer in operation.
  • The Eastern Continental Divide separates the watershed of the Gulf of Mexico from the Atlantic Ocean. It runs from the Twin Tiers of New York and Pennsylvania down the Appalachian Mountains to the tip of Florida, dividing the Atlantic from the Gulf of Mexico. The city of Atlanta sits atop this divide.
  • Canada can be considered to have another continental divide separating the Arctic Ocean watershed from the Hudson Bay watershed, since Hudson Bay is often considered a separate body of water from the Arctic Ocean. This divide, which is sometimes called the Arctic Divide, was a barrier to transportation until the Methye Portage was discovered in 1778, which opened up the Arctic rivers to the fur traders and became part of a transcontinental trade route from Atlantic to Pacific. It was of significance in Canadian history because it marked the northern boundary of Rupert's Land, the trading monopoly area of the Hudson's Bay Company.^[2]

• In South America, the Continental Divide of the Americas lies along the Andes, but the divide does not always run along the highest peaks of the mountain system. In Patagonia many morrain-damed used to drain to the Atlantic rather than the Pacific before the Pleistocene glaciations.
• Australia has less distinct ocean boundaries and fewer prominent mountain ranges, making it hard to define a single divide. Much of the interior of the continent drains into the endorheic Lake Eyre Basin.
• Eurasia has various divides, depending on the definition of "ocean" (for example, the Mediterranean Sea and its various lobes, the Atlantic Ocean, the North Sea, the Baltic Sea, the Arctic Ocean and the Black Sea with Europe). Examples include:
  • Asia:
    • Tibetan Plateau (Himalayas): Indian Ocean, Pacific Ocean
    • Himachal Pradesh (Indus-Ganges): Arabian Sea, Bay of Bengal
    • Lake Baikal (Yenisei-Lena): Kara Sea, Laptev Sea
    • Perm Krai/Urals (Volga-Pechora/Ob): Caspian Sea, Arctic Sea
  • Europe-Asia:
    • Don-Volga: Black Sea, Caspian Sea
  • Europe:
    • Lunghin Pass, Alps (Rhine-Danube-Po): North Sea, Black Sea, Adriatic Sea
• The most significant continental divide in Africa is that between the watersheds of the Nile and the Congo, passing through the area of the African Great Lakes. Between the Congo and the Sahara, a vast area drains into the endorheic Lake Chad, so puncturing the Atlantic-Mediterranean divide. The Mediterranean-Indian Ocean divide is punctured in West Africa by the endorheic lake systems of the Great Rift Valley; in the south of the continent the divide between the Atlantic and Indian Oceans snakes between the watersheds of the Congo, Zambezi, Limpopo and Orange Rivers, with the Okavango terminating in the Kalahari Desert.
• The interior of Antarctica receives very little precipitation, and that in the form of snow, and the continent is entirely surrounded by the Southern Ocean. Therefore, Antarctica is not generally considered to have a continental divide. The Transantarctic Mountains divide the ice streams draining West Antarctica into the Ronne Ice Shelf, toward the Pacific and into the Ross Ice Shelf, from those draining East Antarctica toward the Atlantic and Indian Oceans

[edit] References

[edit] See also

• Drainage basin
• Borders of the continents
• Drainage divide

[edit] External links

• nationalatlas.gov
• Continental Divides in North Dakota and North America by Mark A. Gonzalez. Excellent article on America's other continental divides.PDF (123 KiB)
• Map of drainage basins in Canada