Drainage divide

Adrainage divide,water divide,ridgeline,^[1]watershed,water partingorheight of landis elevatedterrainthat separates neighboringdrainage basins.On rugged land, the divide lies along topographicalridges,and may be in the form of a single range ofhillsormountains,known as adividing range.On flat terrain, especially where the ground ismarshy,the divide may be difficult to discern.

Atriple divideis a point, often asummit,where three drainage basins meet. Avalley floor divideis a low drainage divide that runs across avalley,sometimes created bydepositionorstream capture.Major divides separating rivers that drain to different seas or oceans arecontinental divides.

The termheight of landis used in Canada and the United States to refer to a drainage divide.^[2]It is frequently used in border descriptions, which are set according to the "doctrine ofnatural boundaries".^[3]Inglaciatedareas it often refers to a low point on a divide where it is possible toportagea canoe from one river system to another.^[4]

Types[edit]

Drainage divides can be divided into three types:^[5]

Continental dividesin which waters on each side flow to different oceans, such as theContinental Divide of the Americasand theCongo-Nile Divide.^[6]
Major drainage divides in which waters on each side of the divide never meet but flow into the same ocean, such as the divide between theYellow Riverbasin and theYangtze.Another, more subtle example is the Schuylkill-Lehigh divide atPisgah Mountainin Pennsylvania in which two minor creeks divide to flow and grow east and west respectively joining theLehigh RiverandDelaware Riveror theSusquehanna RiverandPotomac River,with each tributary complex having separate outlets into the Atlantic.
Minor drainage divides in which waters are originally separate but eventually join at a river confluence, such as theMississippi Riverand theMissouri Riverdrainage divides.

Valley-floor divides[edit]

A valley-floor divide occurs on the bottom of a valley and arises as a result of subsequent depositions, such asscree,in a valley through which a river originally flowed continuously.^[7]

Examples include theKartitsch Saddlein theGailvalley inEast Tyrol,which forms the watershed between theDrauand the Gail, and the divides in theToblacher FeldbetweenInnichenandToblachinItaly,where theDrauempties into theBlack Seaand theRienzinto theAdriatic.

Settlements are often built on valley-floor divides in the Alps. Examples areEben im Pongau,Kirchberg in TirolandWaidring(In all of these, the village name indicates the pass and the watershed is even explicitly displayed in the coat of arms). Extremely low divides with heights of less than two metres are found on theNorth German Plainwithin theUrstromtäler,for example, betweenHavelandFinowin theEberswalde Urstromtal.In marsh deltas such as theOkavango,the largest drainage area on earth, or in large lakes areas, such as theFinnish Lakeland,it is difficult to find a meaningful definition of a watershed.

Abifurcationis where the watershed is effectively in a river bed, in a wetland, or underground. The largest watershed of this type is the bifurcation of theOrinocoin the north ofSouth America,whose main stream empties into theCaribbean,but which also drains into the South Atlantic via theCasiquiare canalandAmazon River.

Political boundaries[edit]

Since ridgelines are sometimes easy to see and agree about, drainage divides may formnatural bordersdefining political boundaries, as with theRoyal Proclamation of 1763in British North America which coincided with the ridgeline of theAppalachian Mountainsforming the Eastern Continental Divide that separated settled colonial lands in the east from Indian Territory to the west.^[8]Another instance of a border matching a watershed in modern times involves the western border betweenLabradorandQuebec,as arbitrated by the privy council in 1927.^[9]

Portages and canals[edit]

Drainage divides hinder waterwaynavigation.In pre-industrial times, water divides were crossed atportages.Later,canalsconnected adjoining drainage basins; a key problem in such canals is ensuring a sufficient water supply. Important examples are theChicago Portage,connecting the Great Lakes and Mississippi by theChicago Sanitary and Ship Canal,and theCanal des Deux Mersin France, connecting the Atlantic and the Mediterranean. The name is enshrined at theHeight of Land Portageon the route from theGreat Lakesin the Atlantic drainage basin to theHudson Bay drainage basin.^[10]

References[edit]

^^a ^b"ridgeline. Dictionary.com"(Dictionary.com Unabridged ed.). Random House Inc.Retrieved7 September2013.
^Colombo, John Robert (16 December 2013)."Height of land".The Canadian Encyclopedia.Archivedfrom the original on 20 June 2014.
^Dikshit, Ramesh Dutta (1999).Political Geography: the Spatiality of Politics(3rd ed.). New Delhi: McGraw-Hill. p. 70.ISBN 978-0-07-463578-0.
^Decker, Jody F. (2011)."Portages".InWishart, David J.(ed.).Encyclopedia of the Great Plains.
^"Divide".Resource Library.National Geographic Society.Retrieved3 April2022.
^"Congo-Nile Divide Landscape".Albertine Rift.Wildlife conservation Society.Retrieved3 April2022.
^Leser, Hartmut, ed. (2005).Wörterbuch Allgemeine Geographie,13th ed., dtv, Munich, p. 935.ISBN 978-3-423-03422-7.
^Del Papa, Eugene M. (1975). "The Royal Proclamation of 1763: Its Effects Upon Virginia Land Companies".The Virginia Magazine of History and Biography.83(4). JSTOR: 406–411.JSTOR 4247979.
^McGrath, Patrick T. (1927)."The Labrador Boundary Decision".Geographical Review.17(4): 643–660.Bibcode:1927GeoRv..17..643M.doi:10.2307/208004.ISSN 0016-7428.JSTOR 208004.
^Shelley, Fred M. (2013).Nation Shapes: The Story Behind the World's Borders.Santa Barbara, California: ABC-CLIO. pp. 173, 242.ISBN 978-1-61069-105-5.