Major drainage divides (yellow and red ridgelines[1]) anddrainage basins (green regions) in Europe
Adrainage divide,water divide,ridgeline,[1]watershed,water parting orheight of land is elevatedterrain that separates neighboringdrainage basins. On rugged land, the divide lies along topographicalridges, and may be in the form of a single range ofhills ormountains, known as adividing range. On flat terrain, especially where the ground ismarshy, the divide may be difficult to discern.
Atriple divide is a point, often asummit, where three drainage basins meet. Avalley floor divide is a low drainage divide that runs across avalley, sometimes created bydeposition orstream capture. Major divides separating rivers that drain to different seas or oceans arecontinental divides.
The termheight of land is 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] Inglaciated areas it often refers to a low point on a divide where it is possible toportage a canoe from one river system to another.[4]
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 River basin and theYangtze. Another, more subtle example is the Schuylkill-Lehigh divide atPisgah Mountain in Pennsylvania in which two minor creeks divide to flow and grow east and west respectively joining theLehigh River andDelaware River or theSusquehanna River andPotomac 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 River and theMissouri River drainage divides.
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]
Settlements are often built on valley-floor divides in the Alps. Examples areEben im Pongau,Kirchberg in Tirol andWaidring (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 Plain within theUrstromtäler, for example, betweenHavel andFinow in 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.
Abifurcation is where the watershed is effectively in a river bed, in a wetland, or underground. The largest watershed of this type is the bifurcation of theOrinoco in the north ofSouth America, whose main stream empties into theCaribbean, but which also drains into the South Atlantic via theCasiquiare canal andAmazon River.
Since ridgelines are sometimes easy to see and agree about, drainage divides may formnatural borders defining political boundaries, as with theRoyal Proclamation of 1763 in British North America which coincided with the ridgeline of theAppalachian Mountains forming 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 betweenLabrador andQuebec, as arbitrated by the privy council in 1927.[9]
^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.JSTOR4247979.
