BASIN (OF. basin., from LLat. bachinus, from Lucca, water-vessel). in geology and hydro graphy, a depression of the earth's surface, or of the strata constituting the crust of the earth, also the drainage area of a river system. Basins originate in several ways, and may be grouped under two general heads: those formed by crustal movements and those due to erosion. Most basins are due in part to both agencies. Basins formed by movements of the earth's crust are apt to be of great extent according as they owe their origin to either localized or continental movements. The Mississippi Valley, previous to Tertiary time, was occupied by a shallow arm of the sea that covered the central portion of the North American continent, Elevation of the continent raised the district above sea-level and drained the marine waters from the hand, and orogvnic movements partially surrounded it with mountains, with the result that an extensive interior basin was formed, the larger part of which is now drained by a single river system, that of the Mississippi. Other portions of the same basin are drained by the Saint Lawrence River and by the rivers that flow into the Arctic Ocean. We have here an example of a geologic basin that contains several hydrographic basins.
The inclosed basins of Utah, Nevada, Arizona, and Mexico have been formed by local move ments, folding, and faulting of the crust, that have culminated in the elevation of mountains unclosing plains. The elevation of the moun tains has cut these plains off from the supply of moisture-laden winds, and arid desert con ditions have largely ensued. During a former more humid period in Pliocene time great lakes occupied many of these basins, but the'ir waters have disappeared largely through evaporation and lack of supply by rains, and concentrates of alkali, salt, and gypsum now cover large areas of their desert bottoms, while traces of their former shore lines are to be found far up on the mountain sides. See LAKE BONNEVILLE; LAKE LAIIONTAN; GREAT SALT LAKE.
Another form of basin results when a section of the earth's crust subsides between two nearly parallel great faults or dislocations, to form what is known as a 'rift valley' ((, v.). The most striking basins of this kind are found in Western Asia and Eastern Africa. The lamest basin in this region begins north of the Dead Sea, and ex tends southward through the valleys of the Dead Sea, the river Jordan, and the Red Sea, whence the rift is continued in a southerly direction across Eritrea in Africa. to a point near Mount Kilimanjaro. This may be considered a continu ous single depression or rift-valley basin, with a length of about 3500 miles and a maximum width at the Red Sea of 200 miles. Another such depression extends westward of the Victoria Nyanza for nearly 1500 miles in a general south erly direction, and includes the valleys of the Albert Nyanza, Tanganyika, and Nyassa, ter minating at the coast in the vicinity of the mouth of the Zainhezi River. The amount of dis placement of the bed of this latter rift has been enormous, the vertical throw of the faults having been at least 5000 feet in the vicinity of Lake Tanganyika. In volcanic regions basins tilled by
lakes often occur in the craters of extinct vol canoes. Crater Lake, in Oregon, is a good ex ample.
In coal-mining regions the term basin is ap plied to synclinal depressions of the coal-bearing strata that have been produced by the folding of the beds incident to their elevation into moun tains. Such a stratigraphic basin in the crust of the earth is not usually co-extensive with a hydrographic basin on the surface, for the reason that synclinal depressions generally occupy ridges. The anticlinal upfolds have been worn away by weathering, and are now replaced by valleys or hydrographic basins.
Basins formed by erosion are of two kinds, those formed by the work of rivers and those due to the wearing action of glacier ice. The former are the larger and more common, though the latter often constitute prominent features of the landscape of northern countries. The basin of a river, when considered hydrographically, con sists of the entire area drained by the main stream and its tributaries. This hydrographic basin may be in large part the result of oro genie movements; but its present boundaries have been determined entirely by the cutting power of its streams, and accordingly a hydrographic basin may consist of one or more geologic basins. The Hudson River system furnishes a good example of such a complex basin. The Lower Hudson River, from New York City to Peekskill; the Up per Hudson, from Peekskill to the Adirondacks; the Schroon River in the Adirondacks; the Mohawk River; the Schoharie, Esopu_s, and Cats kill creeks, in the Catskill Mountains; and the Rondont in the Shawangunk Mountains all drain distinct geological basins, the divides between which have been cut down by the rivers, till now the entire region is included in a single hydro graphic system.
Glacial basins have been formed by the eroding power of ice acting on the door and sides of valleys already formed by some other means.
The basins usually hold lakes that form parts of river basins. Such are the 'finger lakes' of cen tral New York. Round lakes occupying shallow, bowl-shaped depressions on glaciated surfaces are common. A basin may also be formed when a landslide or moraine obstructs a valley. Such. a basin and its contained lake is of an evanescent character, the extent of its duration depending upon the rate at which the overflow can cut down or remove the obstruction.
Basins tend in time to become shallower through the accumulation upon their floors of diThris worn from the surrounding mountains. This process would, if continued without inter ruption, eventually fill the basin to its brim with deposits of gravel, sand, and silt, and a plain would result which, under conditions of sufficient moisture, would be of great fertility. Such waste-filled basins are common in the Paci fic States. The discussion of the origin and char acteristics of oceanic basins is reserved for the article on OCEAN. See also articles on RIFT VALLEY; LAKE; RIVER; HYDROGRAPHY ; PHYSI OGRAPHY.