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ARKANSAS, a river of the United States of America, rising in the mountains of central Colorado, near Leadville, in lat. 39° 20′ N., long. 106° 15′ W., and emptying into the Mississippi, at Napoleon, ArKansas, in lat. 33° 40′ N. Its total length is about 2000 m., and its drainage basin (greater than that of the Upper Mississippi) about 185,000 sq. m. It is the greatest western affluent of the Missouri-Mississippi system. It rises in a pocket of lofty peaks at an altitude of 10,400 ft. on a sharply sloping plateau, down which it courses as a mountain torrent, dropping 4625 ft. in 120 m. At Canyon City it passes out of the Rockies through the Grand Canyon of the ArKansas; then turning eastward, and soon a turbid, shallow stream, depositing its mountain detritus, it flows with steadily lessening gradient and veloCity in a broad, meandering bed across the prairies and lowlands of eastern Colorado, Kansas, Oklahoma and ArKansas, shifting its direction sharply to the south-east in central Kansas. The ArKansas ordinarily receives little water from its tributaries save in time of floods. In topography and characteristics and in the difficulties of its regulation the ArKansas is in many ways typical of the rivers in the arid regions of the western states. The gradient below the mountains averages 7.5 ft. per mile between Canyon City and Wichita, Kansas (543 m.), about 1.5 ft. between Wichita and Little Rock (659 m.), and 0.65 of a foot from Little Rock to the mouth (173 m.). The shores are sand, clay or loam throughout some 1300 m., with very rare rock ridges or rapids, and the banks rise low above ordinary water. The waters are constantly rising and falling, and almost never is the discharge at any point uniform. Every year there are, normally, two distinct periods of high water; one an early freshet due mainly to the heavy winter rainfall on the lower river, when the upper river is still frozen hard; the other in the late spring, due to the setting in of rains along the upper courses also, and to the melting of the snow in the mountains. The lowest waters are from August to December. In the summer there are sometimes violent floods due to cloud-bursts. Everywhere along the river there is a never-ending variation of veloCity and discharge, and an equally ceaseless transformation of the river’s bed and contour. These changes become revolutionary in times of flood. All these characteristics are accentuated below Little Rock. The depth of water at this point has been known to vary from 27 ft. to only half-a-foot, and the discharge to fall to 1170 cub. ft. per second. There is often no more than 1.5 ft. of water, and far below Little Rock a depth of 3 ft. on crossings is not infrequent. In many places there are different channels for high and low water, the latter being partly filled by each freshet, and recut after each subsidence; and the river meanders tortuously through the alluvial bottom in scores of great bends, loops and cut-offs. It is estimated that the eating and caving of the shore below Little Rock averages 7.64 acres per mile every year (as against 1.99 acres above Little Rock). By way of the White river cut-off the ArKansas finds an additional outlet through the valley of that river in times of high water, and the White, when the current in its natural channel is deadened by the backwaters of the Mississippi, finds an outlet by the same cut-off through the valley 551 of the Arkansas. This backwater, where it meets and checks the current of the Arkansas, occasions the precipitation of enormous alluvial deposits, and vast quantities of snags. The banks are disintegrated along this part of the river and built up again on the opposite side to their original height in the extraordinarily short time of two or three years, the channel remaining all the while narrow. At the mouth of the White, the Arkansas and the Mississippi the level of recurrent floods is 6 or 8 ft. above the timber-bearing soil along the banks, and all along the lower river the country is liable to overflow; and as the land backward from the stream slopes downward from the banks heaped up by successive flood-deposits, each overflow creates along the river a fringe of swamps. These features, although exaggerated in the portion of the river now in question, are qualitatively characteristic of its entire course below the mountains.

Up to the 30th of June 1907 the government of the United States expended ,384,557 on improvements along the Arkansas. Almost half of this sum was required for snagging operations alone. There is a considerable traffic on the river within the borders of Arkansas in miscellaneous freights, and a slight passenger movement. The river is rarely navigable above Fort Smith, and during a considerable part of the year not above Pine Bluff. Steamer service is maintained the year round between this point and Memphis. Ordinarily there are some 400 m. of channel open to steamers part of the year, and in time of high flood considerably more. To the mouth of the Grand river (460 m.) the river is open about four months in a year for vessels of 4 ft. draft and about eight months for vessels of 2 ft. draft.

Bibliography.—General descriptions of different portions of the river are indicated in the Index to the Reports of the Chief of Engineers, U.S. Army (many volumes, 1879-1900). See also H. Gannett, Profiles of Rivers in the U.S. (U.S. Geolog. Survey, 1901); Greenleaf, “Western Floods,” in Engin. Mag. xii. 945-958; U.S. Geolog. Survey, Bull. 140; I.C. Russell, Rivers of North America (1898); T.J. Vivian, Transportation, Rivers of the Miss. Valley (U.S. Census, 1890, special Rp.).

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