ARKANSAS, a river of the United States of America, rising in the mountains of central Colorado, near Leadville, in lat. 20' N., long. 1o6° 15' W., and emptying into the Mississippi, at Na poleon, Ark., in lat. 33° 4o' N. Its total length is about 2,000m., and its drainage basin (greater than that of the Upper Mississippi) about 185,000 sq. miles. It is the greatest western affluent of the Missouri-Mississippi system. It rises in a pocket of lofty peaks at an altitude of 1 o,400f t. on a sharply sloping plateau, down which it courses as a mountain torrent, dropping 4,625ft. in 120 miles. Above Canyon City it leaves the Rockies through the Grand Canyon of the Arkansas ; then turning eastward, it flows with steadily lessening gradient and velocity in a broad, meander ing 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 regu lation, the Arkansas is in many ways typical of the rivers in the arid regions of the Western States. The gradient below the moun tains averages 7.5ft. per mile between Canyon City and Wichita, Kan. (543m.), about I.5ft. between Wichita and Little Rock (659m.) , and o.65 of a foot from Little Rock to the mouth The shores are sand, clay or loam throughout some 1,300m., with very rare rock ridges or rapids, and the banks rise low above ordinary water. The waters are constantly rising and fall ing, 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. Every where along the course of the river there is never-ending transfor mation of the river's bed and contour. These changes become revo lutionary in times of flood. All these characteristics are accentu ated below Little Rock. The depth of water at this point has been known to vary from 2 7f t. to only lft. and the discharge to fall to 1,170 cu. ft. per second. 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 mean ders tortuously through the alluvial bottom in scores of great bends, loops and cut-offs. It is estimated that the eating and cav ing of the shore below Little Rock averages 7.64ac. per mile every year (as against 1.99ac. 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 of the Arkansas. This backwater, where it meets and checks the current of the Arkansas, occasions the pre cipitation 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 8f t. 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. Prior to the great flood of April and May 1927, much of the swamp area along the Arkansas had been reclaimed by means of levees built by local levee dis tricts in co-operation with the U.S. Government. By the middle of May 1927, the whole Arkansas valley, from Ft. Smith to the Mississippi, had been flooded and much of the levees destroyed. Three breaks in the right levee near the mouth of the river, known as the Pendleton breaks, allowed the waters to race through south eastern Arkansas, and to follow the valley of the Boeuf river into Louisiana. The damage caused by the flood, in common with other portions of the lower Mississippi valley, mounted into millions of dollars. U.S. army engineers believe that adequate flood protection will result from the $325,000,000 Mississippi Valley Flood Control act passed by Congress on May 9, 1928.
After the advent of railways, traffic on the Arkansas decreased rapidly because of the hazards of navigation—snags, sandbars and the lack of a stable flow. However, traffic in 1920 was 30,568 tons valued at $177,000; in 1926 it was 87,72o tons valued at $367,260.
BIBLIOGRAPHY.—General descriptions of different portions of the 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 (1879 seq.) . See also H. Gannett, "Profiles of Rivers in the U.S." (U.S. Geolog. Survey, 19o1) ; Greenleaf, "Western Floods," in Engin. Mag. xii., 945-958 ; U.S. Geolog. Survey, Bull. 14o; and I. C. Russell, Rivers of North America (1898).