Arkansas, a S. W. river of the United States, the largest tributary of the Mississippi except the Missouri. Its extreme sources, which were first explored in 1806 by Lieut. Pike, U. S. A., lie in the Rocky mountains W. of the South Park, in lat. 39° N., lon. 106° W., at an elevation of 10,000 ft. above the sea level, which is reduced one half in the first 150 m. It flows E. through Colorado to near the centre of Kansas, and thence generally S. E. through the Indian territory and Arkansas to its junction with the Mississippi at Napoleon, 275 m. (direct) above New Orleans; length, over 2,000 m. Near its source the river pours with great violence through a deep and narrow fissure in the gneiss rock; after leaving the mountains it traverses a sterile hilly region, sustaining considerable timber. The width of the Arkansas undergoes great variations. From 150 ft. near the mountains, it gradually increases to about a mile as it traverses the sandy desert; and after entering the hilly region it varies from 1,000 to 2,000 ft. Through the prairie region the ordinary depth is about 2 or 3 ft., but there are seasons when the water entirely dis-appears, being absorbed by the immense beds of sand through which it flows.

The range of the river between low and high water is about 45 ft. at Napoleon, 40 at South Bend, 35 at Little Rock, 25 at Fort Smith, and 10 at Fort Gibson, 642 m. from the mouth. It is navi-gable for steamboats of 3 to 4 ft. draught to a point 40 m. above Little Rock, and during the. floods as far as Fort Smith and Fort Gib-son. Below Pine Bluff it has been neces-sary to construct levees to restrain the flood. For a distance of 400 m. from its mouth the Arkansas has many lakes and bayous. Its principal tributary is the Canadian in the Indian territory. The White and Arkansas are connected by a large bayou 6 m. above the junction of the former with the Mississippi, through which the current moves sometimes in one direction and sometimes in another, according to the relative stand of the rivers. The principal points on the Arkansas are Napoleon, Arkansas Post, Pine Bluff, Little Rock, Van Burcn, and Fort Smith in Arkansas, and Fort Gibson in the Indian territory.