At 1,469 miles (2,364 km) it is the sixth longest river in the United States, the second-longest tributary in the Mississippi-Missouri system, and the 45th longest river in the world. Its origin is in the Rocky Mountains in Lake County, Colorado, near Leadville, and its outlet is at Napoleon, Arkansas. The Arkansas River drainage basin covers nearly 195,000 sq mi (505,000 km²). In terms of volume, the river is smaller than both the Missouri and Ohio Rivers, with a mean discharge of 8,460 ft³/s (240 m³/s).
The Arkansas has three distinct sections in its long path through central North America.
At its headwaters the Arkansas runs as a steep mountain torrent through the Rockies in its narrow valley, dropping 4,600 feet (1.4 km) in 120 miles (193 km). This section (including The Numbers, Brown's Canyon, and the Royal Gorge) sees extensive whitewater rafting in the spring and summer.
At Cañon City, Colorado, the Arkansas River valley widens and flattens markedly. Just west of Pueblo, Colorado, the river enters the Great Plains. Through the rest of Colorado, through Kansas, and through northern Oklahoma to Tulsa, it is a typical Great Plains riverway, with wide shallow banks subject to seasonal flooding. Tributaries include the Canadian River and the Cimarron River (both flowing from northeastern New Mexico) and the Salt Fork Arkansas River.
Below Tulsa, and continuing to its mouth, the river is navigable by barges and large river craft thanks to a series of dams that turn it into reservoirs. (Above Tulsa, it is navigable only by small craft such as rafts, canoes, and kayaks.)
Water flow in the Arkansas River (as measured in central Kansas) has dropped from approximately 248 cubic feet per second (7 m³/s) average from 1944-1963 to 53 cubic feet per second (1.5 m³/s) average from 1984-2003, largely because of the pumping of groundwater for irrigation in eastern Colorado and western Kansas.
Through Oklahoma and Arkansas, dams artificially deepen and widen this modest sized river to build it into a commercially navigable body of water. From the mouth of the Verdigris until the McClellan-Kerr system moves over to the White River near Arkansas Post, the Arkansas sustains commercial barge traffic and offers passenger and recreational use and is little more than a series of reservoirs.
From 1819 the Adams-Onís Treaty set the Arkansas as part of the frontier between the United States and Spanish Mexico, which it remained until the annexation of Texas and Mexican-American War in 1846.
Later, the Santa Fe Trail followed the Arkansas through much of Kansas except for the Cimarron Cutoff from Cimarron, Kansas, to Cimarron, New Mexico, via Cimarron County, Oklahoma along the Cimarron River.