High Quality Content by WIKIPEDIA articles! The Arkansas River is a major tributary of the Mississippi River. The Arkansas generally flows to the east and southeast and traverses the U.S. states of Colorado, Kansas, Oklahoma, and Arkansas. 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 mouth 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 from its headwaters down to the 100th meridian west formed part of the US-Mexico border from the Adams-Onís Treaty (in force 1821) until the Texas Annexation or Treaty of Guadelupe Hidalgo.