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The Arkansas River is a major tributary of the Mississippi River. It generally flows to the east and southeast as it traverses the U.S. states of Colorado, Kansas, Oklahoma, and Arkansas. The river's source basin lies in Colorado, specifically the Arkansas River Valley. The headwaters derive from the snowpack in the Sawatch and Mosquito mountain ranges. It flows east into Kansas and finally through Oklahoma and Arkansas, where it meets the Mississippi River. 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. In 1859, placer gold discovered in the Leadville area brought thousands seeking to strike it rich, but the easily recovered placer gold was quickly exhausted. The Arkansas River's mouth is at Napoleon, Arkansas, and its drainage basin covers nearly 170,000 square miles (440,000 km⁲). Its volume is much smaller than the Missouri and Ohio rivers, with a mean discharge of about 40,000 cubic feet per second (1,100 m⁳/s). The Arkansas from its headwaters to the 100th meridian west formed part of the U.S.–Mexico border from the Adams–Onís Treaty until the Texas Annexation or Treaty of Guadalupe Hidalgo.