The Arkansas River is a major tributary of the Mississippi River that stretches for 1,469 miles. Its source lies in Colorado, and it flows into the Midwest via Kansas, and into the American South via Oklahoma and Arkansas. On 29 June 1541, Francisco Vasquez de Coronado's Spanish expedition became the first group of Europeans to see the river, and the French later traded with the local Native Americans. In 1819, the Adams-Onis Treaty set the Arkansas as part of the US-Mexico frontier, and it continued to be so until the Mexican-American War and the annexation of Texas. During the American Civil War, the Union abandoned Fort Gibson and Fort Smith in the Indian Territory, but, following their victories at Pea Ridge and Fort Smith, they reopened the Arkansas River as a supply route.