At 1469 miles in length, the Arkansas River is the second longest tributary in the Mississippi-Missouri system. It was first discovered and explored by Hernando De Soto in 1541. Joliet and Marquette, exploring for France, reached the mouth of the Arkansas on the Mississippi in 1673. They applied the name Riviere des Ark, or Riviere d'Ozark, derived from the Indians who lived on the river's banks, which the French called Arkansea. De Soto's chroniclers had called them Pacaha and Capaha. The Arkansas Post was established by Henry Tonti in 1686 as the first permanent settlement in the region of the Arkansas River. For many years, the Arkansas Post was the administrative center from which the French operate in the region. In the Civil War, it was fortified by the Confederates, who were forced to surrender it by a Union army force and a fleet of ironclads on January 11, 1863. Located in present day Arkansas County, the site is now a National Memorial. The French used the Arkansas as a highway into the Spanish territory of the Southwest, where the headwaters were located. In 1696, the Spanish explorer Uribarri had given the name Rio Napestle to the upper reaches of the river, which was the name that the Spanish continued to use. As a result of the Adams-Onis Treaty of 1819, the Arkansas River west of the 100th meridian became part of the boundary of the United States. As American traders pressed westward, they carried their name for the river until it became the standard throughout the river's length. The Arkansas River Route was a portion of the Santa Fe Trail that avoided the Jornada desert. Although longer, it passed the important Bent's Fort and had easier availability of water. This was the route taken by prospectors rushing to Colorado in the Pikes Peak Gold Rush of 1858.