Arkansas (ärkănˈzəs, ärˈkənsôˌ) [key], river, c.1,450 mi (2,330 km) long, rising in the Rocky Mts., central Colo., and flowing generally SE across the plains to the Mississippi River, SE Ark.; drains 160,500 sq mi (415,700 sq km). The Canadian and Cimarron rivers are its main tributaries. It is the chief waterway for the state of Arkansas, where it drains a broad valley. The upper course of the Arkansas River has many rapids and flows through Royal Gorge, one of the deepest canyons in the United States. More than 25 dams on the river provide flood control, power, and irrigation. During the warm months, because of its extensive use for irrigation, the middle course of the Arkansas is reduced to a trickle. The John Martin dam and reservoir in Colorado is one of the largest water-storage and flood-control units in the river basin. The Arkansas River Navigation System, opened in 1971, makes the river navigable to Tulsa, Okla., 440 mi (708 km) upstream. The Spanish explorers Coronado and De Soto probably traveled along portions of the river in the 1540s. In 1806, Zebulon Pike, an American army officer, explored the river's upper reaches in Colorado. The Arkansas River was an important trade and travel route in the 19th cent.