Araguaía (ärägwĪˈə) [key], river, c.1,600 mi (2,575 km) long, rising in the Serra des Araras, at the border of Goiás and Mato Grosso states, S central Brazil. It flows generally northward into the Tocantins River, forming most of the border between Goiás and the states of Mato Grosso and Pará. Diamonds are washed along its upper tributaries. There are numerous falls on the Araguaía. The island of Bananal (c.200 mi/320 km long; 35 mi/56 km wide), separating the river into two arms, is one of the largest freshwater islands in the world. It is also a national park. The Araguaía region has been made accessible by new highways. The Brazilians have constructed hydroelectric projects along the Araguaía.