Sunset on the Javaé river, Bananal island, Tocantins State, Amazon rain forest, Brazil. Bananal Island (Portuguese: Ilha do Bananal) is a large island formed from the bisection of the Araguaia River, in Tocantins, Brazil. The island is formed by a fork in a very flat section of the Araguaia River. Bananal Island is the largest fluvial island in the world, at 350 km long and 55 km wide