I
River, central Uruguay.
It rises in the southern highlands of Brazil. Flowing southwest across Uruguay, it is dammed at the Rincón del Bonete Reservoir, the largest artificial lake in South America, with an area of roughly 4,000 sq mi (10,400 sq km). The river joins the Uruguay River at Soriano. Although it is 434 mi (698 km) long, it is navigable for only about 45 mi (70 km) upstream from its mouth.
II
or Guainía River
River, northwestern South America.
A major tributary of the Amazon River , it rises in the rainforest of eastern Colombia, where it is known as the Guainía, and forms a section of the border between Colombia and Venezuela. It crosses Brazil and joins the Amazon at Manaus . It is about 1,400 mi (2,250 km) long and is a major transport artery. Its name comes from its jet-black colour, which is caused by the decomposition of organic matter and its low silt content.