Solimões River, Portuguese Rio Solimões, the section of the upper Amazon River in Amazonas estado (state), northwestern Brazil. The Solimões flows from the Brazilian-Peruvian border on the west to its confluence with the Negro River near Manaus. The junction is known as the “meeting of waters,” where the muddy, brown-coloured Solimões waters meet the black Negro waters to form the Amazon River. Islands covered with tropical rainforest separate the maze of channels that constitute the river. The Solimões River flows for about 1,000 miles (1,600 km) through a floodplain about 50 miles (80 km) wide.