The NEGRO is a tributary of the Amazon River, located in Brazil. It is the second largest tributary of the Amazon, with a length of 2, 250 km and a drainage basin of approximately 1, 000, 000 km². The NEGRO is known for its dark waters, which are caused by the high concentration of humic acid from decaying organic matter. The river is home to a variety of fish species, including the pirarucu, one of the largest freshwater fish in the world.