Rio Negro is the largest tributary of the Amazon and the world’s largest black-water river. The name “Rio Negro” itself means “Black River”. The color comes from the presence of dissolved decaying vegetable matter that leached into the water as the river flowed through the rainforest and swamps. A black water river has high acidic levels, and very little sediment. The waters of the Amazon, on the other hand, is thick with sand, mud and slit giving it a brownish appearance.
Because of their different constituents, both rivers have different water density, speed and temperature due to which they are reluctant to mix. The cooler, denser, and faster waters of the Amazon and the warmer, slower waters of the Negro form a distinct boundary. Six kilometers downstream, the turbulent eddies driven by the faster-moving Amazon eventually mix the two, as they merge to become the Lower Amazon River.
Several dozen tour companies offer boat trips to the exact spot where the rivers meet. The best time to take tours is between January and July when the rivers are swollen with water. These tours are typically combined with a highly packaged tour of the Janauary Ecological Park where one can observe and take pictures of the aquatic plant water lilies.