The Rio Negro is South America’s largest blackwater river and supports many endemic fish species.
Flowing through Colombia, Venezuela, and Brazil before joining the Solimões River near Manaus, the Rio Negro gets its dark tea color from dissolved organic compounds released as forest vegetation breaks down. Unlike muddy whitewater rivers, blackwater rivers are typically acidic and low in dissolved minerals.
Those unusual conditions have produced a distinctive aquatic community. About 700 fish species have been documented in the Rio Negro basin, with estimates of 800 to 900 species overall and nearly 100 endemics. Aquarium favorites such as the cardinal tetra are among its best-known fish, but many species remain poorly studied.
The Rio Negro is a major tributary of the Amazon, not a rival river that flows separately to the Atlantic. It meets the pale, sediment-rich Solimões at the famous Meeting of Waters; downstream, the combined river is called the Amazon. Its dark color also does not mean the water is polluted—it is primarily a natural result of tannins and humic substances.