The Amazon river carries a lot of sediment (particles of mud and sand), which gives the water a muddy-brown color. Its largest tributary (branch), the Rio Negro, or black river, is filled with chemicals washed out of soil and plants, making the water very dark.

The "black" water of the Amazon, most notably in the Rio Negro, is caused by high concentrations of tannins and humic acids leached from decomposing leaf litter and vegetation in the surrounding rainforest. As the river flows through dense jungles, the organic matter breaks down in the water, creating a dark, tea-like transparency. Despite its appearance, the water is remarkably low in sediment and is actually quite clean. This contrasts sharply with the "white" rivers (like the Solimões) that carry heavy silt from the Andes. When these two meet, they create a famous "Meeting of Waters" where they flow side-by-side without mixing for miles due to differences in temperature, speed, and density.