The abundance of sediment?bits of rocks, soil, and clay carried by currents or resting on the bottom?is what gives much of the main stem of the Amazon River its milky brown color.

The "brown" or "cafe con leche" color of the Amazon River is primarily caused by suspended sediment and silt eroded from the Andes Mountains. These are known as "white-water" rivers (like the Solimões). As the water flows down from high altitudes, it carries high concentrations of minerals, sand, and clay, which give it its murky, opaque appearance. This is distinct from the "black-water" tributaries like the Rio Negro, which appear like dark tea because of tannins from decaying leaf litter and vegetation that dissolve into the water without the heavy sediment. When these two types of water meet at the famous "Meeting of the Waters" near Manaus, they flow side-by-side for miles without mixing due to differences in temperature, speed, and density. By 2026, environmental monitoring shows that seasonal flooding significantly increases this "brown" intensity as more topsoil is washed into the main channel, providing the vital nutrients that sustain the entire basin's ecosystem.