Erosion from river banks brings soil into the river, changing its color. After heavy storms, many rivers run brown from all the runoff flowing into the river. Clay can cause rivers to be murky muddy brown, or yellow. Algal blooms are naturally occurring overgrowths of algae caused by sunlight, slow water, or nutrients.

The brown color of river water is primarily caused by suspended sediment and dissolved organic matter. When heavy rain or rapid snowmelt occurs, the increased volume of water erodes soil, clay, and silt from the riverbanks and the surrounding landscape, carrying these particles downstream. This is known as "turbidity." Additionally, rivers often carry "tannins"—organic compounds that leach from decaying leaves, peat, and wood, much like tea steeping in a pot. These tannins can turn the water a dark, tea-like brown or even black. In some regions, the presence of iron oxides in the soil can give the water a rusty or orange-brown tint. While brown water might look "dirty," it is often a sign of a highly productive ecosystem carrying vital nutrients to downstream deltas. However, excessive brownness can also be a result of human activity, such as runoff from construction sites, logging, or industrial farming, which can introduce unnaturally high levels of sediment that can smother fish spawning grounds and block the sunlight needed for underwater plant growth.