Brown water can be caused by the corrosion of water pipes as materials flake away and find their way into water supplies, making the water an orange-brown color (from iron pipes) or even black (from lead pipes). Certain elements from sulfur dioxide can cause water to turn brown when combined with hot water.

Brown water in rivers, lakes, or coastal areas is usually a natural phenomenon and not necessarily a sign of pollution. The most common cause is the presence of tannins, which are organic compounds derived from the decomposition of plant material like leaves, peat, and roots; this essentially "steeps" the water like tea. Another frequent cause is sediment and silt being stirred up by heavy rainfall, storms, or fast-moving currents, a process known as turbidity. In 2026, increased frequency of extreme weather events often leads to more "runoff" from soil and clay, which turns river mouths brown. Additionally, certain types of algal blooms or high concentrations of iron and manganese in groundwater can contribute to a murky or rusty hue. While it may look unappealing for swimming, brown water caused by tannins or sediment is a natural part of the ecosystem's nutrient cycle.