There are four major ways that freshwater can be polluted:

• domestic pollution, for example, sewage and other organic matter, detergents and other household cleaners

• soil pollution where land erosion is washed into the watercourse, this collects as silt often in estuaries

• industrial and chemical pollution when toxic substances and other wastes are discharged either with permission or accidentally

• thermal pollution from power station cooling towers and some industrial effluents are warmer than the receiving waters