Thermal pollution is the degradation of water quality caused by artificial changes in water temperature, most commonly from the discharge of heated water from power plants and industrial facilities.
Thermal pollution occurs when human activities alter the natural temperature of water bodies, primarily through the discharge of heated cooling water from thermoelectric power plants, industrial processes, and nuclear facilities. Power plants are the largest source, using vast quantities of water for condenser cooling and returning it to rivers, lakes, or coastal waters at temperatures 5-15 degrees Celsius above ambient. Elevated water temperatures decrease dissolved oxygen solubility, increase metabolic rates of aquatic organisms, and can exceed the thermal tolerance of sensitive species such as cold-water fish (trout, salmon). Warm water discharges can create thermal plumes that attract some species while repelling others, altering community composition and disrupting spawning cycles. The Clean Water Act Section 316(a) allows thermal variances for power plants that demonstrate their discharges do not harm balanced populations of aquatic organisms. Cooling technologies designed to reduce thermal pollution include cooling towers, cooling ponds, and closed-cycle recirculating systems. Urban runoff from heated impervious surfaces such as parking lots and rooftops can also contribute to thermal pollution in streams during summer storms.