Thermal pollution is the degradation of water quality by any process that changes ambient water temperature. Many industries use high temperature furnace and boilers in their manufacturing processes. The resulting hot effluents are directly released in to a nearby water source. This is called thermal pollution. It destroys the aquatic life due to the thermal shock. A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers. When water used as a coolant is returned to the natural environment at a higher temperature, the change in temperature decreases oxygensupply and affects ecosystem composition. Urban runoff–stormwater discharged to surface waters from roads and parking lots–can also be a source of elevated water temperatures.
When a power plant first opens or shuts down for repair or other causes, fish and other organisms adapted to particular temperature range can be killed by the abrupt change in water temperature known as "thermal shock."