Thermal pollution is the degradation of water quality by any process that changes ambient water temperature. 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 sudden change in temperature decreasesoxygen supply and affects ecosystem composition. Fish and other organisms adapted to particular temperature range can be killed by an abrupt change in water temperature (either a rapid increase or decrease) known as "thermal shock."
Urban runoff—stormwater discharged to surface waters from roads and parking lots—can also be a source of elevated water temperatures.
When thermal pollution happens, the water temperature in streams, rivers, lakes, or oceans changes. It is one of the most serious types of pollution.
Thermal pollution can have many different causes. Power station are one factor. They use water in the nearby lakes or rivers to cool their equipment, which heats up the water. Then they dump the hot water back into its source. Thermal pollution may also happen due to the discharge of cold water from reservoirs into warm rivers.
Thermal pollution can have dramatic effects. Most aquatic creatures need a certain temperature to survive. Warmer water has less oxygen in it, and this can harm fish populations. Besides, warmer water can cause harmful algal blooms. This can change the colour of the water like in the first picture and, more seriously, the algea poisons the fish.
So, in many places, we build cooling towers like in the second picture to cool down the water from power stations.
