Thermal pollution is basically the form of water pollution that refers to degradation of water quality by any process that changes ambient water temperature. The main cause of thermal pollution is our industry, or to be more precise power plants that use water as a coolant. After this water has been used as coolant it is returned to its natural environment at a higher temperature. This change in water temperature decreases the amount of oxygen in the water which can lead to many negative ecological effects.
Less oxygen in the water can harm fish population, for instance it can increase the metabolic rate of fish and other aquatic animals so they eat lot more food in a shorter time than if their environment were not changed. This can lead to imbalance in food chain resulting in significant damage to many aquatic ecosystems.
Warmer water temperatures can also lead to reproduction problems for many aquatic animals, and can cause huge bacteria and plant growth. It can even lead to algae bloom resulting in even less oxygen in the water.
Industry doesn't always return water at higher temperatures, sometimes the water can return to its natural environment at lower temperatures. Colder water can have even more serious consequences to fish population than warmer water, as it can completely disrupt their reproduction ability.
There have been several cases around the globe where colder water created total extinction of native fish species which lead to the drastic changes in these ecosystems.
In United States power plants currently account for more than three quarters of total thermal pollution. The thermal pollution levels are still relatively high in United States despite the controlling measures like the implementation of cooling ponds and cooling towers in some power plants. This is because many power plants in United States use less efficient cooling solutions like once-through cooling (OTC).
Urban runoff can also become the source of thermal pollution especially in smaller streams once stormwater passes over hot parking lots and roads and enters the water body.
Thermal pollution still doesn't get much of public attention. In most cases it is being scarcely mentioned when discussing the general water pollution issue.