Thermal pollution is the change in the water temperatures of lakes, rivers, and oceans caused by man-made structures. These temperature changes may adversely affect aquatic ecosystems especially by contributing to the decline of wildlife populations and habitat destruction. Any practice that affects the equilibrium of an aquatic environment may alter the temperature of that environment and subsequently cause thermal pollution. There may be some positive effects, though, to thermal pollution, including the extension of fishing seasons and rebounding of some wildlife populations. Thermal pollution may come in the form of warm or cold water being dumped into a lake, river, or ocean. Increased sediment build-up in a body of water affects its turbidity or cloudiness and may decrease its depth, both of which may cause a rise in water temperature. Increased sun exposure may also raise water temperature. Dams may change a river habitat into a lake habitat by creating a reservoir (man-made lake) behind the dam. The reservoir water temperature is often colder than the original stream or river. The sources and causes of thermal pollution are varied, which makes it difficult to calculate the extent of the problem. Because the thermal pollution caused by Hydropower Plants (HPPs) may not directly affect human health, it is neglected in general. Therefore, sources and results of thermal pollution in HPPs are ignored in general. This paper aimed to reveal the causes and results of thermal pollution and measures to be taken in HPPs.