The most likely source of thermal pollution in a river ecosystem would be one harmful effect that this type of pollution has is to killing fish and other native animals.

The most likely source of thermal pollution in a river ecosystem would be the use of water as a coolant by power plants and industrial manufacturers. One harmful effect that this type of pollution has is a sudden change in water temperature (either a rapid increase or decrease) known as "thermal shock.", killing fish and other native animals.
>Thermal pollution is defined as the degradation of water quality by any means that changes ambient water temperature.