Thermal pollution, which is the act of changing the temperature of a water body by unnatural means, is most often caused by the heat generated from industrial processes. It is also sometimes caused by deforestation or urbanization practices such as removing trees from the shoreline or laying asphalt.
Industrial processes that use water as a cooling agent are most likely to cause thermal pollution. Factories sometimes use water from lakes or streams to cycle through the plant and reduce the temperature of machinery. The heat given off by the machinery is absorbed by the water, raising the overall temperature of the body of water when it is pumped back into the river or lake.
The contributions of deforestation and urbanization to thermal pollution are smaller than that of industrial processes, but they are still significant. When trees are removed from the shoreline, more sunlight reaches the water body, raising its temperature. Urbanization leads to thermal pollution in several ways. Asphalt, which is commonly used in cities, is dark in color and thus absorbs a lot of heat from sunlight. Water that collects on asphalt and runs off into rivers and lakes is hot, and it raises the overall temperature of the water body. Urbanization also often causes shoreline soils to be disturbed and soil to erode into water. Water that is cloudy because it contains soil particles absorbs more heat from the sun's rays than clear water.