The temperature of stream water is influenced by both natural processes and human activities. Climatic zone, altitude, air temperature, and season of the year produce variation in water temperature. Other natural factors include shade provided by streamside vegetation, depth, flow rate, snow melt, and mixing with ground water.
Human activities can introduce thermal pollution into streams in several ways.
Industries and power plants may use water to cool machinery and then discharge the warmed water into a stream. In the summer, storm water warmed by urban surfaces, such as roads, roofs, and parking lots, can flow into nearby streams. Water temperature rises when trees and tall vegetation providing shade are cut down. Soil erosion caused by construction, removal of streamside vegetation, poor farming practices, overgrazing, and recreation increases the amount of suspended solids in the water. The suspended particles absorb the sun's rays and also increase water temperature.