The water temperature of a river is very important for water quality. Most of the physical, biological, and chemical characcteristics of a river are directly affecte by temperature. Temperature influences ammount of oxygen that can be dissolved in water (see dissolved oxygen), rate of photosynthesis, metabolic rates of aquatic organisms, and sensitivity of organisms to toxic wastes, parasites, and diseases.

Humans affect water temperature mainly through thermal pollution.This is caused by the addition of relatively warm water to the river. This comes from industries such as power plants that may be discharging water used to cool machinery. The cutting down of trees also affects the temperature since thre trees help shade to water.

As water temperature rises, the rate of photsynthesis and plant gwoth increases. More plants grow then die, which increases the decomposition by bacteria that consume oxygen. Therefore, the need for oxygen in the water (see Biochemical Oxygen Demand) is also increased.