Temperature influences the amount of oxygen that can be dissolved in water. Cold water holds more oxygen than warm water. The more oxygen in the water, the more living things it can support. Temperature also influences the rate of photosynthesis by algae and larger aquatic plants. As water temperature rises, the rate of photosynthesis by algae and larger aquatic plants increases. The faster plants grow, the faster plants die. As plants die, they are decomposed by bacteria that consume, or use up, oxygen. Therefore, when the rate of photosynthesis is increased, the demand for oxygen in the water (BOD) is also increased.

Temperature influences the metabolic rates of aquatic organisms. Metabolic rate is the speed at which cells conduct the chemical processes of life. Most aquatic animals are cold-blooded. Their metabolic rate is slower in cold water but faster in warm. Therefore, they need more food and oxygen in warm water. They will also release more wastes.

Temperature also influences the sensitivity of organisms to toxic wastes, parasites, and diseases. Bacteria and other disease-causing organisms grow faster in warm water.

One of the most serious ways humans can harm a river is by causing thermal pollution. Thermal pollution is an increase in water temperature caused by adding warm water to a river. Industries, such as nuclear power plants, may cause thermal pollution by releasing water used to cool machinery. Thermal pollution may also result from storm water running off warm urban surfaces, such as streets, sidewalks and parking lots.

People also affect water temperature by cutting down trees. Trees help shade a river. Water exposed to direct sunlight can be as much as 20°F warmer than water that is shaded. Cutting trees often increases soil erosion. Soil erosion can contribute to warm water temperatures. Increased amounts of suspended solids carried by the river make the water cloudy or turbid. Turbid water absorbs more of the sun's rays which causes the water's temperature to rise.