The water temperature of a river is very important for water quality. Many of the physical, biological, and chemical characteristics of a river are directly affected by temperature. For example, temperature influences:

1. the amount of oxygen that can be dissolved in water;
2. the rate of photosynthesis by algae and larger aquatic plants;
3. the metabolic rates of aquatic organisms;
4. the sensitivity of organisms to toxic wastes, parasites, and diseases.

Remember, cool water can hold more oxygen than warm water, because gases are more easily dissolved in cool water.

Human-Caused Changes in Temperature

One of the most serious ways that humans change the temperature of rivers and lakes is through thermal pollution. Thermal pollution is an increase in water temperature caused by adding relatively warm water to a body of water. Industries, such as nuclear power plants, may cause thermal pollution by discharging water used to cool machinery. Thermal pollution may also come from stormwater running off warmed urban surfaces, such as streets, sidewalks, and parking lots.

People also affect water temperature by cutting down trees that help shade the river, exposing the water to direct sunlight.

Soil erosion can also contribute to warmer water temperatures. Soil erosion can be caused by many types of activities, including the removal of streamside vegetation, overgrazing, poor farming practices, and construction. Soil erosion raises water temperatures because it increases the amount of suspended solids carried by the river, making the water cloudy (turbid). Cloudy water absorbs the sun's rays, causing water temperature to rise.

Changes in Aquatic Life

As water temperature rises, the rate of photosynthesis and plant growth also increases. More plants grow and die. As plants die, they are decomposed by bacteria that consume oxygen. Therefore, when the rate of photosynthesis is increased, the need for oxygen in the water (BOD) is also increased.

The metabolic rate of organisms also rises with increasing water temperatures, resulting in even greater oxygen demand. The life cycles of aquatic insects tend to speed up in warm water. Animals that feed on these insects can be negatively affected, particularly birds that depend on insects emerging at key periods during their migratory flights.

Most aquatic organisms have adapted to survive within a range of water temperatures, Some organisms prefer cooler water, such as trout, stonefly nymphs, while others thrive under warmer conditions, such as carp and dragonfly nymphs. As the temperature of a river increases, cool water species will be replaced by warm water organisms. Few organisms can tolerate extremes of heat or cold.

Temperature also affects aquatic life's sensitivity to toxic wastes, parasites, and disease. For example, thermal pollution may cause fish to become more vulnerable to disease, either due to the stress of rising water temperatures or the resulting decrease in dissolved oxygen.

Sampling Procedure

The temperature test measures the change in water temperature between two points, the test site and a site one mile upstream. By detecting significant temperature changes along a section of the river, we can begin to uncover the sources of thermal pollution.

Because the temperature test compares the difference in water temperature at two different stream sites, it is important to match as closely as possible the physical conditions at these sites - current speed, amount of sunlight reaching the water, and the depth of the stream.

To reduce errors, the same thermometer should be used at both sites. Rubber gloves should be worn if there is any chance that hands might come in contact with the water.

Temperature Testing Procedure