The rates of biological and chemical processes depend on temperature. Aquatic organisms from microbes to fish are dependent on certain temperature ranges for their optimal health. Optimal temperatures for fish depend on the species: some survive best in colder water, whereas others prefer warmer water. Benthic macroinvertebrates are also sensitive to temperature and will move in the stream to find their optimal temperature. If temperatures are outside this optimal range for a prolonged period of time, organisms are stressed and can die. Temperature is measured in degrees Fahrenheit (F) or degrees Celsius (C).

Temperature affects the oxygen content of the water (oxygen levels become lower as temperature increases); the rate of photosynthesis by aquatic plants; the metabolic rates of aquatic organisms; and the sensitivity of organisms to toxic wastes, parasites, and diseases.

Causes of temperature change include weather, removal of shading streambank vegetation, impoundments (a body of water confined by a barrier, such as a dam), discharge of warm water from industrial facilities, urban storm water, and groundwater inflows to the stream.

1. The thermometers have an adhesive back. Adhere them to the kit container or another object to make grasping them easier. The temperature is indicated by a liquid crystal number on the Low Range thermometer and a green display on the High range thermometer.

2. While wearing protective gloves, place the thermometer four inches below the water surface for one minute.

3. Remove the thermometer from the water, read the temperature and record the temperature as ° C.

4. Repeat the test approximately 1 mile upstream as soon as possible.

5. The difference between the temperature upstream and the temperature at the sampling site is the change in temperature.