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Stream Water Quality - Importance of Temperature

Why is Temperature Important?

Temperature is a critical water quality and environmental parameter because it governs the kinds and types of aquatic life, regulates the maximum dissolved oxygen concentration of the water, and influences the rate of chemical and biological reactions.

The organisms within the ecosystem have preferred temperature regimes that change as a function of season, organism age or life stage, and other environmental factors. With respect to chemical and biological reactions, the higher the water temperature the higher the rate of chemical and metabolic reactions and the lower the amount of dissolved gases it can contain.

Seasonal variations in stream temperature may be caused by changing air temperature, solar angle, meteorological events, and a number of physical aspects related to the stream and watershed. These physical features include stream origin, velocity, vegetation types and coverage, stream configuration, land-use, and percentage of impervious area in the watershed. For example, a narrow, deep well-shaded shoreline reduces the impact of warming by the sun; whereas, a wide shallow stream would be more impacted by solar heating.

In warm water streams, the temperatures should not exceed 89 °F. Cold water streams should not exceed 68 °F. Often summer heat can cause fish kills in ponds because high temperatures reduce available dissolved oxygen in the water.

Temperature (Water Quality Index Calculator)
Based On Temperature Change from a Reference Site

Chart of temperature results

Source of Image: NSF Consumer Resources

Taking the Temperature of a Stream and Calculating a Water Quality Index

The temperature of a stream is taken at two locations. The control site is upstream of any source of thermal pollution such as the cooling water effluent from a power plant or warmer water from a stream tributary.  The test site is downstream of a suspected source of thermal pollution. Comparing the water temperatures between the two sites would document the presence and magnitude of the thermal pollution.  

At an upstream control site, place a thermometer about 0.5 inches from the unshaded stream bottom or a few inches below the water surface. Keep the thermometer in the water until a constant reading is attained (approximately two minutes). Then, at the same site, try to take the water temperature in a portion of the stream that is shaded. Average the two temperatures; the averaged temperature becomes the temperature of the control site. 

Record your measurement in Celsius. (To convert from Fahrenheit to Celsius, subtract 32 and multiply by 5/9.)

Repeat the procedure at your downstream test site. Try to collect temperature data under similar conditions, using the same thermometer which should be readable to 0.1 °C.


Subtract the upstream temperature (the control site) from the temperature downstream (the test site) and record the result as temperature change (°C). Convert the temperature change (°C) to a water quality index using the Water Quality Index Calculator.

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