Seasons aren't the only thing causing temperature variations in streams! The heat of the day can warm them up; and stormwater running off of hot summer stormpavement can cause dramatic temperature increases in streams. Read more about how temperature can impact streams here.


Why is it Important?

Besides being important to swimmers and fishermen, most aquatic organisms are poikilothermic - i.e., "cold-blooded" - which means they are unable to internally regulate their core body temperature. Therefore, temperature exerts a major influence on the biological activity and growth of aquatic organisms. To a point, the higher the water temperature, the greater the biological activity. Fish, aquatic insects, zooplankton and other aquatic species all have preferred temperature ranges. As temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are few, or none. For example, we would generally not expect to find a thriving trout fishery in ponds or shallow lakes because the water is too warm throughout the ice-free season. In particular, warm water impairs their reproduction- trout eggs need cool, well oxygenated water to develop properly, but it may also directly stress adult fish.

This is also a major concern for the brook trout populations in Duluth's urban streams since water temperatures during heat waves and low flow periods in summer are physiologically stressful. In Miller Creek, in the Miller mall area of northern Duluth, intensive studies have shown that trout are near their thermal maximum in summer and that runoff water that heats up on parking lots and in shallow sedimentation ponds contribute to the problem. The removal of shoreline trees and shrubs associated with commercial development also contributes to the problem by removing natural shading. Go here to find out more about Miller Creek studies.

Changes in the growth rates of cold-blooded aquatic organisms and many biochemical reaction rates can often be approximated by this rule which predicts that growth rate will double if temperature increases by 10°C (18°F) within their "preferred" range.

Changes in the growth rates of cold-blooded aquatic organisms and many biochemical reaction rates can often be approximated by this rule which predicts that growth rate will double if temperature increases by 10°C (18°F) within their "preferred" range.

Temperature is also important because of its influence on water chemistry. The rate of chemical reactions generally increases at higher temperature, which in turn affects biological activity. Warmer temperatures increase the solubility of salts in water but decrease (just the opposite !) the solubility of gasses in water. Warm soda pop going flat is an example of this effect. Another important example of the effects of temperature on water chemistry is its impact on oxygen. Warm water holds less oxygen that cool water, so it may be saturated with oxygen but still not contain enough for survival of aquatic invertebrates or certain fish. Some compounds are also more toxic to aquatic life at higher temperatures. Temperature is reported in degrees on the Celsius temperature scale (°C). Aquatic scientists usually measure temperature with electronic thermometers such as a thermistor, a semiconducting material that smoothly decreases in electrical resistance with increasing temperature. Water temperature can be precisely determined by maintaining a constant current through the thermistor and measuring the voltage drop across it.

Although air temperatures reported in the news media in the U.S. are given in degrees Fahrenheit, scientists and the rest of he world usually record temperatures in Celsius, because this is the unit designated by the International System of Units. To convert from ° F to ° C, use this equation:

T °C = [T °F - 32 °F] * [5 °C / 9 °F]

Where T °C = temperature in Celsius and T° F = temperature in Fahrenheit

For example, the freezing temperature of water is 32 °F; this translates to 0 °C. The boiling temperature of water is 212 °F, or 100 °C.

10° C translates to 50 °F
20° C translates to 68 °F (just about perfect for most Duluthians)
30° C translates to 86 °F
40° C translates to 104 °F (Whew ! - the melting point of most Duluthians is 31 °C)

Reasons for Natural Variation

The most obvious reason for temperature change in streams, wetlands and lakes is the change in seasonal air temperature. Daily variation also may occur, especially in the surface layers, which warm during the day and cool at night.

Streams, unless very large and slow moving, differ from lakes in that water density is usually uniform enough with depth that little depth variation temperature is evident. This greatly contrasts with deeper lakes (typically greater than 5 meters for small lakes and 10 meters for larger ones) that thermally stratify. People who want to know a lot more about how lakes work should visit Water-on-the-Web (WOW) at

Expected Impact of Pollution

Thermal pollution (i.e., artificially high temperatures) in larger streams usually occurs as a result of discharge of municipal or industrial effluents. Except in very large lakes, it is rare to have an effluent discharge. In urban areas with smaller streams such as Duluth, runoff that flows over hot asphalt and concrete pavement before entering a stream or pond will be artificially heated and can cause significant warming. In running waters, particularly small urban streams during low flow periods, elevated temperatures from road and parking lot runoff can be a serious problem for populations of cool or cold-water fish already stressed from the other contaminants in urban runoff. During summer, temperatures may approach their upper tolerance limit. Higher temperatures also decrease the maximum amount of oxygen that can be dissolved in the water, leading to oxygen stress if the water is receiving high loads of organic matter. Since trout eggs require cool, well oxygenated water, reproduction may be directly impaired by this pollutant in addition to its effects on adult and juvenile fish survival. Another less well understood indirect effect relates to disease. Since bacteria and other disease causing organisms grow faster in warm water, the susceptibility of aquatic organisms to disease in warm water increases as well. Water temperature fluctuations in streams may be further worsened by cutting down trees, which provide shade, and by absorbing more heat from sunlight due to increased water turbidity.


Michaud, J.P. 1991. A citizen's guide to understanding and monitoring lakes and streams. Publ. #94-149. Washington State Dept. of Ecology, Publications Office, Olympia, WA, USA (360) 407-7472. Moore, M.L. 1989. NALMS management guide for lakes and reservoirs.
North American Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443, USA ( Holdren, C., W. Jones and J. Taggart. 2001. Managing lakes and reservoirs. North American Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443, USA and Terrene Institute, 4 Herbert Street, Alexandria, VA 22305, USA (