A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 3 - Streams

Temperature of Streams

Why Is It Important?

Temperature is important because it governs the kinds of aquatic life that can live in a stream. Fish, insects, zooplankton, phytoplankton, and other aquatic species all have a preferred temperature range. If temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are none.

Temperature also is important because it influences water chemistry. The rate of chemical reactions generally increases at higher temperatures, which in turn affects biological activity. An important example of the effects of temperature on water chemistry is its impact on oxygen. Warm water holds less oxygen than cool water, so it may be "saturated" with oxygen but still not contain enough for survival of aquatic life. Some compounds are also more toxic to aquatic life at higher temperatures.

Reasons for Natural Variation

In addition to seasonal variations in stream temperature caused by changing air temperatures, many other physical aspects of a stream cause natural variation in temperature. The origin of the stream – whether it flows from a glacier, a lowland lake, or a spring or wetland – determines its initial temperature. Tributaries may alter the stream temperature as they mix with the mainstream. Velocity also influences temperature. A particle of water in a fast-moving stream is exposed to sunlight for a shorter time than that in a slow-moving stream.

The physical character of the stream and shoreline also are important. A well-shaded shoreline reduces the impact of warming by the sun. In a wide shallow stream, even a forested shoreline will permit lots of sunlight to fall upon the stream. A narrow, deep stream with a well-vegetated bank would remain cooler.

The character of the watershed also affects temperature. If the watershed is forested and steep or hilly, runoff water will move quickly and the sun won’t have much time to warm the runoff before it reaches the stream. Conversely, in a flat and sparsely vegetated watershed, the water moves more slowly, with more time to absorb heat from the ground surface and the sunlight.

Expected Impact of Pollution

We usually think of thermal pollution in terms of the discharges of heated municipal and industrial discharges. However, the process of watershed development also can affect temperatures in nearby streams. Streambank vegetation often is lost when land is cleared, thereby exposing the stream to increased warming by sunlight. A less obvious impact is that runoff water may be warmer, especially during the summer months when it flows over hot asphalt or concrete. Although temperature-induced impacts from development are important, they are difficult to measure as part of a typical stream monitoring program. It usually is more informative to note the loss of the shade trees and shoreline vegetation or the increase in paved areas in the watershed as indicators of likely temperature effects.

Temperature is reported in degrees on the Celsius temperature scale (C). The Washington State water quality standard for temperature varies according to the stream classification. Temperature can not exceed 16^o C in Class AA streams, 18^o C in Class A streams, 21^o C in Class B streams, and 22^o C in Class C streams. In addition to the maximum allowed temperatures, the total amount of change in temperature as caused by human activities also is regulated. In other words, even if the temperature in a Class AA stream remains below 15^o C, if human disturbance causes too much change) as determined by a series of equations described in the regulation) then the standard will have been violated.

The  table summarizes temperature data for three Puget Sound area streams. The streams were selected to represent a range in land use and water quality conditions. The Cedar River watershed is almost 90% forested and has "very good" water quality. The Newaukum Creek watershed is primarily forest, and agricultural land – mostly dairy farming – and is considered to have "fair’ water quality. Land use in the Springbrook Creek watershed is commercial and industrial with some agriculture; the water quality is considered to be "poor." These same streams will be used throughout this section on water quality parameters to provide an example of the normal range that can be expected in each type of data.

The next section discusses dissolved oxygen in streams.

Chapter Four provides information on how to measure temperature in streams.

Stream Temperature | Stream Dissolved Oxygen | Stream pH | Stream Nutrients | Stream TSS and Turbidity | Stream Fecal Coliforms | Return to Table of Contents