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Stream temperature is an important ecological variable for a variety of riverine organisms. Some salmon and trout, for example, are very sensitive to water temperature at certain life stages, particularly as fertilized eggs and fry. In the Pacific Northwest, salmonids require rather cold water during the developmental stage to maximize survivability. Water temperature affects the amount of dissolved oxygen and the microorganisms upon which the fry and fingerlings feed, among others.

Stream temperature is controlled by a large number of factors, including (but not limited to) the temperature of rainfall, the amount of runoff and/or groundwater originating from melting snow and ice, the elevation of the drainage basin above sea level, the time of year, the water depth, how fast the water is moving and the amount of direct sunlight on the stream. This latter variable is dictated by the density and completeness of the canopy over the stream. Recognizing the role of direct sunlight in warming streams, Washington State's new logging regulations require more extensive streamside buffers of vegetation.

The Center for Urban Water Resources Management (CUWRM) and the Center for Streamside Studies (CSS) at the University of Washington in Seattle collected a large amount of data about lowland and foothills streams in 1998 and 1999, including discharge, water temperature, vegetation, etc. The UW collects this data in just a few days, in order to provide a "snapshot" of the streams without the effects of season, weather, etc. We have extracted a subset of the 1999 data.

The table and histograms show stream temperatures under a closed canopy of vegetation and in open sky with no shielding vegetation. The student should recognize immediately that widely different size of the two samples (n = 178 and n = 67) might introduce problems in comparing the distribution of values. As can be seen from the histograms and the student's computations, the means for these two groups are different, with colder water in the shaded streams. However, the standard deviations of both distributions are fairly large compared to this difference in mean temperatures. Is this difference in mean temperatures significant?

The student should be asked if these data are sufficient to test the idea that canopy plays a significant role in keeping the stream cool. For example, what was the amount of shading upstream from the measurement?

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Source: Center for Urban Water Resources Management (CUWRM) & the Center for Streamside Studies (CSS) at the University of Washington in Seattle.