Stream order

FIGURE 5.12 Maximum daily temperature ranges in relation to stream order in temperate streams. (Reproduced from Vannote and Sweeney 1980.)

increase the residence time of water and the surface area exposed to solar radiation (McRae and Edwards 1994). A study of small dams on rivers in Michigan found a modest warming (up to 5°C) during summer to be common. Cold-water fish species generally were less abundant below than above dams, but overall fish diversity increased below dams due to the influx of warm-water species (Lessard and Hayes 2003). Large, deep impoundments that release water from the bottom of the dam have a very different effect. Because the hypolimnion of reservoirs is relatively cool throughout the year, deep-release dams result in downstream temperatures that are cooler in summer and warmer in winter than the norm (Stanford and Ward 1979). Temperatures in the lower Colorado River below Glen Canyon Dam are 9-12°C year-round, compared to the historic temperature range of 2-26°C (Petersen and Paukert 2005). Restoration of warmer summer temperatures via surface releases would likely result in higher growth rates of the federally endangered humpbacked chub Gila cypha and reduce the "window of vulnerability'' of young chub to predators. By reengineering the dam release, so that water can be drawn from warmer or cooler depths, it may be possible to manage the temperature of released water to recreate pre-dam conditions (Figure 5.13).

Future climate change presents additional scenarios for altered streamwater temperatures (see also Section 13.2.5). Using present-day latitudinal distributions as a guide, a warming of 4°C is predicted to result in approximately a 600 km poleward shift in the distribution of macroinver-tebrates (Sweeney et al. 1992). It is worth noting, however, that temperature changes of a similar or greater magnitude occur in forested headwater streams due to riparian clearing.

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