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IntroductionPotential global warming caused by an increase in some atmospheric gases, especially carbon dioxide and methane, will influence the thermal regime of aquatic environments. These changes will impact the organisms living in the aquatic environments. For example, if maximum water temperature in a stream reach exceeds the thermal tolerance of a fish species, that fish species is likely to disappear in that stream reach [Eaton and Scheller, 1996]. Similarly, if stream temperature drops below a threshold, it may cause the demise of a fish species through osmoregulatory dysfunction. There is also evidence that fish species stop feeding or feed only sporadically at weekly stream temperatures below a threshold, for example, on average, 8øC for warm water fish species [Scheller et al., 1999].To project fish habitat changes under a 2 x CO2 climate scenario, i.e., a climate after doubling of carbon dioxide in the atmosphere, it is necessary to know the thermal constraints of different fish species and to have access to the current and projected stream temperature time series. A fish survival model, based on the stream thermal regime and thermal constraints of warm water fish species, has been developed by Scheller et al. [1999]. The fish survival model has a weekly Regressions between water temperatures at individual stream gaging stations and air temperatures at nearby weather stations provide the easiest practical method to estimate stream temperatures for the entire United States. They are also attractive for climate change effect studies because only one input variable, air temperature, is used, and general circulation models (GCMs) simulate this variable better than they simulate other climate variables [Lau et al., 1996]. Among regression models, linear models are more common and easier to utilize. However, a linear function of air temperature is usually not sufficient to determine stream temperatures yearround. It has been shown that because of evaporative cooling, stream temperature starts leveling off as air temperature exceeds ---20øC. The overall stream temperature-air temperature 3723

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