Upper Bounds for Stream Temperatures in the Contiguous United States

Water temperatures mediate the rate of chemical reactions and constrain the habitat space of freshwater organisms in streams. Stream temperatures are therefore of great interest for stream water quality and stream ecology. There is evidence that evaporative cooling imposes an upper bound on stream temperatures. Maximum stream temperatures observed in the United States were explored in this paper. A frequency-based method was employed for the estimation of probable maximum stream temperatures. The method utilizes an enveloping standard deviate, $K_E\text{,}$ and the first two sample moments of a maximum stream temperature series. The parameter $K_E$ was evaluated by analysis of 100 independent, maximum (weekly) stream temperature series from Idaho, Maine, Minnesota, Oklahoma, and Washington and found to be 4.88. Probable maximum weekly stream temperatures were determined for 798 stream temperature records obtained from USGS water quality monitoring stations located throughout the United States. Projections of probable maximum stream temperatures at these stations using $K_E=4.88$ are presented. As to be expected, all probable maximum stream temperatures exceeded observed stream temperatures. Calculated probable maximum stream temperatures are representative of the past (before 1990), but for streams with recorded annual maximum temperatures of about 30°C or higher they may also be indicative for warmer future climate conditions. Effects of latitude and elevation on probable maximum stream temperatures of the past are illustrated in this paper.