Increasing synchrony of high temperature and low flow in western North American streams: double trouble for coldwater biota?

Flow and temperature are strongly linked environmental factors driving ecosystem processes in streams. Stream temperature maxima (T (max_w)) and stream flow minima (Q (min)) can create periods of stress for aquatic organisms. In mountainous areas, such as western North America, recent shifts toward an earlier spring peak flow and decreases in low flow during summer/fall have been reported. We hypothesized that an earlier peak flow could be shifting the timing of low flow and leading to a decrease in the interval between T (max_w) and Q (min). We also examined if years with extreme low Q (min) were associated with years of extreme high T (max_w). We tested these hypotheses using long-term data from 22 minimally human-influenced streams for the period 1950-2010. We found trends toward a shorter time lag between T (max_w) and Q (min) over time and a strong negative association between their magnitudes. Our findings show that aquatic biota may be increasingly experiencing narrower time windows to recover or adapt between these extreme events of low flow and high temperature. This study highlights the importance of evaluating multiple environmental drivers to better gage the effects of the recent climate variability in freshwaters.