Variability of water temperatures in the braided Ashley and Rakaia rivers

Abstract Water temperatures were measured throughout the day in selected branch channels of the Ashley and Rakaia Rivers, on days near midwinter, midsummer, and the equinox. At any 1 time, water temperatures in the channels varied over several degrees; the maximum range observed was between 17.2 and 35°C in the Ashley River at 1400 h in December 1982. Without interchange with groundwater, rate of downstream increase in temperature is theoretically proportional to F net , the net heat exchange at the water surface, and Q ‐0.067 , where Q is discharge, so that smaller channels should have a greater rate of temperature increase (and therefore, higher temperatures) than larger channels. However, it was commonly observed that small channels had lower temperatures than the main channel, because of the seepage of cool underflow from the streambed. Up to 40% of the total water surface area may have temperatures several degrees cooler than the main channel; this may represent potential refuges for fish for which the high temperatures elsewhere would be lethal.