Rate of Natural Stream Temperature Fluctuation in Pennsylvania Streams

The Susquehanna River Basin Commission monitors temperature, dissolved oxygen, turbidity, pH, and conductivity at 5 min intervals at 50 stations in streams in the Susquehanna River watershed.  Forty of the stations are located in Pennsylvania, which regulates thermal discharges to a maximum rate of change in stream temperature, in either direction, of 1.1° C/hr.  Of the Pennsylvania streams in the monitoring network, 30 are designated trout streams (stocked or wild) and 10 are not.  The 99th percentile of hourly temperature changes in individual streams in the 2011 data set ranged  from +0.50 to +2.95° C/hr for streams not designated as trout streams, and +0.35 to +1.45° C/hr for designated trout streams.  The 1st percentiles of hourly temperature changes ranged from -0.30 to -01.85° C/hr for non-trout streams and -0.25 to -1.70° C/hr for trout streams.  For the non-trout streams temperature change rates in excess of 1.1° C/hr occurred on average 2% of the time or more from March through August, with maximums of 5% of the time in April and July.  For designated trout streams, frequency of exceeding of 1.1° C/hr was greater than 2% of the time only in May and July.  Increasing temperatures exceeded the regulatory limit more frequently than decreasing temperatures for both trout and non-trout streams.  For the entire 2011 data set, which does not contain equal sampling for all streams or months, the non-trout streams exceeded the regulatory criteria due to rising temperatures 1.3% of the time, and due to falling temperatures 0.3% of the time.  For trout streams the criteria were exceeded 0.9% of the time by rising temperatures and 0.2% of the time by falling temperatures.