A spatiotemporal exploration of stream temperature surge occurrence, magnitude, and duration in the United States between 2008-2018

Stormwater discharge, especially when combined with the thermal influence of urbanization, can cause stream temperature surges that result in negative impacts on stream ecosystem structure. A spatial and temporal understanding of stream temperature surges is critical for managing the ecology and uses of these streams. This study assessed the frequency, magnitude, and duration of stream temperature surges, defined as a rise in stream temperature of 2 °C for 30 minutes, for 200 United States Geological Survey sites across the contiguous United States. Using stream temperature observations for 2008-2018 at high temporal resolution (every 15 minutes) we found that 114 out of the 200 sites experienced at least one stream temperature surge between 2008 through 2018. There were a total of 3153 surges during the study period at these 114 sites. The average frequency of stream temperature surges for sites with at least one stream temperature surge was 2.51 surges per year, with a standard deviation of 6.33. Surge occurrence and duration were highest during the summer over the 81 active sites and lowest during the winter over the 43 active sites, while surge magnitude was the largest during the fall over the 73 active sites and shortest during the winter. Surge occurrences were generally less common in coastal regions, apart from Florida, and were most common in the central United States, particularly in Northeast Texas. Average surge occurrence has increased by 0.17 surges and average surge magnitude has increased by 0.44 °C from 2008-2018; however, no clear change in average surge duration was found. This study shows that stream temperature surge occurrence and magnitude are increasing across the United States, and this increase is likely due to urbanization. Our work highlights the need for low impact development practices to mitigate stream temperature surges in urbanizing watersheds, especially during summer when stream temperature is closest to the upper thermal limits for aquatic organisms. In addition, our work also highlights the need for a larger systematic collection of stream temperatures nationwide, as many USGS site locations either collect stream temperature records over a relatively short period of time, are irregularly collected, or are not collected at all.