Describing complexity in thermal regimes in Pacific Northwest streams

Background/Question/Methods

There is no current consensus in the literature or among scientists as to how best to classify thermal regimes; however, these thermal regimes are essential for analysis of Pacific Northwest streams and are changing in response to climate and anthropogenic actions.  As field data loggers for water temperature are increasingly deployed in riverine systems across the region, efficient calibration, deployment, and storage of water temperature data are needed along with a robust set of metrics to accurately describe thermal regimes. Metrics classifying thermal regimes should describe mean temperatures, thermal variance, and biologically relevant aspects of stream temperatures such as days above species-specific lethal thresholds and variance of winter temperatures during egg incubation.  We use an eight-year time series of water temperature collected every 30 minutes across multiple channels of a floodplain on the Sauk River to test and identify a collection of water temperature metrics that is sufficient for describing thermal regimes in Pacific Northwest streams.  

Results/Conclusions

Metrics differed in the degree to which they were correlated with other metrics, the effect of sampling frequency on accuracy, robustness to missing data, and the aspect of thermal complexity captured.  Our analyses identified a suite of recommended water temperature metrics for winter, for summer, and for specific salmon species that minimize correlation and are robust.  By capturing patterns in mean temperatures and in thermal variance across multiple time scales, this suite of metrics can be used to capture essential elements of naturally complex riverine thermal regimes.  Identification of a standardized and concise set of metrics will also increase opportunities to collaborate across projects, rivers, and regions.