Recent Research toward Understanding Spatial, Temporal, and Climatic Variation in Stream Temperatures across the Northwest U.S.

Global air temperature increases raise concerns about effects on thermal regimes of the Earth's rivers and streams. These concerns are acute in the Northwest U.S. due to legislatively mandated water quality standards and the importance of recreational and commercial fisheries for cold-water species such as salmon and trout. Efforts to study climate effects on stream thermal regimes are limited by sparse long-term monitoring records, resulting in a lack of information on historical spatial and temporal variation from which to measure departure. We present research from the last five years that begins to address these shortcomings, including: 1) estimation of stream warming rates in recent decades associated with long-term climate change (+0.11 °C/decade for mean annual temperatures; +0.22 °C/decade for summer temperatures), 2) development of an inexpensive protocol for monitoring full-year temperatures in dynamic mountain streams, 3) rapid expansion of an informal regional monitoring network from < 1,000 stream sites to > 3,000 sites in the last three years, 4) development and use of high-resolution (i.e., 100's of meters) air temperature microclimate models to understand variation in stream temperatures, 5) development of NorWeST, a comprehensive stream temperature database consisting of > 45,000 summers of temperature measurement at > 15,000 unique stream sites, and 6) use of new spatial statistical stream network models with NorWeST to krige predictions at unsampled locations and develop thermal information for most of the region's 350,000 stream kilometers. There is much yet to be learned regarding thermal regimes in rivers and streams but the accelerating pace of knowledge discovery driven by inexpensive sensors, computational improvements, geospatial technologies, and new analyses suggests that many important remaining unknowns will be resolved in the next five years.