Stream Thermal Sensitivities in Southern Alaska

This project characterized stream temperature regimes in the Cook Inlet, Copper River, and Prince William Sound regions. We aggregated 1,548 summertime stream temperature time series from 355 monitoring locations across southwestern and southcentral Alaska and calculated a suite of metrics related to the magnitude, frequency, duration, timing, and variability of stream temperatures. We categorized streams into one of six different thermal regimes. A comparison of stream thermal regimes among regions showed that cold habitats with later timing of maximum temperatures were most common in all regions, cold stable habitats were more common in the Copper River and Prince William Sound regions, and all regions included all six thermal regimes described in our classification.

We also calculated stream thermal sensitivity (𝛕) across monitoring sites to describe how closely stream temperatures track air temperatures. Estimated 𝛕 was highest in the Cook Inlet and Kodiak regions, followed by Bristol Bay and Copper River, and was lowest in Prince William Sound. We modeled variation in 𝛕 using geomorphic, hydrologic, climatic, and landcover covariates. The model was used to map thermal sensitivities across 966 salmon streams for high and low scenarios of spring snowpack and summer precipitation. Thermal sensitivities decreased under higher summertime precipitation and changed minimally between years with low and high snowpack. The strongest control on 𝛕 was watershed slope. Streams draining steeper watersheds had lower 𝛕, which may be due to snowmelt contributions later in the summer period, shorter water residence times, and deeper flowpaths that experience less solar radiation. Chum and pink salmon habitats had the lowest 𝛕, followed by spawning habitats. Rearing habitats and Chinook, coho, and sockeye salmon habitats all had higher 𝛕. In a warming future, salmon may need to balance physical habitat preferences for low gradient systems with adaptations for cold water.