Data  collection

Stream temperature data collected in the basin between 1991 - 2023 was compiled. Each data point was assigned the annual maximum of the 7-day average of daily maximum stream temperatures at the site (Max7DADM, °C). This rolling average is a common regulatory metric for stream temperature and helps assess impacts to cold water species.

A number of watershed conditions related to stream temperature were also derived. For making stream temperature predictions, we include projected future air temperatures. 

Stream temperature analysis and modeling

For our analysis, we use a geostatistical model which draws upon spatial patterns in our data based on in-stream distances, flow accumulation and flow direction. Watershed variables are also included. Stream sampling locations are incorporated into a virtual landscape network. Reach contributing areas are assigned for each stream, with the numerous predictor variables summarized for each.

A standardized approach is used to select the optimum set of watershed variables required for our analysis, which include streamflow, topography, underlying geology, land cover, and air temperature. Using this model, stream temperature predictions are generated across the entire basin for each year, 1991-2023. Using the results of the annual models, we can derive annual trend for stream temperature. We next model future stream temperatures using air temperatures from IPCC low and high emissions models for the mid-century and end-of-century time periods. 

Modeling physical and thermal habitat

Salmon thermal tolerances are based on performance curves under a range of temperatures. This models the relationship between stream temperature and biological performance, with maximum performance within optimal thermal ranges and decreasing as temperatures approach sub-lethal levels. With this information, the quality of thermal habitat can be mapped across the watershed under current and future conditions.

Finally, stream flow and channel topography are used to model streams' physical habitat within each species' range. Intrinsic potential habitat scores are based on empirical relationships between stream attributes and juvenile fish use, with highest considered having high intrinsic potential.