Findings

Using spatial stream network models and CMIP6 climate projections under SSP2-4.5 and SSP5-8.5 scenarios, we projected mid- (2045–2074) and late-century (2075–2100) stream temperature changes integrated with thermal tolerances and intrinsic habitat metrics to assess future habitat suitability for key species, including steelhead, coho, Chinook, and bull trout.

Stream temperature trends from 1991 to 2023 reveal basin-wide warming, with high-elevation tributaries demonstrating more resilience to climate impacts while lower-elevation subwatersheds face greater vulnerability. Future projections indicate a basinwide maximum 7-day average of daily maximum stream temperature increases of 1.42 °C–2.04 °C for the midcentury, and 1.84 °C–3.24 °C by the end of the 21st century.

Above: Spatial changes in Max7DADM relative to baseline conditions for four future scenarios: (a) Low emissions, 2060s, (b) Low emissions, 2080s, (c) High emissions, 2060s, (d) High emissions, 2080s. (Figure: Krochta et al., 2025)

Left: Distribution of basinwide Max7DADM predictions under current and future emissions scenarios. Density plots show temperature distributions for current conditions and projections for low emissions (2060s and 2080s) and high emissions (2060s and 2080s). Mean temperature predictions for each scenario are symbolized as dashed grey lines with values included to the left of its distribution. The increase in mean temperatures from current conditions is highlighted on the right. (Figure: Krochta et al., 2025)

These projected increases in stream temperature reduce the most thermally suitable habitats for steelhead, coho, Chinook, and bull trout by an average of 27 %–36 % during the mid-century time period, and 35 %–51 % by the end-of-century time period. The figure to the right (Krochta et al., 2025) shows Thermal suitability for salmonid species (Chinook, coho, steelhead) across the Clackamas River watershed under (a) Current; (b) Low emissions, 2060s; (c) Low emissions, 2080s; (d) High emissions, 2060s; and (e) High emissions, 2080s scenarios. Delineated areas include reaches within known species occurrences for at least one salmonid species of interest (Chinook, coho, steelhead).

By linking climate-driven temperature projections with habitat metrics, we identify priority subwatersheds based on projected thermal suitability where targeted conservation and restoration efforts can maximize impact.

Left: Habitat scores representing cumulative lengths of HIP below fish passage barriers thermally suitable to coho, steelhead, and Chinook salmon under (a) Current; (b) Low emissions, 2060s; (c) Low emissions, 2080s and High emissions, 2060s; and (d) High emissions, 2080s scenarios.

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