Hydrologic Alterations from Climate Change Inform Assessment of Ecological Risk to Pacific Salmon in Bristol Bay, Alaska

Abstract

We developed an integrated hydrologic model of the upper Nushagak and Kvichak watersheds in the Bristol Bay region of southwestern Alaska, a region under substantial development pressure from large-scale copper mining. We incorporated climate change scenarios into this model to evaluate how hydrologic regimes and stream temperatures might change in a future climate, and to summarize indicators of hydrologic alteration that are relevant to salmon habitat ecology and life history. Model simulations project substantial changes in mean winter flow, peak flow dates, and water temperature by 2100. In particular, we find that annual hydrographs will no longer be dominated by a single spring thaw event, but will instead be characterized by numerous high flow events throughout the winter. Stream temperatures increase in all future scenarios, although these temperature increases are moderated relative to air temperatures by cool baseflow inputs during the summer months. Projected changes to flow and stream temperature could influence salmon through alterations in the suitability of spawning gravels, changes in the duration of incubation, increased growth during juvenile stages, and increased exposure to chronic and acute temperature stress. These climate-modulated changes represent a shifting baseline in salmon habitat quality and quantity in the future, and an important consideration to adequately assess the types and magnitude of risks associated with proposed large-scale mining in the region.

Fig 1. Site Location Map.

Salmon presence as documented in the anadromous waters catalog [14].

Fig 2. Comparison of NARR vs. Measured Temperature Data over the Period 2006–2009.

Fig 3. Monthly Changes in Temperature and Precipitation Projected for the 5 Models Used in Climate Change Simulations.

Fig 4. Modeled vs. Observed (a) Hydrograph and (b) Stream Temperatures at USGS Gage Site on Upper Talarik Creek (See Map for Location).

Fig 5. Summary of Flow Changes in Future Climate Scenarios.

A) Ratio of peak flow, by model node, in future scenarios to peak flow in baseline scenarios. Peak flow values are calculated without respect to date of peak flow. B) Ratio of average winter flow, by model node, in future scenarios to average winter flow in baseline. C) Ratio of average summer flow, by model node, in future scenarios to average summer flow in baseline.

Fig 6. Changes in Hydrograph for Upper Talarik Creek Gage Site in 2100, for Lowest (MIROC, Green Line) and Highest (MPI, Red Line) Temperature Scenarios.

Note the loss of the spring freshet in both future climate simulations.

Fig 7. Summary of Baseline and Projected Daily Temperatures (a), and Fraction of Winter Storms Occurring when Temperatures are Above Freezing (b).

Purple shading in (a) highlights temperatures below freezing.

Fig 8. Modeled Change in Annual Average Stream Temperatures in 2100 for (a) “Cool” Scenario (CNRM-CM5, RCP 4.5) and (b) “Hot” Scenario (MPI-ESM-8.5 RCP 8.5).

Fig 9. Summary of Temperature Data from All Model Results.

A) Mean annual stream temperature, by model node, for each of the modeled scenarios. B) Average annual number of days above 20°C by model node, for each of the modeled scenarios. C) Average annual number of days above 25°C by model node, for each of the modeled scenarios. Box and whisker plots show distribution of average temperatures

Fig 10. Comparison of Modeled TDD for Baseline vs. 2100 at the Three USGS Gages Assuming Lowest (MIROC, Green Line) and Highest (MPI, Cyan Lines) Future Temperature Scenarios.

Thin colored lines show results from each of the 10 years in the simulations. Thick blue lines show cumulative TDD calculated from measured temperatures at the same sites. Black dash-dot lines represent estimated TDD required for sockeye to hatch [8].

Fig 11. Modeled Number of Days Above 20°C in 2100 for (a) “Cool” Scenario (CNRM-CM5, RCP 4.5) and (b) “Hot” Scenario (MPI-ESM-8.5 RCP 8.5).

Colored dots represent the average number of days above the threshold for each node in the model domain, averaged over the 10-year model run. Groundwater dominated reaches adapted from [15].