Quantifying the effects of beaver dam analogs on stream temperature

Background/Question/Methods

Beaver Dam Analogs (BDAs) are an increasingly popular stream restoration tool that mimic beaver dams in streams. Like natural beaver dams, BDAs will theoretically increase hydrologic residence time thus increasing water storage, late-season base-flows, and groundwater recharge. Despite the reported success of BDAs in restoring degraded streams, research regarding their effects on stream temperature is limited. Many cold-water species in Western streams are declining due to climate-induced stream warming making this knowledge gap a point of contention as BDA projects are planned. If BDAs increase hydrologic residence time without increasing the occurrence of cooling hyporheic flow-paths, but instead expose the stream to prolonged solar radiation, there is potential for BDAs to exacerbate climate effects by warming streams beyond the thermal capacity of cold-water dependent native trout. To inform the debate surrounding the effects of beaver dam analogs on stream temperatures, we conducted a Before-After-Control-Impact (BACI) study on three stream pairs in Western Montana. Each pair contains one treatment stream where BDAs were installed and one nearby control. We measured discharge and stream temperature upstream and downstream of restoration and control reaches, one summer before (2019) and two summers after (2020, 2021) installing BDAs.

Results/Conclusions

We observed increases in mean residence time (2020: 68.5 days, 2021: 206.7 days) and surface water volume (2020:124%, 2021: 14%) in restored stream reaches compared with controls post-restoration. We also observed decreases in downstream mean maximum temperature (2020: -0.93°C, 2021: -1.12°C) and less longitudinal warming (2020: -1.05 °C, 2021: -.91 °C) in restored stream reaches compared to controls. Despite similar trends in effects of restoration to stream hydrology and temperature, we observed considerable variation in the magnitude of effects across stream pairs and years. This is likely due to variation in the geologic and topologic domains, with some reaches located in areas with higher groundwater potential and degree of degradation, as well as variation in precipitation. Our results indicate the potential for BDAs to create climate resilience in degraded aquatic habitats. Our results also highlight the importance of considering local geologic and topologic controls of stream hydrology when setting expectations of the effects of BDAs on stream temperature.