Strategic Groundwater Augmented Streamflow for Temperature Reduction Reduced Salmonid Mortality

Survival of ecological systems pivot on critical moments. In the Middle Fork of the John Day River (MFJD) of Eastern Oregon, after two decades of restoration of the stream and floodplain recurring brief periods of extreme temperature increase have resulted in salmon mortality. Spring run salmon awaiting the fall spawn are restricted by excess temperature to the upper 40 km, which is the second longest unregulated river in the USA (behind the Yellowstone). We evaluate alleviating temperature stress during extreme events by strategically adding groundwater to the stream. Thermal mixing show that adding 0.03 to 0.14 cubic m3/s of groundwater at 10°C would have lowered extreme MFJD stream temperatures associated with large die-offs in 2007 and 2013 by 0.93-2.9 oC, while impacting the late-season base-flow of the river by less than 1%. Previous studies suggest that as much as 75% of thermal benefit is retained at 1 km downstream (Hall et al., 2020) providing critical thermal refugia for salmonids. Strategic placement of addition sites would aid salmonids in upstream movement through thermal hopscotch during extreme periods. However, legal structures of water resources in Oregon, designed to protect riparian systems from overexploitation and agencies from litigation, limit the use of such methods. If groundwater use was legally permitted, net present value evaluation of MFJD projects shows that groundwater additions are at least ten times more cost effective in temperature change per dollar spent. These comparisons are in terms of thermal relief provided in situations that result in fish kills but do not address other important benefits such as habitat and aesthetics but overall offer a potential stopgap to allow complementary restoration such as shade develop. Hall, A., Y-C Chiu, J.S. Selker, Coupling High-Resolution Monitoring and Modeling to Verify Restoration-based Temperature Improvements, River Research and Applications, https://doi.org/10.1002/rra.366812, 2020.