THE FUTURE OF FINE-SEDIMENT DISCHARGE IN THE HUMBOLDT BAY-EEL RIVER REGION, CALIFORNIA

The future of fine-sediment (<63 microns) discharge (Q_ss) in the Humboldt Bay – Eel River (HBER) study region was assessed using a combination of empirical and statistical models and an ensemble of future climate projections. Streamflow was simulated using a deterministic water-balance model, and Q_ss from fluvial sources was computed using statistical sediment-transport models. A probabilistic framework to capture California’s uniquely variable climate. Five global climate models, selected from the full ensemble of IPCC-CMIP5 models on the basis of their ability to represent California-specific climate features, and two greenhouse gas scenarios were selected. Mean climate change projections for the HBER region included 2 ^oC of warming by the mid-century (2040–2069), 3 ^oC of warming by the end-of-century (2070–2099), the persistence of natural climate variations, and increases in the magnitude and frequency of climatic extremes. For the Humboldt Bay basins, mid-century ensemble projections indicate an 8% increase in mean annual streamflow, a 14% increase in the number of peak flow days, and a 27% increase in Q_ss, whereas the end-of-century projections indicate a 16% increase in mean annual streamflow, a 26% increase in the number of peak flow days, and a 58% increase in Q_ss. For the Eel River basin, the mid-century ensemble projections indicate a 9% increase in mean annual streamflow, a 26% increase in the number of peak flow days, and a 53% increase in Q_ss, whereas end-of-century projections indicate a 17% increase in mean annual streamflow, a 37% increase in the number of peak flow days, and a 99% increase in Q_ss. These results demonstrate positive climate feedback caused by the interplay between hydrologic and geomorphic processes in the HBER region. When climate change (temperature and precipitation) was translated into hydrologic responses (runoff, infiltration, and recharge), there were disproportionally large responses in fine-sediment discharge. Resource managers can use these future projections that account for time-varying sediment discharge to inform climate change adaptation.