Long-term monitoring reveals muted, seasonally distinct water quality responses to wildfire across a stream network in the Pacific Northwest, USA

The occurrence of large, high severity wildfires has increased across the Pacific Northwest, with potential effects on watershed processes and water quality. However, quantifying the magnitude and persistence of wildfire impacts remains challenging due to the unpredictable nature of wildfires and limited long-term and pre-fire reference data. In our study we used a unique dataset of 19 years of water quality data spanning 14 years pre-fire and 5 years post-fire across eight sites in the Cascade Mountains of western Oregon to evaluate wildfire effects on carbon, nitrogen, phosphorus, and sediment concentrations. Bayesian generalized linear mixed models were used to quantify wildfire effects on concentration differences across time (pre-/post-fire) and burn status (burned/unburned). Contrary to much of the literature from wildfires in other regions, we found muted and uncertain wildfire effects. However, seasonal impacts were observed. During the dry summer period, dissolved organic carbon increased (174 %) while orthophosphate (-53 %) and nitrate (-113 %) decreased, suggesting increased productivity following wildfire. The fall wetting-up period showed small increases in total suspended solids (140 %) and total organic carbon (35 %), which may reflect short-term increases in available sediment from dry ravel, enhanced bank erosion, or entrainment of channel sediment due to enhanced streamflow. We also found that site-level climate and landscape characteristics were critical for accurately assessing wildfire impacts, with daily precipitation strongly influencing constituent concentrations. Our findings underscore the value of long-term monitoring of reference sites for disentangling water quality responses to wildfire relative to natural climatic variability.