River heatwaves (RHWs) are defined as events with prolonged periods of anomalously warm stream temperatures, and these events are emerging as an essential thermal extreme for freshwater ecosystems. In this study, a continuous 23-year (2002-2024) daily river water temperature (RWT) dataset was reconstructed for four USGS stations along the Willamette River (Albany, Keizer, Newberg, and Portland) to assess long-term warming, RHW characteristics, and post-heatwave recovery patterns across a rural-to-urban gradient. The reconstruction was done using a hybrid gap-filling framework on the observed temperature records, combining linear interpolation for short gaps (≤ 2 consecutive days) and a Daymet-driven multiple regression model using daily maximum air temperature, minimum air temperature, precipitation, and day length as predictors for longer gaps (≥ 2 consecutive days). The marineHeatWaves algorithm was then applied to the final dataset to identify the RHWs.
The findings reveal a consistent warming trend and significant interannual variability among all four stations, with Sen's slope estimates indicating a discernible yet statistically nominal warming rate of 0.13-0.28 °C decade⁻¹ (Mann-Kendall p > 0.34). The most pronounced seasonal warming was observed in summer (0.66 °C decade⁻¹, p = 0.10) and autumn (0.45 °C decade⁻¹, p = 0.064) at the urban Portland station. RHWs occurred frequently at all sites (50-56 events per station). The mean heatwave duration per year for all stations showed an increasing trend at Newberg and Portland (0.09 days yr⁻¹) and a declining trend at Albany and Keizer (-0.05 days yr⁻¹). The annual total heatwave duration showed significant cross-basin correlation (Spearman's ρ = 0.61–0.73, p < 0.05). Trends in RHW intensity were tenuous, with only Keizer manifesting significant increases. Post-RHW recovery times varied substantially among events (mean 18-29 days), but nonparametric tests detected no significant differences among stations.
Despite statistical limitations, this study provides the foundational RHW climatology for one of the most ecologically, culturally, and economically important rivers in the Pacific Northwest, delineating that RHWs along the Willamette mainstem are synchronized and predominantly influenced by regional climate, underscoring the need for basin-scale, climate-adaptive management strategies.