Watershed management may overcome effects of climate change on stream water quality in the Portland metropolitan area

Urbanization and climate change may alter stream temperature and turbidity such that they become stressful or deadly for salmonids and important freshwater species. Cities are responding by improving watershed development practices and pursuing restoration activities but few empirical studies have compared their effects to those induced by urbanization and climate change. In our study, we combined data on stream water quality from thirteen USGS stations between the water years 2008-2022 throughout the Portland metropolitan area in Oregon, United States, with data on land cover, watershed restoration, low-impact development, and beaver activity; we sought to clarify their effects on water quality and to contextualize them given regional trends in climate and urbanization. While improved water quality metrics were positively correlated with forest cover, wetland and open water cover, and human- and beaver-related watershed management, we found that year-to-year change in water quality was more strongly and frequently attributable to climate factors. Across all watersheds, we nonetheless found evidence that watershed management efforts may mitigate pressures from climate on longer timescales and benefit salmonid conservation efforts. We discussed study results with regional watershed councils and managers to identify possible causes for detected correlations and to refine future watershed management efforts to match changes driven by climate change. The framework developed in the current study could be applicable to other watersheds' restoration and infrastructure construction in tackling similar challenges of climate change and urbanization.