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Water temperature is an important physical variable that affects most aquatic ecosystem functions, including biogeochemical reaction rates, phenology, and species fitness. We analyzed >60 water temperature records from US national parks in the Western US and Appalachian Mountains to identify trends in stream temperatures from 2008-2023, characterize the seasonality of stream temperature change, and assess the contribution of several hydroclimatic variables (e.g., air temperature and discharge) to observed stream temperature patterns. A series of site-specific generalized additive models (GAMs) were used for trend analysis and quantifying the effect of covariates. Several case studies in representative and notable records are highlighted. Estimates of historical stream temperature change will provide park researchers and resource managers with physical data to understand observed ecological changes, while identification of important covariates will inform planned projections of stream temperature across the NPS system.