Climate change is altering temperature and precipitation patterns across the United States, shifting water temperatures in streams and rivers across the National Park Service (NPS) system. Water temperature is a biological βmaster variableβ that affects almost all ecosystem functions, carrying widespread implications for aquatic ecosystems that may be increasingly pressured in a warmer, less snowy, and potentially drier future. Many NPS stream temperature records are only now becoming sufficiently long to identify long term temporal trends. Here, we analyze stream temperature trends at 46 NPS sites in the mountain West and Appalachian Mountains. Warming was most pronounced in the summer, fall and early winter months (July β December). Cooling trends were mostly found in February, May and June. Significant trends in July, August and September (n = 9) ranged from 0.47 β 3.16 Β°C / decade.