Archival measurements and a statistical model show that average water temperature in a major US West Coast river has increased by 1.8 °C since 1850, at a rate of 1.1 °C per century. The largest factor driving modeled changes are warming air temperatures (nearly 75 %). The remainder is primarily caused by depth increases and other modifications to the river system. Near-freezing conditions, common historically, no longer occur, and the number of warm water days has significantly increased.