Large wildfires generate smoke that greatly compromises air quality over a wide area. Limited studies have suggested that smoke constituents may enter natural water bodies. In an 18-year water monitoring study, we examined whether smoke from distant wildfires had a detectable effect on ion content in a mountain river in an unburned watershed. Significant local wildfire smoke occurred in six years as traced by MODIS satellite data of fires, regional and local atmospheric fine particulate matter (PM_2.5), and the amount of potassium (K⁺) in PM_2.5 as a marker of vegetation combustion. Rainwater had elevated K⁺ and calcium (Ca²⁺, also associated with wildfire smoke) in smoke years compared to no-smoke years, and was the primary route of atmospheric deposition. Similarly, river water in smoke years had elevated concentrations of K⁺ and Ca²⁺, with a higher ratio of K⁺ to Ca²⁺ compared to no-smoke years. River concentrations were generally unrelated to river discharge and observed K⁺ concentrations in smoke and no-smoke years could be accounted for atmospheric deposition. Our study provides early evidence that wildfires affect water quality far beyond the watersheds where they occur. Wildfires are increasing in frequency and extent worldwide, widely distributing vast quantities of smoke containing nutrients, toxins and microbes. Potassium is a routinely-measured water quality parameter that can act as a sentinel of smoke inputs. Further work is needed on the patterns and processes by which wildfire smoke enters water as well as on the consequences for ecosystems and human health.