Public water supplies are threatened by wildfire, which is becoming more frequent and intense worldwide. After a fire, rain and wind can carry contaminants from ash and exposed soils into reservoirs, potentially affecting water quality for months or even years. In this study, mud from the bottom of a reservoir that supplies drinking water to 400,000 people on Vancouver Island in British Columbia, Canada, was examined for evidence of fire-related impacts and contamination. Changes in charcoal, chemical elements, pollen, and the remains of algae revealed that some past fires likely made the water cloudier and introduced toxic contaminants. These impacts probably lasted for years or even decades. Considering this, practical recommendations were developed for water supply managers to safeguard water resources from wildfire effects. The new approach developed in this study can be used to assess fire risk to water supply in other reservoirs.