A major new study has revealed that wildfires continue to pollute rivers and streams across the western United States for up to eight years after they occur far longer than previously believed. The research, published in Nature Communications Earth & Environment analyzed over 100,000 water samples from more than 500 watersheds and provides the first large-scale assessment of post-wildfire water quality in the region.
The study found that key contaminants such as organic carbon, nitrogen, phosphorus and sediment remain elevated in rivers long after the fires are extinguished. In many areas, these pollutants significantly worsen water quality for several years, particularly in heavily forested regions where the impact is most severe.
The research team compared data from burned watersheds to similar unburned ones to measure changes in water quality over time. Their findings showed that organic carbon, phosphorus, and water cloudiness (turbidity) peak within one to five years after a fire. Nitrogen and sediment levels remain high for as long as eight years. The study also revealed that the extent of pollution varies widely between locations, depending on factors like terrain, vegetation, soil type, and the fire’s proximity to water sources.
In some watersheds, major pollution events were delayed until heavy rains occurred, washing fire-related debris into rivers and triggering sudden spikes in contamination. This variability makes it difficult for water managers to predict when and where the worst impacts will be felt.
Despite the complexity, the study offers clear data that can be used by local governments, utilities, and environmental agencies to better prepare for and respond to wildfire-driven water pollution. It provides concrete timelines for how long post-fire water quality degradation might last, helping guide investment in filtration, monitoring, and watershed restoration efforts.
By filling a crucial gap in large-scale environmental monitoring, the findings highlight the need for long-term strategies to protect water resources in a fire-prone and rapidly warming climate.