Post-fire forest loss increases summer streamflow in snow-dominated watersheds

A new study published in Forest Ecosystems reports that the 2021 White Rock Lake Wildfire in the Okanagan Valley, British Columbia, Canada, significantly increased summer low flows, the most critical period for water supply in dry regions, by reducing evapotranspiration and altering the way snowmelt and groundwater contribute to river systems. By combining hydrometric monitoring with geochemical tracing, researchers not only documented post-fire changes in streamflow but also uncovered the underlying mechanisms driving those changes.

The Okanagan Valley depends heavily on summer low flows for drinking water, irrigation, and ecosystem health, yet climate change is shrinking snowpacks and lengthening dry seasons. After the wildfire burned more than 83,000 hectares of forest, the research team compared burned and unburned watersheds and found that summer low flows increased significantly in nearly all burned catchments. They also showed that snowmelt contributed a much larger share of streamflow in early summer, while groundwater became the dominant source of flow later in the season.

The researchers explain that forests normally return large amounts of water to the atmosphere through evapotranspiration, but wildfire sharply reduces this water loss by removing tree cover. With fewer trees using water, more snowmelt infiltrates into soils and aquifers and is stored underground, where it is slowly released back to streams throughout the summer. Geochemical tracers such as stable water isotopes and dissolved ions revealed that burned watersheds received a much larger contribution from snowmelt than unburned ones, directly linking wildfire to enhanced summer water supply.

As climate change drives more frequent and severe wildfires, understanding how fire alters hydrological processes is becoming increasingly important for water managers and policymakers. This study provides a practical framework for predicting post-fire water availability. The researchers caution, however, that the observed increase in summer low flows is only a temporary surplus that is unlikely to persist as the forest recovers. Therefore, long-term, continuous monitoring of discharge in burned watersheds is crucial for tracking the dynamics of low flows.“Wildfire is not a solution to water scarcity,” the study emphasizes, “but understanding these mechanisms helps us prepare for a future where fire and climate change are integral parts of mountain hydrology.”

The research was conducted in partnership with the Okanagan Indian Band and the Okanagan Nation Alliance, whose leadership in watershed stewardship supported this work. The research team respectfully acknowledge that the research was conducted on the traditional, ancestral, and unceded territory of the Syilx Okanagan Nation.