As Wildfires Burn, Rainfall Can Strip Oxygen From Rivers, Killing Fish

The study, conducted in cooperation with the Karuk Tribe, the Yurok Tribe and two universities, is the first published documentation of a rain-on-wildfire event directly linked to lethal water-quality conditions in a large river.

The research highlights how continuous water-quality monitoring, uncommon in fire-prone areas, is critical for detecting sudden changes. Using high-frequency monitoring data, scientists revealed how the initial flood wave of stormwater moved quickly downstream while a concentrated pulse of fire-scar material, including ash, charcoal, and sediment left behind by burned hillslopes, arrived later. This caused dissolved oxygen in the water to drop to zero for more than five hours, creating a condition called anoxia, a total loss of oxygen that can quickly kill fish.

"This research provides resource managers and emergency responders with critical information about how post-fire landscapes can affect water quality in ways we haven't fully understood before," said Jennifer Curtis, USGS research geologist and lead author of the study. "Understanding these oxygen depletion events helps managers better anticipate and potentially mitigate similar impacts as wildfires become more frequent and intense across the West."

The study draws on high-frequency streamflow and water-quality data collected at USGS and Tribal monitoring stations jointly operated with Tribal partners. Sensors measured turbidity, dissolved oxygen, pH, conductivity, and temperature before, during, and after the event, providing the data needed to connect fire, rain, and fish mortality.

"We are seeing increases in fire frequency, size and burn intensity making it more important to understand risks to native fish populations and water quality," said Toz Soto, Karuk Tribe Department of Natural Resources, Fisheries Program manager. "The Karuk Tribe is committed to long-term water quality monitoring to continue learning about large fires and floods."

The limited number of studies documenting anoxia after wildfires may reflect insufficient monitoring rather than the rarity of such events. Because oxygen can be depleted rapidly and prove deadly to aquatic life, continuous sensors are often the only way to capture these conditions in real time. This study demonstrates that rain-on-wildfire events represent a unique type of runoff event in which stormwater carries heavy loads of sediment, ash, and debris - the first flush of material from a freshly burned landscape. Rain-on-wildfire events are more likely than ordinary storms to cause extreme downstream impacts.

With wildfires expected to become more severe across the western United States and more intense monsoon storms frequently occurring in late summer, rain-on-wildfire events may become more common. The Klamath River basin, despite being California's wettest region, is already experiencing significant increases in fire size, severity, and frequency. By contrast, smaller, lower-intensity fires may help reduce fuel loads without severely affecting downstream water quality.

"We are proud to play a role this important research project.," said Yurok Tribe Environmental Department Water Program Manager Josh Cahill. "Through this unique partnership between USGS, the Yurok Tribe and the Karuk Tribe, we are able to track the Klamath River's ever-changing conditions in real time."

This study emphasizes the importance of sustained, high-frequency monitoring in fire-prone areas to capture short-lived yet consequential water-quality impairments and provide insights to inform efforts to build ecosystem resilience.