PNNL

The Science

Wildfires can change how water moves through a watershed and what it carries into streams, but how the wet forests of Oregon and Washington respond to wildfire has not been synthesized and may be different from the response in other regions. A multi-institutional team of researchers reviewed recent literature and assembled available monitoring records to describe what was known and where gaps remained. They found that stream networks were monitored widely for flow, and many sites also tracked five primary water quality indicators—temperature, turbidity (water cloudiness), specific conductance (dissolved ions), dissolved oxygen level, and pH (acidity). Even with these data streams available, few analyses have been published that link fires to changes in these indicators. The review highlighted the limited evidence regarding how fires affect metal transport, nutrient pulses, organic matter, and aquatic organisms over multiple timescales.

The Impact

The 2020 Labor Day fires included six large fires that burned 334,549 hectares in a few weeks, highlighting the importance of understanding how postfire landscapes influence streams and downstream water uses in the Pacific Northwest. This analysis paired a regional synthesis of postfire hydrology, chemistry, and ecology with an inventory of locations where long-term stream monitoring already existed. The results showed that the region had many continuous and discrete sampling locations, yet much of the information had not been pulled together to characterize postfire responses. By identifying especially thin coverage for dissolved oxygen level and pH, and the mismatch between where monitoring is dense and where fires often burn, the review highlights that future studies should connect flow events to chemical pulses and biological effects in the region.

Summary

Researchers from multiple institutions reviewed existing research and monitoring efforts focused on forested watersheds west of the Cascade Range in Oregon and Washington, emphasizing studies from 2015–2025, with special attention to work following the 2020 Labor Day fires. While existing monitoring was extensive, published analyses lagged behind data availability. About half of stream gauges in the two states recorded major water quality parameters, and the region also had hundreds of locations with discrete sampling for many constituents; however, monitoring was often not in the same areas as where wildfires occurred. Postfire responses were analyzed in connection with shifts in watershed water balance and event timing. In the first two years after recent fires, runoff increased and evapotranspiration declined in burned watersheds, while early responses after the 2020 fires were muted because precipitation was below average and extreme storms did not occur. For nutrients, studies were mostly limited to the first three years and often reported higher nitrate and total dissolved nitrogen levels, although one study observed lower total dissolved nitrogen levels with higher burn severity during the first postfire rain event. The analysis highlighted major unknowns for dissolved oxygen and pH behavior, metal mobilization beyond mercury, and how chemical pulses translate into changes in stream ecosystem function.

The initial draft of the text above was created using ChatGPT (version 5.5 or lower, OpenAI). The language and content were subsequently edited by the author for grammar, clarity, and accuracy, and the final document was reviewed by the author. 

Research Contact(s)

Allison Myers-Pigg, River Corridor SFA co-principal investigator

Pacific Northwest National Laboratory

allison.myers-pigg@pnnl.gov

James Stegen, River Corridor SFA principal investigator

Pacific Northwest National Laboratory

james.stegen@pnnl.gov

Funding

This research was supported by the Department of Energy, Office of Science, Biological and Environmental Research program, Environmental System Science program. This contribution originates from the River Corridor Scientific Focus Area (SFA) project at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle Memorial Institute for the Department of Energy.

Related Links

River Corridor Scientific Focus Area PNNL web page

U.S. Environmental Protection Agency and U.S. Geological Survey data analysis:

The data that support the findings of this study are openly available at the following URL/DOI: https://waterservices.usgs.gov/docs/site-service/; https://doi.org/10.5066/P9QRKUVJ