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Post-fire Water Quality: An Investigation of Determinants and Recovery Processes in Burned Watersheds in the Western U.S. - JFSP Final Report

Author(s): Terri S. Hogue, John McCray
Year Published: 2019

Wildfires are a natural disturbance that are increasing in size and severity in forested landscapes across the Western United States. Forest fires affect water quality in the disrupted watershed, which can significantly alter the aquatic ecosystem, including sensitive trout (Salmonidae) and macroinvertebrate species. However, the type and duration of the water quality impact from fire is unpredictable. Previous studies on individual fires have observed an increase in various forms of nutrients, ions, sediments, and metals in stream water for different post-fire time periods. While wildfires clearly are growing in size and severity, less is known about the type of stream water quality and community impacts that should be anticipated from these growing fires.

This study explored water quality response from fire. First, we investigated how water quality and the aquatic community were disrupted by a large Colorado fire. The West Fork Fire Complex consumed 110,000 acres of forest in the state of Colorado during the summer of 2013. The recent fire surrounded the Rio Grande, affecting water quality and habitat critical to insects and fish. The water quality of the Rio Grande (above and below the burn) and some of the effected tributaries was monitored for three years after the fire. All water quality parameters remained the same above and below the fire except turbidity and total suspended solids. Steep, severely burned hillslopes experienced erosion and were the source of sediment loading into the surface water. Despite elevated turbidity levels that persisted for three years in close proximity to the fire, the ecosystem showed resiliency and aquatic macroinvertebrate populations and trout populations have recovered.

To examine how water quality generally impacts water quality, data was compiled for over 24,000 fires across the western United States to evaluate post-fire water quality response. Data from 159 of these fires in 153 burned watersheds were used to identify common water quality response during the first five years after a fire. Within this large dataset, a subset of ten fires was examined further to identify trends in water quality response. Change-point analysis was used to identify moments in the post-fire water quality data where significant shifts in analyte concentrations occurred. Evidence from this analysis reveals significant increases in nutrient flux (different forms of nitrogen and phosphorus), major-ion flux, and metal concentrations are the most common changes in stream water quality within the first five years after fire. Assembling this unique and extensive data set provided the opportunity to determine the most common post-fire water quality changes in the large and diverse Western U.S.

After looking at this large number of fires, the physical determinants driving water quality response and recovery were evaluated. Through the use of the conditional inference tree technique and Spearman correlation analysis, the drivers of post-fire water quality response were exposed. Results show that the geology of the burn area, which influences the soil characteristics, burn severity, and the rate of vegetation recovery are the key determinants of water quality change and resiliency.

Results from this project expand the scientific knowledge of how wildfires impact surface water quality and the communities that depend on it. Through compiling a large dataset, it was possible to identify common post-fire water quality responses and the watershed features that control them. This research also provides critical information on water quality behavior to land and water managers in anticipation of, and following, a wildfire.

Citation: Hogue TS and McCray J. 2019. Post-fire Water Quality: An Investigation of Determinants and Recovery Processes in Burned Watersheds across the Western U.S. JFSP Final Report. PROJECT # 14-1-06-14 Golden, Colorado: Colorado School of Mines, 47 p.
Topic(s): Fire Effects, Ecological - Second Order, Aquatic Life, Water
Ecosystem(s): None
Document Type: Technical Report or White Paper
NRFSN number: 19472
Record updated: May 13, 2019