Evaluating the effects of wildfire on water quality

In September 2010, the Fourmile Canyon Fire burned 26 square km (6400 acres), destroyed 169 homes, and caused over $217 million in damages in the Boulder Creek Watershed.

The wildfire burned through rugged terrain, and deforestation of steep slopes has left the area at risk of flooding and erosion, including debris flows (Ruddy et al., 2010). Past studies of streams after wildfire are conflicting: some show increases in pH, turbidity, nutrients, organic carbon, sulfate, major ions, and metals, while and others report little impact (Neary et al., 2005; Smith et al., 2010). However, many of these studies sampled water chemistry at intervals that do not capture high temporal variability in stream discharge. This study will involve high-frequency stream sampling, and will evaluate how upland hydrologic and biogeochemical processes affected by fire will influence downstream water quality.

The Boulder Creek CZO and the USGS are collaborating to evaluate the effects of the wildfire on stream water quality, and on hillslope and in-stream processes. Stream discharge, organic carbon, nutrients, major ions, and metals and stream ecosystem characteristics (macroinvertebrates, biofilms) were measured in Fourmile Creek immediately after the wildfire and through the winter, and monitoring will continue during snowmelt and summer convective storms at daily and shorter frequencies. Runoff from burned north- and south-facing drainages will also be monitored to assess differences in hillslope contributions to the stream. Hillslopes have been instrumented to capture changes in hydrologic response and chemical evolution of solution waters through the soil profile (i.e., N and C species, anions, and cations). Stream water and soil water chemistry will be compared to the nearby, unburned CZO catchments. This study will contribute to our understanding of wildfire's effects on critical zone processes and stream systems, and assess the importance of increased temporal and spatial sampling.

References

Neary, D.G., Ryan, K.C., DeBano, LF, eds., 2005. Wildland fire in ecosystems: effects of fire on soils and water. USDA General Technical Report RMRS-GTR-42-vol.4. Ogden, Utah. USDA Forest Service Rocky Mountain Research Station. 250 p.

Ruddy, B.C., Stevens, M.R., Verdin, K.L., Elliott, J.G., 2010. Probability and volume of potential postwildfire debris flows in the 2010 Fourmile burn area, Boulder County, Colorado: U.S. Geological Survey Open-File Report 2010–1244, 5 p.

Smith, H.G., Sheridan, G.J., Lane, P.N.J., Nyman, P., Haydon, S., 2011. Wildfire effects on water quality in forest catchments: A review with implications for water supply. Journal of Hydrology 396, 170-192.

Written by Sheila Murphy, USGS. For more information, contact Sheila at sfmurphy at usgs.gov