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[[File:DragonBravoFire.jpg|thumb|600px|Figure 1. Smoke plume from the Dragon Bravo Fire over the Grand Canyon on July 11, 2025. (Photo: National Park Service/M. Quinn)]]
[[File:DragonBravoFire.jpg|thumb|600px|Figure 1. Smoke plume from the Dragon Bravo Fire over the Grand Canyon on July 11, 2025. (Photo: National Park Service/M. Quinn)]]


Watersheds across the western United States are being increasingly impacted by wildfires. Since 2000, West-wide average annual burned area has more than doubled, and the number of "megafires" (>100,000 acres) has increased from about one per year to over five per year<ref name="williams2025" />. Overall, wildfires are also spreading faster, burning at higher overall severity, and at higher average elevations than in the 1980s and 1990s. In the Colorado River Basin, many large high-severity wildfires have impacted runoff-generating watersheds (Table 1, below), from the subalpine spruce-fir and lodgepole forests of Colorado, Utah, and Wyoming, to the ponderosa pine and mixed-conifer forests of Arizona (Figure 1) and New Mexico.<ref name="mcgrath2023" /><ref name="kampf2022" /><ref name="alizadeh2021" />
Watersheds across the western United States are being increasingly impacted by wildfires. Since 2000, West-wide average annual burned area has more than doubled, and the number of "megafires" (>100,000 acres) has increased from about one per year to over five per year<ref name="williams2025" />. Overall, wildfires are also spreading faster, burning at higher overall severity, and at higher average elevations than in the 1980s and 1990s. <ref name="parks2023" /><ref name="alizadeh2023" /> In the Colorado River Basin, many large high-severity wildfires have impacted runoff-generating watersheds (Table 1, below), from the subalpine spruce-fir and lodgepole forests of Colorado, Utah, and Wyoming, to the ponderosa pine and mixed-conifer forests of Arizona (Figure 1) and New Mexico.<ref name="mcgrath2023" /><ref name="kampf2022" /><ref name="alizadeh2021" />
The dramatic shifts in wildfire occurrence and severity in the past several decades have multiple causes, including warming and drying of the atmosphere and vegetation due to anthropogenic climate change, increases in fuel loading and forest density from past forest uses and fire suppression, and increases in development in the wildland-urban interface (WUI) and in wildland recreation, leading to additional structural fuels and many more ignition opportunities.
The dramatic shifts in wildfire occurrence and severity in the past several decades have multiple causes, including warming and drying of the atmosphere and vegetation due to anthropogenic climate change, increases in fuel loading and forest density from past forest uses and fire suppression, and increases in development in the wildland-urban interface (WUI) and in wildland recreation, leading to additional structural fuels and many more ignition opportunities.


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<ref name="williams2025">Williams, A. P., Juang, C. S., & Short, K. C. (2025). The Western United States MTBS-Interagency database of large wildfires, 1984–2024 (WUMI2024a). Earth Syst. Sci. Data, 17(12), 7359–7372. https://doi.org/10.5194/essd-17-7359-2025</ref>
<ref name="williams2025">Williams, A. P., Juang, C. S., & Short, K. C. (2025). The Western United States MTBS-Interagency database of large wildfires, 1984–2024 (WUMI2024a). Earth Syst. Sci. Data, 17(12), 7359–7372. https://doi.org/10.5194/essd-17-7359-2025</ref>
<ref name="alizadeh2023">Alizadeh, M. R., Abatzoglou, J. T., Adamowski, J., Modaresi Rad, A., AghaKouchak, A., Pausata, F. S. R., & Sadegh, M. (2023). Elevation-dependent intensification of fire danger in the western United States. Nature Communications, 14(1), 1773. https://doi.org/10.1038/s41467-023-37311-4</ref>
<ref name="parks2023">Parks, S. A., Holsinger, L. M., Blankenship, K., Dillon, G. K., Goeking, S. A., & Swaty, R. (2023). Contemporary wildfires are more severe compared to the historical reference period in western US dry conifer forests. Forest Ecology and Management, 544, 121232. https://doi.org/10.1016/j.foreco.2023.121232</ref>




</references>
</references>

Revision as of 16:40, 16 July 2026

Overview

Figure 1. Smoke plume from the Dragon Bravo Fire over the Grand Canyon on July 11, 2025. (Photo: National Park Service/M. Quinn)

Watersheds across the western United States are being increasingly impacted by wildfires. Since 2000, West-wide average annual burned area has more than doubled, and the number of "megafires" (>100,000 acres) has increased from about one per year to over five per year[1]. Overall, wildfires are also spreading faster, burning at higher overall severity, and at higher average elevations than in the 1980s and 1990s. [2][3] In the Colorado River Basin, many large high-severity wildfires have impacted runoff-generating watersheds (Table 1, below), from the subalpine spruce-fir and lodgepole forests of Colorado, Utah, and Wyoming, to the ponderosa pine and mixed-conifer forests of Arizona (Figure 1) and New Mexico.[4][5][6] The dramatic shifts in wildfire occurrence and severity in the past several decades have multiple causes, including warming and drying of the atmosphere and vegetation due to anthropogenic climate change, increases in fuel loading and forest density from past forest uses and fire suppression, and increases in development in the wildland-urban interface (WUI) and in wildland recreation, leading to additional structural fuels and many more ignition opportunities.

Wildfires can affect the basin's water resources and complicate their management in two interrelated ways: (1) altering water supply by changing snowpack accumulation and timing, plant water use, soil infiltration, and overall runoff efficiency, and (2) impairing water quality by mobilizing sediment, organic carbon, metals, and nutrients into the stream. Preventative forest management (e.g., fuels mitigation) can be undertaken to reduce wildfire impacts. These forest treatments can themselves affect watershed hydrology, depending on their intensity and scale.

Notable recent wildfires in the Colorado River Basin

The table below summarizes selected large wildfires since 2000 that have impacted the basin's water resources.

Table 1. Selected Large Wildfires in the Colorado River Basin since 2000

Fire Year Size (acres) Location Water-related impacts
Wallow 2011 538,049 White Mountains, Arizona (Salt River) Altered summer streamflow, high peak flows and flood events, water quality impacts.
Rodeo–Chediski 2002 468,638 White Mountains/Mogollon Rim, Arizona (Salt River) Changes in evapotranspiration in previously forested areas; elevated nitrogen (N), phosphorus (P), arsenic (As), lead (Pb), copper (Cu), and iron (Fe) documented in the Salt River at Roosevelt Lake.
Whitewater–Baldy 2012 297,845 Gila National Forest, New Mexico (Gila River) Stream degradation and debris flows in the Gila River following the fire; destruction of native Gila trout habitat.
East Troublesome 2020 193,812 Grand County, Colorado (Colorado River) Sediment and debris flows, harmful algal blooms, increased water treatment demands, and elevated peak runoff following precipitation events.
Dragon–Bravo 2025 145,504 North Rim, Grand Canyon, Arizona (Colorado River) Reduced water supply by approximately 50%; destroyed the North Rim's only potable water system; post-fire debris flows.
Grizzly Creek 2020 32,632 Glenwood Canyon, Colorado (Colorado River) High turbidity following the fire; debris flows blocked the mainstem Colorado River channel.
Dollar Ridge 2018 68,869 Strawberry River, Utah Debris flows and total dissolved solids (TDS) transported into the Strawberry River and Starvation Reservoir; changes in TDS and pH impaired the fishery.
East Fork 2020 89,765 Duchesne River, Utah Debris flows, damage to water lines at Moon Lake Campground, and high total dissolved solids (TDS) influxes.

Water supply impacts

Snowpack and timing of snowmelt and runoff

Wildfires can alter the amount and timing of streamflows primarily by impacting the accumulation and melt of the seasonal snowpack. Large wildfires that kill some portion of the canopy (i.e., crown fire) consistently result in decreased snow-water storage and earlier timing of snowmelt.[7][8][9] The loss of forest canopy alters the snowpack energy balance by increasing solar radiation reaching the snow surface, causing enhanced melt and sublimation throughout the winter and spring.[4][5][7][9] Additionally, black carbon and burned woody debris shed onto the snow post-fire darkens its surface, causing greater absorption of solar radiation and accelerating melt rates.[4][9][7][5] These changes in snow accumulation and melt timing are long-lived[4], persisting for at least 10 years[7][9][8], and need to be factored into for water supply forecasting, planning, and management.

Plant water use

By causing both immediate and delayed mortality in non-woody plants, shrubs, and trees wildfires typically result in reductions to total water use (i.e., transpiration) by vegetation in the burn area.[10][11][12] The timing of onset, magnitude, and duration of these changes depend strongly on burn severity, vegetation type, and forest recovery mechanisms. During the period that transpiration is reduced, runoff from the burned area may increase, depending on the fire's effects on other elements of the area's water balance.

Reduced transpiration following wildfire vegetation mortality has been documented across many forest types.[13][14][15][16][17] The duration of these reductions varies, but typically lasts from 2 to 15 years.[18][17][13] In some cases, vigorous regrowth of trees post-fire ultimately leads to greater water use than in the pre-fire condition.[19][20][21][22]

Soil infiltration and runoff

Wildfires also often change how water from rain and snowmelt moves through–or doesn't move through–the soil. Fires strip away the protective and water-absorbent surface litter layer, exposing bare soil, and reduce tree canopy cover that could have prevented raindrops from directly striking that soil.[23] High-severity fires can also create a strongly water-repellent (hydrophobic) layer beneath the surface.[24][25][26] This hydrophobic layer inhibits rain and snowmelt from soaking into the soil, so that more of that water reaches the stream through surface runoff rather than slower groundwater processes.[23][27][9][28] This leads to more abrupt and higher peak stream discharges after snowmelt and rain events, with much higher risk for damaging flash floods and debris flows, and potentially also to increases in total annual water yield, depending on other factors.[23]

Water quality impacts

After a wildfire, large areas of soil are newly exposed to erosion. The resulting jump in sediment flux to streams and rivers, along with ash and other burned debris, causes many contaminants to exceed pre-fire concentrations by orders of magnitude.[29] Among these contaminants is dissolved organic carbon (DOC), which can combine with the chlorine used in water treatment to form harmful chlorinated by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs).[30][31] while also reducing coagulation efficiency in treatment plants.[31][32]

Levels of nutrients such as nitrogen (N) and phosphorus (P) can also spike.[29] causing algal blooms and other biological impacts.[29][31] Metals such as iron (Fe), manganese (Mn), arsenic (As), lead (Pb), and copper (Cu) are often mobilized with the sediment and ash as well.[30][33][34][35] While most of these water-chemistry impacts subside within five years of a wildfire, some effects, such as the persistence of nitrogen and DOC, can extend for 15 years or more following high-severity events.[31][29]

Beyond the chemical contaminants, the dramatically increased sediment loading in streams and rivers is itself problematic for water treatment.[29][30] High turbidity makes disinfection and filtration less efficient and more costly.[30][29][36] and in extreme conditions water intakes may have to be shut down completely.[31][37][29] The influx of ash and sediment also has acute effects on aquatic life.[38][29] often leading to declines in fish populations.[29][39] The altered physical and chemical conditions post-fire can drive shifts in macroinvertebrate communities towards more opportunistic species.[29]

Land management to reduce fire risk and impacts

Mechanical thinning and prescribed fire have been used for many years to reduce the risk of high-severity fires, particularly in mid-elevation ponderosa pine and mixed-conifer forests that have become much denser due to post-1900 fire suppression. These treatments can also improve snowpack retention and modestly increase annual streamflow by reducing canopy interception and evapotranspiration, though sustained increases in water supply are uncertain. Several water providers, including Denver Water (Forests to Faucets) and Salt River Project (SRP Healthy Forests Initiative), have partnered with land managers to plan, fund, and carry out forest treatments in their Colorado River watersheds to reduce the risk of severe wildfires and their impacts to water supplies.

More recently, it has been recognized that stream and wetland restoration can bring substantial co-benefits for wildfire mitigation. Restoring the water- and sediment-retention functions of riparian and wetland areas and enlarging their footprint on the landscape can slow fire spread through "wet fuel breaks", reduce burn severity, and alleviate post-fire impacts on water quality. These restoration techniques, collectively called process-based restoration (PBR), include beaver reintroduction, emplacing logs and wood structures in stream channels such as beaver-dam analogs (BDAs), riparian plantings, meadow restoration, and, when necessary, earthmoving to reestablish natural channels and stream connectivity.

PBR projects in the basin are often supported by water agencies in collaboration with land managers and environmental NGOs. For example, the Kawuneeche Valley Restoration Collaborative (KVRC) in the Colorado River headwaters in north-central Colorado receives direct funding and staff involvement from Northern Water as well as grants from the Colorado Water Conservation Board, Colorado River District, and Reclamation.

Data and Tools

USDA MTBS (Burn Severity) Data Explorer

Monitoring Trends in Burn Severity (MTBS) is an interagency program to map the burn severity and extent of large wildfires (>1,000 acres in the West) based on satellite imagery. When opened, the MTBS Data Explorer will show the perimeter and per-pixel burn severity of all large fires from 1984 through 2024. To see the name, ignition date, and acreage of a fire, select Tools > Pixel Tools > Query Visible Map Layers and then click within a fire perimeter.

NIFC WildfireSA Public Map

This interactive map from the National Interagency Fire Center (NIFC) shows all current and recent wildfires (‘incidents’). Click on a fire to bring up more information about that fire. From the Layers button in the upper right, you can also select ‘Fuels Treatments’ to show all recent (>2000) management actions to reduce fuels.

USGS Post-Fire Debris Flow Hazard Assessment Viewer

The USGS produces rapid postfire debris-flow hazard assessments for select fires in the Western U.S., using geospatial data on basin slopes, burn severity, soil properties, and rainfall characteristics to estimate the likelihood and volume of debris flows that may occur in response to a defined rainstorm (typically 0.25”-0.5” in 15 minutes, corresponding to a once-in-1-year event).

Additional Resources

A review of post-wildfire shifts in streamflow

This 2025 review paper by Ebel et al. synthesizes existing research on how wildfire alters hydrologic processes and streamflow generation in the western United States. It presents a narrative review and conceptual model highlighting the key factors that shape post-fire changes in streamflow such as seasonal precipitation, the timing overlap between precipitation and potential evapotranspiration, shifts in interception and evapotranspiration relative to overall precipitation, and vegetation-related changes.

Notes

  1. ↑ Williams, A. P., Juang, C. S., & Short, K. C. (2025). The Western United States MTBS-Interagency database of large wildfires, 1984–2024 (WUMI2024a). Earth Syst. Sci. Data, 17(12), 7359–7372. https://doi.org/10.5194/essd-17-7359-2025
  2. ↑ Parks, S. A., Holsinger, L. M., Blankenship, K., Dillon, G. K., Goeking, S. A., & Swaty, R. (2023). Contemporary wildfires are more severe compared to the historical reference period in western US dry conifer forests. Forest Ecology and Management, 544, 121232. https://doi.org/10.1016/j.foreco.2023.121232
  3. ↑ Alizadeh, M. R., Abatzoglou, J. T., Adamowski, J., Modaresi Rad, A., AghaKouchak, A., Pausata, F. S. R., & Sadegh, M. (2023). Elevation-dependent intensification of fire danger in the western United States. Nature Communications, 14(1), 1773. https://doi.org/10.1038/s41467-023-37311-4
  4. ↑ 4.0 4.1 4.2 4.3 McGrath, D., Zeller, L., Bonnell, R., Reis, W., Kampf, S., Williams, K., ... & Rittger, K. (2023). Declines in peak snow water equivalent and elevated snowmelt rates following the 2020 Cameron Peak wildfire in northern Colorado. Geophysical Research Letters, 50(6), e2022GL101294.
  5. ↑ 5.0 5.1 5.2 Kampf, S. K., McGrath, D., Sears, M. G., Fassnacht, S. R., Kiewiet, L., & Hammond, J. C. (2022). Increasing wildfire impacts on snowpack in the western US. Proceedings of the National Academy of Sciences, 119(39), e2200333119.
  6. ↑ Alizadeh, M. R., Abatzoglou, J. T., Luce, C. H., Adamowski, J. F., Farid, A., & Sadegh, M. (2021). Warming enabled upslope advance in western US forest fires. Proceedings of the National Academy of Sciences, 118(22), e2009717118.
  7. ↑ 7.0 7.1 7.2 7.3 Smoot, E. E., & Gleason, K. E. (2021). Forest fires reduce snow-water storage and advance the timing of snowmelt across the Western US. Water, 13(24), 3533.
  8. ↑ 8.0 8.1 Giovando, J., & Niemann, J. D. (2022). Wildfire impacts on snowpack phenology in a changing climate within the western US. Water Resources Research, 58(8), e2021WR031569.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Koshkin, A. L., Hatchett, B. J., & Nolin, A. W. (2022). Wildfire impacts on western United States snowpacks. Frontiers in Water, 4, 971271.
  10. ↑ Bär, A., Michaletz, S. T., & Mayr, S. (2019). Fire effects on tree physiology. New Phytologist, 223(4), 1728–1741.
  11. ↑ Niccoli, F., Pacheco-Solana, A., Delzon, S., et al. (2023). Effects of wildfire on growth, transpiration and hydraulic properties of Pinus pinaster Aiton forest. Dendrochronologia, 79, 126086.
  12. ↑ O'Brien, J. J., Kevin Hiers, J., Mitchell, R. J., Varner III, J. M., & Mordecai, K. (2010). Acute physiological stress and mortality following fire in a long-unburned longleaf pine ecosystem. Fire Ecology, 6(2), 1–12.
  13. ↑ 13.0 13.1 Dore, S., Montes-Helu, M., Hart, S. C., et al. (2012). Recovery of ponderosa pine ecosystem carbon and water fluxes from thinning and stand-replacing fire. Global Change Biology, 18(10), 3171–3185.
  14. ↑ Nolan, R. H., Lane, P. N., Benyon, R. G., et al. (2014). Changes in evapotranspiration following wildfire in resprouting eucalypt forests. Ecohydrology, 7(5), 1363–1377.
  15. ↑ Poon, P. K., & Kinoshita, A. M. (2018). Spatial and temporal evapotranspiration trends after wildfire in semi-arid landscapes. Journal of Hydrology, 559, 71–83.
  16. ↑ Cooper, C. E., Aparecido, L. M., Muir, J. P., et al. (2019). Transpiration in recovering mixed loblolly pine and oak stands following wildfire in the Lost Pines region of Texas. Ecohydrology, 12(1), e2052.
  17. ↑ 17.0 17.1 Ma, Q., Bales, R. C., Rungee, J., et al. (2020). Wildfire controls on evapotranspiration in California's Sierra Nevada. Journal of Hydrology, 590, 125364.
  18. ↑ Häusler, M., Nunes, J. P., Soares, P., et al. (2018). Assessment of the indirect impact of wildfire (severity) on actual evapotranspiration in eucalyptus forest based on the surface energy balance estimated from remote-sensing techniques. International Journal of Remote Sensing, 39(20), 6499–6524.
  19. ↑ Langford, K. J. (1975). Change in yield of water following a bushfire in a forest of Eucalyptus regnans. Report No. MMBW-W-0003.
  20. ↑ Kuczera, G. (1987). Prediction of water yield reductions following a bushfire in ash-mixed species eucalypt forest. Journal of Hydrology, 94(3–4), 215–236.
  21. ↑ Buckley, T. N., Turnbull, T. L., Pfautsch, S., et al. (2012). Differences in water use between mature and post-fire regrowth stands of subalpine Eucalyptus delegatensis R. Baker. Forest Ecology and Management, 270, 1–10.
  22. ↑ Meili, N., Beringer, J., Zhao, J., & Fatichi, S. (2024). Aerodynamic effects cause higher forest evapotranspiration and water yield reductions after wildfires in tall forests. Global Change Biology, 30(1), e16995.
  23. ↑ 23.0 23.1 23.2 Wine, M. L., & Cadol, D. (2016). Hydrologic effects of large southwestern USA wildfires significantly increase regional water supply: Fact or fiction? Environmental Research Letters, 11(8), 085006.
  24. ↑ Mataix-Solera, J., & Doerr, S. H. (2004). Hydrophobicity and aggregate stability in calcareous topsoils from fire-affected pine forests in southeastern Spain. Geoderma, 118(1–2), 77–88.
  25. ↑ Huffman, E. L., MacDonald, L. H., & Stednick, J. D. (2001). Strength and persistence of fire-induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range. Hydrological Processes, 15(15), 2877–2892.
  26. ↑ Shakesby, R. A., & Doerr, S. H. (2006). Wildfire as a hydrological and geomorphological agent. Earth-Science Reviews, 74(3–4), 269–307.
  27. ↑ Williams, A. P., Livneh, B., McKinnon, K. A., Hansen, W. D., Mankin, J. S., Cook, B. I., ... & Lettenmaier, D. P. (2022). Growing impact of wildfire on western US water supply. Proceedings of the National Academy of Sciences, 119(10), e2114069119.
  28. ↑ Beeson, P. C., Martens, S. N., & Breshears, D. D. (2001). Simulating overland flow following wildfire: Mapping vulnerability to landscape disturbance. Hydrological Processes, 15(15), 2917–2930.
  29. ↑ 29.00 29.01 29.02 29.03 29.04 29.05 29.06 29.07 29.08 29.09 Paul, M. J., LeDuc, S. D., Lassiter, M. G., Moorhead, L. C., Noyes, P. D., & Leibowitz, S. G. (2022). Wildfire induces changes in receiving waters: A review with considerations for water quality management. Water Resources Research, 58(9), e2021WR030699.
  30. ↑ 30.0 30.1 30.2 30.3 Smith, H. G., Sheridan, G. J., Lane, P. N., Nyman, P., & Haydon, S. (2011). Wildfire effects on water quality in forest catchments: A review with implications for water supply. Journal of Hydrology, 396(1–2), 170–192.
  31. ↑ 31.0 31.1 31.2 31.3 31.4 Hohner, A. K., Rhoades, C. C., Wilkerson, P., & Rosario-Ortiz, F. L. (2019). Wildfires alter forest watersheds and threaten drinking water quality. Accounts of Chemical Research, 52(5), 1234–1244.
  32. ↑ Hohner, A. K., Terry, L. G., Townsend, E. B., Summers, R. S., & Rosario-Ortiz, F. L. (2017). Water treatment process evaluation of wildfire-affected sediment leachates. Environmental Science: Water Research & Technology, 3(2), 352–365.
  33. ↑ Burton, C. A., Hoefen, T. M., Plumlee, G. S., Baumberger, K. L., Backlin, A. R., Gallegos, E., & Fisher, R. N. (2016). Trace elements in stormflow, ash, and burned soil following the 2009 Station Fire in Southern California. PLOS ONE, 11(5), e0153372.
  34. ↑ Allen, E. W., Prepas, E. E., Gabos, S., Strachan, W. M., & Zhang, W. (2005). Methyl mercury concentrations in macroinvertebrates and fish from burned and undisturbed lakes on the Boreal Plain. Canadian Journal of Fisheries and Aquatic Sciences, 62(9), 1963–1977.
  35. ↑ Kelly, E. N., Schindler, D. W., St. Louis, V. L., Donald, D. B., & Vladicka, K. E. (2006). Forest fire increases mercury accumulation by fishes via food web restructuring and increased mercury inputs. Proceedings of the National Academy of Sciences, 103(51), 19380–19385.
  36. ↑ Emelko, M. B., Silins, U., Bladon, K. D., & Stone, M. (2011). Implications of land disturbance on drinking water treatability in a changing climate: Demonstrating the need for "source water supply and protection" strategies. Water Research, 45(2), 461–472.
  37. ↑ Writer, J. H., Hohner, A., Oropeza, J., Schmidt, A., Cawley, K. M., & Rosario-Ortiz, F. L. (2014). Water treatment implications after the High Park wildfire, Colorado. Journal ‐ American Water Works Association, 106(4), E189–E199.
  38. ↑ Earl, S. R., & Blinn, D. W. (2003). Effects of wildfire ash on water chemistry and biota in south-western USA streams. Freshwater Biology, 48(6), 1015–1030.
  39. ↑ Rust, A. J., Saxe, S., McCray, J., Rhoades, C. C., & Hogue, T. S. (2019). Evaluating the factors responsible for post-fire water quality response in forests of the western USA. International Journal of Wildland Fire, 28(10), 769–784.
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