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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, runoff efficiency, and soil infiltration 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.
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, runoff efficiency, and soil infiltration 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.
Wildfires are increasingly impacting watersheds across the western United States. Since 2000, annual burned area has increased several-fold West-wide, many more very large fires (>10,000 acres) burning at higher overall severity than in the 1980s and 1990s. In the Colorado River Basin, large and severe wildfires since 2000 have impacted runoff-generating watersheds from the subalpine spruce-fir and lodgepole forests of Colorado, Utah, and Wyoming, to the ponderosa pine and mixed-conifer forests of the Mogollon Rim and White Mountains and the Gila Wilderness in New Mexico.<ref name="mcgrath2023" /><ref name="kampf2022" /><ref name="alizadeh2021" /> The dramatic shifts in wildfire occurrence and severity since the 1980s have multiple causes, including warming and drying of the atmosphere and vegetation with 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 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, runoff efficiency, and soil infiltration 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.
==Notes==
<references>
<ref name="mcgrath2023">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.</ref>
<ref name="kampf2022">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.</ref>
<ref name="alizadeh2021">Alizadeh et al. (2021).</ref>
</references>
Wildfires are increasingly impacting watersheds across the western United States. Since 2000, annual burned area has increased several-fold West-wide, many more very large fires (>10,000 acres) burning at higher overall severity, than in the 1980s and 1990s. In the Colorado River Basin, large and severe wildfires since 2000 have impacted runoff-generating watersheds from the subalpine spruce-fir and lodgepole forests of Colorado, Utah, and Wyoming, to the ponderosa pine and mixed-conifer forests of the Mogollon Rim and White Mountains and the Gila Wilderness in New Mexico.  (McGrath et al., 2023; Kampf et al., 2022, Alizadeh et al. 2021). The dramatic shifts in wildfire occurrence and severity since the 1980s have multiple causes, including warming and drying of the atmosphere and vegetation with 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 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, runoff efficiency, and soil infiltration 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===
===Notable recent wildfires in the Colorado River Basin===
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===Snowpack and timing of snowmelt and runoff===
===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 (Smoot & Gleason, 2021; Giovando & Niemann, 2022; Koshkin et al., 2022). 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 (McGrath et al., 2023; Kampf et al., 2022; Smoot & Gleason, 2021; Koshkin et al., 2022). 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 (McGrath et al., 2023; Koshkin et al., 2022; Smoot & Gleason, 2021; Kampf et al., 2022). These changes in snow accumulation and melt timing are long-lived (McGrath et al., 2023), persisting for at least 10 years (Smoot & Gleason, 2021; Koshkin et al., 2022; Giovando & Niemann, 2022), and need to be factored into for water supply forecasting, planning, and management.
 
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.<ref name="smoot2021" /><ref name="giovando2022" /><ref name="koshkin2022" /> 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.<ref name="mcgrath2023" /><ref name="kampf2022" /><ref name="smoot2021" /><ref name="koshkin2022" /> 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.<ref name="mcgrath2023" /><ref name="koshkin2022" /><ref name="smoot2021" /><ref name="kampf2022" /> These changes in snow accumulation and melt timing are long-lived<ref name="mcgrath2023" />, persisting for at least 10 years<ref name="smoot2021" /><ref name="koshkin2022" /><ref name="giovando2022" />, and need to be factored into for water supply forecasting, planning, and management.


===Vegetation and Evaporation Changes===
===Vegetation and Evaporation Changes===


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 (Bär et al. 2019, Niccoli et al. 2023, O'Brien et al. 2010). 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.
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.<ref name="bar2019" /><ref name="niccoli2023" /><ref name="obrien2010" /> 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 (Dore et al. 2012, Nolan et al. 2014, Poon and Kinoshita 2018, Cooper et al. 2019, Ma et al. 2020). The duration of these reductions varies, but typically lasts from 2 to 15 years (Hausler et al., 2018; Ma et al., 2020; Dore et al., 2012). In some cases, vigorous regrowth of trees post-fire ultimately leads to greater water use than in the pre-fire condition (Langford 1976, Kuczera 1987, Buckley et al. 2012, Meili et al. 2024).
 
Reduced transpiration following wildfire vegetation mortality has been documented across many forest types.<ref name="dore2012" /><ref name="nolan2014" /><ref name="poon2018" /><ref name="cooper2019" /><ref name="ma2020" /> The duration of these reductions varies, but typically lasts from 2 to 15 years.<ref name="hausler2018" /><ref name="ma2020" /><ref name="dore2012" /> In some cases, vigorous regrowth of trees post-fire ultimately leads to greater water use than in the pre-fire condition.<ref name="langford1975" /><ref name="kuczera1987" /><ref name="buckley2012" /><ref name="meili2024" />


===Soils===
===Soils===


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 (Wine & Cadol, 2016). High-severity fires can also create a strongly water-repellent (hydrophobic) layer beneath the surface (Mataix-Solera & Doerr, 2004; Huffman et al., 2001; Shakesby & Doerr, 2006). 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 (Wine & Cadol, 2016; Williams et al., 2022; Koshkin et al., 2022; Beeson et al., 2001). 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 (Wine & Cadol, 2016).
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.<ref name="wine2016" /> High-severity fires can also create a strongly water-repellent (hydrophobic) layer beneath the surface.<ref name="mataix2004" /><ref name="huffman2001" /><ref name="shakesby2006" /> 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.<ref name="wine2016" /><ref name="williams2022" /><ref name="koshkin2022" /><ref name="beeson2001" /> 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.<ref name="wine2016" />


===Water Quality Impacts===
===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 (Paul et al., 2022). Among those 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) (Smith et al., 2011; Hohner et al., 2019), while also reducing coagulation efficiency in treatment plants (Hohner et al., 2019; Hohner et al., 2017).
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.<ref name="paul2022" /> Among those 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).<ref name="smith2011" /><ref name="hohner2019" /> while also reducing coagulation efficiency in treatment plants.<ref name="hohner2019" /><ref name="hohner2017" />
Levels of nutrients such as nitrogen (N) and phosphorus (P) can also spike (Paul et al., 2022), causing algal blooms and other biological impacts (Paul et al., 2022; Hohner et al., 2019). Metals such as iron (Fe), manganese (Mn), arsenic (As), lead (Pb), and copper (Cu) are often mobilized with the sediment and ash as well (Smith et al., 2011; Burton et al., 2016; Allen et al., 2005; Kelly et al., 2006). 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 (Hohner et al., 2019; Paul et al., 2022).
 
Beyond the chemical contaminants, the dramatically increased sediment loading in streams and rivers is itself problematic for water treatment (Paul et al., 2022; Smith et al., 2011). High turbidity makes disinfection and filtration less efficient and more costly (Smith et al., 2011; Paul et al., 2022; Emelko et al., 2011), and in extreme conditions water intakes may have to be shut down completely (Hohner et al., 2019; Writer et al., 2014; Paul et al., 2022). The influx of ash and sediment also has acute effects on aquatic life (Earl & Blinn, 2003; Paul et al., 2022), often leading to declines in fish populations (Paul et al., 2022; Rust et al., 2019). The altered physical and chemical conditions post-fire can drive shifts in macroinvertebrate communities towards more opportunistic species (Minshall et al., 1997; Paul et al., 2022).
Levels of nutrients such as nitrogen (N) and phosphorus (P) can also spike.<ref name="paul2022" /> causing algal blooms and other biological impacts.<ref name="paul2022" /><ref name="hohner2019" /> Metals such as iron (Fe), manganese (Mn), arsenic (As), lead (Pb), and copper (Cu) are often mobilized with the sediment and ash as well.<ref name="smith2011" /><ref name="burton2016" /><ref name="allen2005" /><ref name="kelly2006" /> 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.<ref name="hohner2019" /><ref name="paul2022" />
 
Beyond the chemical contaminants, the dramatically increased sediment loading in streams and rivers is itself problematic for water treatment.<ref name="paul2022" /><ref name="smith2011" /> High turbidity makes disinfection and filtration less efficient and more costly.<ref name="smith2011" /><ref name="paul2022" /><ref name="emelko2011" /> and in extreme conditions water intakes may have to be shut down completely.<ref name="hohner2019" /><ref name="writer2014" /><ref name="paul2022" /> The influx of ash and sediment also has acute effects on aquatic life.<ref name="earl2003" /><ref name="paul2022" /> often leading to declines in fish populations.<ref name="paul2022" /><ref name="rust2019" /> The altered physical and chemical conditions post-fire can drive shifts in macroinvertebrate communities towards more opportunistic species.<ref name="minshall1997" /><ref name="paul2022" />


===Forest Treatments===
===Forest Treatments===


Mechanical thinning and prescribed fire can lower the risk of future high severity fires 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 from such treatments are uncertain.
Mechanical thinning and prescribed fire can lower the risk of future high severity fires 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 from such treatments 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 Basin watersheds to reduce the risk of severe wildfires and their impacts to water supplies.
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 Basin watersheds to reduce the risk of severe wildfires and their impacts to water supplies.


==Data and Tools==
==Notes==
<references>
 
<ref name="smoot2021">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.</ref>
 
<ref name="giovando2022">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.</ref>
 
<ref name="koshkin2022">Koshkin, A. L., Hatchett, B. J., & Nolin, A. W. (2022). ''Wildfire impacts on western United States snowpacks.'' Frontiers in Water, 4, 971271.</ref>
 
<ref name="bar2019">Bär, A., Michaletz, S. T., & Mayr, S. (2019). ''Fire effects on tree physiology.'' New Phytologist, 223(4), 1728–1741.</ref>
 
<ref name="niccoli2023">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.</ref>
 
<ref name="obrien2010">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.</ref>
 
<ref name="dore2012">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.</ref>
 
<ref name="nolan2014">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.</ref>
 
<ref name="poon2018">Poon, P. K., & Kinoshita, A. M. (2018). ''Spatial and temporal evapotranspiration trends after wildfire in semi-arid landscapes.'' Journal of Hydrology, 559, 71–83.</ref>
 
<ref name="cooper2019">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.</ref>
 
<ref name="ma2020">Ma, Q., Bales, R. C., Rungee, J., et al. (2020). ''Wildfire controls on evapotranspiration in California's Sierra Nevada.'' Journal of Hydrology, 590, 125364.</ref>
 
<ref name="hausler2018">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.</ref>
 
<ref name="langford1975">Langford, K. J. (1975). ''Change in yield of water following a bushfire in a forest of Eucalyptus regnans.'' Report No. MMBW-W-0003.</ref>


<onlyinclude>
<ref name="kuczera1987">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.</ref>
===[https://lcms-viewer.fs2c.usda.gov/mtbs 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.


===[https://egp.wildfire.gov/maps/incidents NIFC WildfireSA Public Map]===
<ref name="buckley2012">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.</ref>
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. 


===[https://apps.usgs.gov/landslides/pwfdf/ USGS Post-Fire Debris Flow Hazard Assessment Viewer]===
<ref name="meili2024">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.</ref>
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).
</onlyinclude>


==Additional Resources==
<!-- Continue with the remaining references: Wine2016, Mataix2004, Huffman2001, Shakesby2006, Williams2022, Beeson2001, Paul2022, Smith2011, Hohner2019, Hohner2017, Burton2016, Allen2005, Kelly2006, Emelko2011, Writer2014, Earl2003, Rust2019, etc. -->


===[https://pacinst.org/wp-content/uploads/2013/05/pacinst-crb-ag-1.pdf Water to Supply the Land: Irrigated Agriculture in the Colorado River Basin]===
<ref name="minshall1997">Minshall et al. (1997).</ref>


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.
</references>

Revision as of 13:19, 13 July 2026

Overview

Figure 1. Smoke plume from the Dragon Bravo Fire over the Grand Canyon on 7-11-25, NPS Photo/M. Quinn

Overview

Figure 1. Smoke plume from the Dragon Bravo Fire over the Grand Canyon on 7-11-25, NPS Photo/M. Quinn

Wildfires are increasingly impacting watersheds across the western United States. Since 2000, annual burned area has increased several-fold West-wide, many more very large fires (>10,000 acres) burning at higher overall severity than in the 1980s and 1990s. In the Colorado River Basin, large and severe wildfires since 2000 have impacted runoff-generating watersheds from the subalpine spruce-fir and lodgepole forests of Colorado, Utah, and Wyoming, to the ponderosa pine and mixed-conifer forests of the Mogollon Rim and White Mountains and the Gila Wilderness in New Mexico.[1][2][3] The dramatic shifts in wildfire occurrence and severity since the 1980s have multiple causes, including warming and drying of the atmosphere and vegetation with 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 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, runoff efficiency, and soil infiltration 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 water resources in the Colorado River Basin.

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

Major Wildfires and Water-Related Impacts in the Colorado River Basin
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.

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.[4][5][6] 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.[1][2][4][6] 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.[1][6][4][2] These changes in snow accumulation and melt timing are long-lived[1], persisting for at least 10 years[4][6][5], and need to be factored into for water supply forecasting, planning, and management.

Vegetation and Evaporation Changes

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.[7][8][9] 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.[10][11][12][13][14] The duration of these reductions varies, but typically lasts from 2 to 15 years.[15][14][10] In some cases, vigorous regrowth of trees post-fire ultimately leads to greater water use than in the pre-fire condition.[16][17][18][19]

Soils

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.[20] High-severity fires can also create a strongly water-repellent (hydrophobic) layer beneath the surface.[21][22][23] 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.[20][24][6][25] 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.[20]

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.[26] Among those 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).[27][28] while also reducing coagulation efficiency in treatment plants.[28][29]

Levels of nutrients such as nitrogen (N) and phosphorus (P) can also spike.[26] causing algal blooms and other biological impacts.[26][28] Metals such as iron (Fe), manganese (Mn), arsenic (As), lead (Pb), and copper (Cu) are often mobilized with the sediment and ash as well.[27][30][31][32] 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.[28][26]

Beyond the chemical contaminants, the dramatically increased sediment loading in streams and rivers is itself problematic for water treatment.[26][27] High turbidity makes disinfection and filtration less efficient and more costly.[27][26][33] and in extreme conditions water intakes may have to be shut down completely.[28][34][26] The influx of ash and sediment also has acute effects on aquatic life.[35][26] often leading to declines in fish populations.[26][36] The altered physical and chemical conditions post-fire can drive shifts in macroinvertebrate communities towards more opportunistic species.[37][26]

Forest Treatments

Mechanical thinning and prescribed fire can lower the risk of future high severity fires 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 from such treatments 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 Basin watersheds to reduce the risk of severe wildfires and their impacts to water supplies.

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Cite error: Invalid <ref> tag; no text was provided for refs named mcgrath2023
  2. ↑ 2.0 2.1 2.2 Cite error: Invalid <ref> tag; no text was provided for refs named kampf2022
  3. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named alizadeh2021
  4. ↑ 4.0 4.1 4.2 4.3 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.
  5. ↑ 5.0 5.1 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.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Koshkin, A. L., Hatchett, B. J., & Nolin, A. W. (2022). Wildfire impacts on western United States snowpacks. Frontiers in Water, 4, 971271.
  7. ↑ Bär, A., Michaletz, S. T., & Mayr, S. (2019). Fire effects on tree physiology. New Phytologist, 223(4), 1728–1741.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 10.0 10.1 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.
  11. ↑ 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.
  12. ↑ Poon, P. K., & Kinoshita, A. M. (2018). Spatial and temporal evapotranspiration trends after wildfire in semi-arid landscapes. Journal of Hydrology, 559, 71–83.
  13. ↑ 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.
  14. ↑ 14.0 14.1 Ma, Q., Bales, R. C., Rungee, J., et al. (2020). Wildfire controls on evapotranspiration in California's Sierra Nevada. Journal of Hydrology, 590, 125364.
  15. ↑ 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.
  16. ↑ Langford, K. J. (1975). Change in yield of water following a bushfire in a forest of Eucalyptus regnans. Report No. MMBW-W-0003.
  17. ↑ 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.
  18. ↑ 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.
  19. ↑ 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.
  20. ↑ 20.0 20.1 20.2 Cite error: Invalid <ref> tag; no text was provided for refs named wine2016
  21. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named mataix2004
  22. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named huffman2001
  23. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named shakesby2006
  24. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named williams2022
  25. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named beeson2001
  26. ↑ 26.00 26.01 26.02 26.03 26.04 26.05 26.06 26.07 26.08 26.09 Cite error: Invalid <ref> tag; no text was provided for refs named paul2022
  27. ↑ 27.0 27.1 27.2 27.3 Cite error: Invalid <ref> tag; no text was provided for refs named smith2011
  28. ↑ 28.0 28.1 28.2 28.3 28.4 Cite error: Invalid <ref> tag; no text was provided for refs named hohner2019
  29. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named hohner2017
  30. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named burton2016
  31. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named allen2005
  32. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named kelly2006
  33. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named emelko2011
  34. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named writer2014
  35. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named earl2003
  36. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named rust2019
  37. ↑ Minshall et al. (1997).
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