Wildfire and water

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. 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.

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. 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

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

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 (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.

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. 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).

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).

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). 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).

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.

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

Water to Supply the Land: Irrigated Agriculture in the Colorado River Basin

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.