U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3036

February 2008

Post-Wildfire Hydrologic Hazards in the 

Wildland Urban Interface of Colorado and the 

Western United States

Following a wildfire, such as the 2002 Missionary Ridge fire (fig. 1), a number of hydrologic hazards may develop 

that can have an important impact on water resources, businesses, homes, reservoirs, roads, and utilities in the wildland 

urban interface (areas where homes and commercial developments are interspersed with wildlands) in mountainous areas 

of the Western United States. This fact sheet describes these 

hazards and identifies approaches to quantify them, thus 

enabling land and resource managers to plan for and mitigate 

the effects of these hazards. The fact sheet has been produced 

in association with the U.S. Geological Survey (USGS) Fire 

Science Thrust program and the Colorado Front Range Demonstration Project (CFRDP). The current (2007) focus of the 

CFRDP is on the Three Lakes watershed in Grand County, 

Colorado (fig. 2), which has applicability to many similar forested, mountain areas in the Western United States. 

Wildfires are natural, recurring events in most terrestrial 

ecosystems in the United States. Historical land-management 

practices (including fire suppression) and expansion of urban 

development into wildland areas have altered these ecosystems so 

that present-day wildfires tend to be larger and more destructive 

than decades ago. For example, the Hayman fire of 2002, near 

Deckers, Colorado (about 35 miles southwest of Denver, Colorado), is the largest known wildfire in Colorado history, burning 

nearly 138,000 acres (Graham, 2003) and affecting many nearby 

communities, including the Denver metropolitan water supply. 

Post-wildfire effects may persist for years and can 

negatively affect ecology, flood-control, and drinking-water 

resources, and pose hazards to life and property in burned watersheds (figs. 2–3). Water-supply reservoirs for metropolitan and 

small rural communities in or downstream from burned areas 

are at risk when forested watersheds burn. 

Figure 1. Missionary Ridge fire threatens 

Vallecito Reservoir, near Durango, 

Colorado. Photograph courtesy of NOAA 

National Weather Service

http://www.crh.noaa.gov/Image/gjt/images/

ImageGallery/WildFires/misfire8.jpg. 

Printed on recycled paper

Introduction
Post-Wildfire Hazards

Post-wildfire hydrologic hazards can be divided into three general categories: flooding, debris flows, and water-quality changes. These hazards are described in the following 

section and methods used by the USGS to quantify them are discussed. 

Figure 2. Locations 

of major Colorado 

wildfires in 2002, and the 

Colorado Front Range 

Demonstration Project 

area in Grand County, 

Colo.

Figure 3. Flooding in 

a watershed burned by 

the Hayman fire near 

Deckers, Colo., threatens 

homes along Fourmile 

Creek. Photograph by 

M.R. Stevens, USGS, 2003.

Post-Wildfire Floods

Post-wildfire flooding can damage property and infrastructure through 

inundation, erosion, and deposition of debris (figs. 3–5). The USGS has used 

flood-frequency equations to predict post-wildfire flood magnitudes and to 

delineate changes in flood elevation and flood-plain boundaries for several 

watersheds burned by Colorado fires in 2002, including the Hayman, Coal 

Seam, Missionary Ridge, and Million fires (Elliott and others, 2005) (fig. 2). 

To estimate pre-fire peak flows, flood-frequency equations were used 

with watershed properties, such as watershed slope, drainage area, and 

mean-annual precipitation. Pre-fire streamflows were used to calibrate 

hydrologic models, such as the Hydrologic Engineering Center−Hydrologic 

Modeling System (U.S. Army Corps of Engineers, 2001a), which 

were further modified to reflect burn severity, allowing an estimate 

of post-wildfire peak streamflows. Among Hayman fire subwatersheds 

having greater than 50 percent of the area described as moderately to severely burned, postwildfire peak streamflows are expected to be 28 to 91 times higher than pre-fire peak streamflows, depending 

on the subwatershed (Elliott and others, 2005). Based on year-to-year changes in peak flows, the USGS has estimated that 

burned watersheds require about 4 to 5 years to return to stable hydrologic conditions (Elliott and others, 2005). 

Predicted post-wildfire streamflows are usually much higher than pre-fire streamflows. These predictions are used 

for practical purposes, such as flood-plain studies and flood-insurance mapping. Flood profiles can be developed using a 

stream-channel routing model (Hydrologic Engineering Center–River Analysis System) to estimate the height of the water 

surface along the stream (U.S. Army Corps of Engineers, 

2001b). High peak streamflows would cause water-surface elevations during floods to be higher and flood-plain 

boundaries wider compared to pre-fire streamflows. This 

information is critical to residents, community planners, 

and emergency personnel who must be prepared to respond 

to storm and flood warnings and also to determine where 

future development in areas adjacent to flood plains might 

occur or be restricted.

Deckers U 

U Glenwood Springs

U Durango

U Grand Lake

U South Fork

Three Lakes Study Area

Coal Seam Fire

Missionary Ridge Fire

Million Fire

Hayman Fire
Figure 4. Highway 

destroyed by postwildfire flooding in 

the West Creek area 

of the Hayman fire 

near Deckers, Colo. 

Photograph by M.R. 

Stevens, USGS, 2006. 

Figure 5. Post-wildfire flooding and debris-covered bridge 

on Trail Creek in the Hayman fire burn area, near Deckers, 

Colo. Photograph by M.R. Stevens, USGS, 2002.

Post-Wildfire Debris Flows

Debris flows are fast-moving mixtures of water, sediment, and debris caused by periods of 

intense rain or rapid snowmelt on steep hillsides that are mantled with loose material. Debris flows 

can move huge amounts of material and be can be very destructive (fig. 6). 

The USGS has used predictive models (Rupert and others, 2003; Cannon and others, 2004, 

2006) to estimate the probability of debris flows and the volume of water and material that could 

be moved in areas recently burned by wildfire. Logistic regression statistical methods were used to assess the probability of a given watershed 

to produce post-wildfire debris flows as a function of a combination of 

soil properties, watershed characteristics, burn severity, and rainfall conditions (Rupert and others, 2003). Multiple regression methods were used 

to develop a predictive model that estimates the volume of debris-flow 

material expected at a watershed outlet (Gartner, 2005). Variables used in 

this model are the area of the watershed burned, watershed gradient, soil 

properties, and rainfall intensity.

Other models, such as the FLO-2D model (O’Brien, 2006), can be 

used to predict and delineate the inundation area that a flood or debris flow 

will affect as it emerges from a burned tributary watershed onto an alluvial 

fan or flood-plain surface. The model uses a specified input hydrograph, 

volumetric sediment concentrations, and watershed topography to route a 

debris flow from the originating tributary to the depositional zone. 

Using the results from these models, emergency responders can 

set priorities in areas recently burned or that have the potential to burn. 

Managers can focus on areas that pose the greatest danger to human life 

and infrastructure. 

Post-Wildfire Water Quality

Wildfire may negatively affect water quality by releasing large quantities 

of nutrients (nitrogen and phosphorus); metals, such as lead, manganese, and 

mercury; and organic carbon (total and dissolved) into the environment where 

they can persist for years (Ranalli, 2004). The large post-wildfire increases 

in the concentration of these chemicals as well as sediment in streams and 

reservoirs may negatively affect aquatic life, aquatic habitat, and drinkingwater supplies. Large increases in concentrations of nutrients in post-wildfire 

runoff (2 to 100 times greater than pre-wildfire conditions) (Ranalli and Stevens, 2004) may cause algal blooms, which can deplete dissolved oxygen to 

levels below those that are needed by fish. Increased mercury that can result 

from post-wildfire runoff is a concern because it is toxic when 

it is converted to methylmercury under conditions of low oxygen, such as in lake and reservoir sediments in summer. Manganese is a concern to the water-treatment industry because at 

high concentrations it can create esthetic problems and requires 

intensive treatment (National Drinking Water Clearing House, 

1998). Organic carbon is composed of particulate and dissolved organic matter and is a concern to the water-treatment 

industry because post-wildfire increases of organic matter can 

increase treatment costs and can form disinfection byproducts, 

such as trihalomethanes, when water is chlorinated, posing 

potential health risks (Rostad, 2004). Organic compounds also 

may be formed during a wildfire, some of which may be toxic 

to aquatic life and humans.
Since 2003, the USGS has been monitoring the water quality of Fourmile Creek, 

which drains a watershed near Deckers, Colo., 

that burned during the Hayman fire, and a 

nearby stream, Pine Creek, which drains a 

watershed that was not burned. The concentrations of total manganese in Fourmile Creek 

and Pine Creek are shown in figure 7. These 

plots show that the concentrations can be 

orders of magnitude higher in streams draining burned watersheds, and that the high 

concentrations can persist. Also, concentrations of total mercury were as much as 80 times greater in Fourmile Creek (burned) than in Pine 

Creek (unburned). More than 100 tentatively identified organic compounds were detected in Fourmile Creek but only a few were detected in Pine Creek (Ranalli and Stevens, 2004). The results of 

all chemical analyses obtained as part of this study reside in the National Water Information System 

(NWIS) database and can be accessed using the USGS Web interface to water data at URL http://

waterdata.usgs.gov/co/nwis/qw/.

Watershed modeling tools, such as the Precipitation Runoff Modeling System (PRMS) developed by the USGS (Leavesley and others, 1983), can be used to estimate the potential for increased 

post-wildfire flooding, erosion, and sediment deposition. Reservoir models also can be used to simulate and predict reservoir-water circulation, transformation of nutrient species, dissolved-oxygen concentrations, transport of sediment within the reservoir, and the timing and potential magnitude of nuisance algal blooms, which can help water-supply managers develop plans to mitigate post-wildfire 

water-quality impacts. 

Summary

Although wildfires are natural and recurring events, historic land-management practices and urban development into wildland areas are resulting in wildfires that tend to be larger and more 

destructive than those in the past. Post-wildfire conditions can produce a number of hydrologic hazards, including floods, debris flows, and impaired water quality, which can have harmful effects 

on the wildlands-urban interface and on water supplies that can be many miles downstream of the 

burned areas. The USGS and others have developed approaches to quantify these hazards, which will 

help land managers plan for and mitigate the effects of these hazards even before watersheds burn.

Figure 7. Post-Hayman fire total manganese 

concentrations at Fourmile Creek and Pine 

Creek near Deckers, Colo. 

Figure 6. Debris flow following the Missionary 

Ridge fire, near Durango, Colo. Photograph 

by Butch Knowlton, La Plata County Office of 

Emergency Management, 2002. 

1

10

100

1,000

10,000

100,000

1,000,000

Explanation Fourmile Creek Pine Creek

Apr-03

May-03

Jun-03

Jul-03

Aug-03

Sep-03

Apr-04

May-04

Jun-04

Aug-04

Sep-04

Oct-04

Apr-05

May-05

Jun-05

Sep-05

TOTAL MANGANESE, 

IN MICROGRAMS PER LITER
References 

Cannon, S.H., Gartner, J.E., Rupert, M.G., and Michael, J.A., 

2004, A method for rapid assessment of post-fire debris flow 

hazards: Geological Society of America Abstracts with Program, v. 36, no. 5, p. 414.

Cannon, S.H., Gartner, J.E., Rupert, M.G., and Michael, J.A., 

2006, A method for the rapid assessment of the probability of 

post-wildfire debris flow from recently burned basins in the 

inter-mountain West, U.S.A.: European Geophysical Society 

XXVII General Assembly, Wildfire effects on soil carbon 

dynamics, soil degradation and soil redistribution, Vienna, 

Austria, April 2006. 

Elliott, J.G., Smith, M.E., Friedel, M.J., Stevens, M.R., 

Bossong, C.R., Litke, D.W., Parker, R.S., Costello, C., 

Wagner, J., Char, S.J., Bauer, M.A., and Wilds, S.R., 2005, 

Analysis and mapping of post-fire hydrologic hazards for the 

2002 Hayman, Coal Seam, and Missionary Ridge wildfires, 

Colorado: U.S. Geological Survey Scientific Investigations 

Report 2004-5300, 104 p.

Gartner, J.E., 2005, Relations between wildfire-related debrisflow volumes and basin morphology, burn severity, material 

properties and triggering storm rainfall: Boulder, University 

of Colorado, master’s thesis, 76 p.

Graham, R.T., ed., 2003, Hayman fire case study: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station, General Technical Report RMRSGTR-114, 396 p.

Leavesley, G.H., Lichty, R.W., Troutman, B.M., and Saindon, 

L.G., 1983, Precipitation runoff modeling system. User’s 

manual: U.S. Geological Survey Water-Resources Investigations Report 83-4238, 207 p.

National Drinking Water Clearing House, 1998, Iron and manganese removal: National Drinking Water Clearing House 

Technical Brief Fact Sheet, 4 p.

O’Brien, J.S., 2006, FLO-2D user’s manual, version 2006.01: 

Nutrioso, Ariz., FLO-2D Software, Inc., CD-ROM.

Ranalli, A.J., 2004, A summary of the scientific literature on 

the effects of fire on the concentration of nutrients in surface 

waters: U.S. Geological Survey Open-File Report 2004-1296, 

23 p., http://pubs.usgs.gov/of/2004/1296/.

Ranalli, A.J., and Stevens, M.R., 2004, Streamwater-quality 

data from the 2002 Hayman, Hinman, and Missionary Ridge 

wildfires, Colorado: U.S. Geological Survey Data Series 

Report 2004-109, http://pubs.usgs.gov/ds/ds109/.

Rostad, C.E., 2004, Studies on disinfection by-products 

and drinking water: U.S. Geological Survey Fact Sheet 

2004-3032, 4 p. 

Rupert, M.G., Cannon, S.H., and Gartner, J.E., 2003, Using 

logistic regression to predict the probability of debris flows 

occurring in areas recently burned by wildland fires: U.S. 

Geological Survey Open-File Report 2003-500, http://pubs.

usgs.gov/of/2003/ofr03500/.

U.S. Army Corps of Engineers, 2001a, Hydrologic Modeling 

system HEC−HMS: U.S. Army Corps of Engineers Hydrologic Engineering Center, Davis, Calif., 188 p. 

U.S. Army Corps of Engineers, 2001b, HEC–RAS River 

Analysis System version 3.0: U.S. Army Corps of Engineers 

Hydrologic Engineering Center, Davis, Calif., variously 

paginated. 

Adobe/Getty Images

By M.R. Stevens, C.R. Bossong, M.G. Rupert, A.J. Ranalli, 

E.W. Cassidy, and A.D. Druliner
A.D. Druliner 

 Colorado Water Science Center

 U.S. Geological Survey

 P.O. Box 25046, Mail Stop 415

 Lakewood, CO 80225

 Phone: (303) 236-4882 

 Email: addrulin@usgs.gov

 http://co.water.usgs.gov

Contact for additional information

Adobe/IPNStock