Wildfire Effects on Source-Water Quality—Lessons from 

Fourmile Canyon Fire, Colorado, and Implications for 

Drinking-Water Treatment 

Study Rationale and Approach 

The degree to which wildfire degrades water quality and 

supply depends on wildfire extent and intensity, postwildfire 

precipitation, watershed topography, and local ecology. Potential effects of wildfire on municipal water supplies and downstream aquatic ecosystems include the following:

• Changes in the magnitude and timing of snowmelt runoff, 

which influence filling of water-supply reservoirs,

• Increased loading of streams by nutrients, dissolved organic 

carbon, major ions, and metals,

• Postfire erosion and transport of sediment and debris to 

downstream water-treatment plants, water-supply reservoirs, 

and aquatic ecosystems, and

• Changes in source-water chemistry that can alter drinkingwater treatment. 

The Fourmile Canyon fire in Boulder County, Colorado 

(September 6–10, 2010) burned 23 percent of the Fourmile 

Creek Watershed (10 square miles or 2,600 hectares), destroyed 

more than 160 homes, and was one of the costliest wildfires 

in Colorado history (figs. 1 and 2). The wildfire left the area at 

risk of substantial erosion (Ruddy and others, 2010), potentially 

degrading the water supply of the communities of Pinebrook 

Hills and Lafayette, which draw water from Fourmile Creek and 

Boulder Creek downstream of the burned area (fig. 2). Mine 

tailings and waste rock from historical placer and lode mining 

U.S. Department of the Interior

U.S. Geological Survey

are widely distributed in the Fourmile Creek watershed, and 

increased erosion could potentially transport these materials into 

downstream water bodies. 

Water-quality monitoring sites were established on Fourmile Creek at 5 locations, 2 upstream (FCCR, FCLG) and 3 

downstream (FCWM, FCLM, FCBC) of the burned area (fig. 2). 

Because water quality is closely tied to the hydrologic cycle, 

water samples were collected monthly during base flow, twice 

weekly during snowmelt runoff, and more frequently during storm events. Water samples were analyzed for dissolved 

organic carbon (DOC), nutrients, turbidity, major ions, and 

metals associated with mine waste (such as cadmium, lead, iron, 

manganese, and zinc). Water level, temperature, and specific 

Fact Sheet 2012–3095

July 2012 Printed on recycled paper

Forested watersheds provide high-quality source water for many communities in the Western United States. These watersheds 

are vulnerable to wildfires, and wildfire size, fire severity, and length of fire season have increased since the middle 1980s (Westerling and others, 2006). Burned watersheds are prone to increased flooding and erosion, which can impair water-supply reservoirs, 

water quality, and drinking-water treatment processes. Limited information exists on the degree, timing, and duration of the effects 

of wildfire on water quality, making it difficult for drinking-water providers to evaluate the risk and develop management options. 

In order to evaluate the effects of wildfire on water quality and downstream ecosystems in the Colorado Front Range, the 

U.S. Geological Survey initiated a study after the 2010 Fourmile Canyon fire near Boulder, Colorado. Hydrologists frequently 

sampled Fourmile Creek at monitoring sites upstream and downstream of the burned area to study water-quality changes during 

hydrologic conditions such as base flow, spring snowmelt, and summer thunderstorms.

This fact sheet summarizes principal findings from the first year of research. Stream discharge and nitrate concentrations 

increased downstream of the burned area during snowmelt runoff, but increases were probably within the treatment capacity of 

most drinking-water plants, and limited changes were observed in downstream ecosystems. During and after high-intensity thunderstorms, however, turbidity, dissolved organic carbon, nitrate, and some metals increased by 1 to 4 orders of magnitude within 

and downstream of the burned area. Increases of such magnitude can pose problems for water-supply reservoirs, drinking-water 

treatment plants, and downstream aquatic ecosystems. 

Figure 1. Fourmile Canyon wildfire burned 160 homes and 

degraded the source-water supply of two communities. 

(Photograph courtesy of Dan Lack, Cooperative Institute for 

Research in Environmental Sciences (CIRES), September 5, 2010).
conductivity were continuously monitored. Selected findings from the study are summarized here; for more detail, see 

McCleskey and others (in press); Murphy and others (2012); 

and Writer and others (2012).

Results 

Hydrologic Response to Postfire Precipitation Events

Stream runoff in spring 2011 rose later than in previous 

years, primarily due to low snowfall in the winter and early 

spring (fig. 3A). Low-intensity rain and mixed snow-and-rain 

events in May and June 2011 (fig. 3B) generally increased 

discharge similarly upstream and downstream of the burned 

area. In contrast, thunderstorms on July 7 and July 13, 2011, 

shed less total rain but did so at much higher intensities. These 

storms produced short-term flash floods; discharge downstream 

of the burned area was as much as 8,000 percent above prestorm discharge. Upstream of the burned area, stream discharge 

during these storms increased by less than 50 percent. During 

the July 13 thunderstorm, stream discharge increased from 30 

to 800 cubic feet per second in less than 5 minutes (fig. 4A), and 

peak discharge was three times as great as previously recorded 

for Fourmile Creek (U.S. Geological Survey, 2011). The July 

storms were generally typical of Colorado Front Range thunderstorms (storms of this size have a 20–50 percent chance of 

occurring each year), yet the stream’s response to these storms 

provides clear evidence that burned watersheds are prone to 

flash floods that transport substantial amounts of sediment to 

downstream water bodies. This phenomenon has been observed 

in other burned watersheds in the Colorado Front Range 

(Moody and Martin, 2001). 

Water-Quality Response to Postfire Precipitation Events 

Variable source-water quality presents challenges for 

drinking-water providers. Drinking-water treatment processes 

operate more effectively when source-water quality is constant, DOC concentrations are below 5 milligrams per liter 

(mg L–1), and turbidity (an indicator of suspended material) 

is less than 20 nephelometric turbidity units (NTU). Elevated 

nitrate concentrations can promote algal growth in water supply 

reservoirs, which can increase DOC concentrations and lead to 

disagreeable taste and odor. 

“First flush” storm. The first substantial postwildfire 

precipitation event in the burned area fell on October 12, 2010, 

one month after the fire. The storm delivered relatively little 

precipitation and was of low intensity (fig. 3B). DOC concentrations downstream of the burned area increased from a baseflow value of 1.5 mg L–1 to as much as17 mg L–1 (fig. 4B); the 

higher value exceeded the 5 mg L–1 treatment threshold. Nitrate 

concentrations increased from base-flow levels of less than 0.1 

to as much as 1.3 mg L–1 (fig. 4C). Turbidity was not measured, 

but suspended material was visible in water samples (though at 

much lower concentrations than during thunderstorms in the following summer). Although no primary drinking-water standards 

for dissolved metals were exceeded, secondary drinking-water 

standards (which are related to aesthetic considerations, such as 

taste, color, and odor) for aluminum, iron, and manganese were 

exceeded downstream of the burned area. 

Spring snowmelt. During snowmelt, DOC concentrations 

were similar (1–5 mg L–1) at sites upstream and downstream 

of the burned areas. Nitrate concentrations increased from less 

than 0.02 mg L–1 to 0.75 mg L–1 downstream of the burned area 

during both spring snowmelt and low-intensity storms. Turbidity values were generally similar at upstream and downstream 

sites, except during the rain and mixed rain-and-snow events in 

May and June 2011, when turbidity downstream of the burned 

area increased from less than 10 NTU to as much as 800 NTU 

(fig. 4D). No increases in metal concentrations were observed 

that exceeded primary or secondary water standards. 

Summer thunderstorms. The thunderstorms on July 7 and 

July 13, 2011, transported substantial amounts of sediment from 

hillslopes to Fourmile Creek (fig. 5). Large increases in concentrations of DOC (greater than 70 mg L–1) and of nitrate (greater 

than 9 mg L–1) and in turbidity (as much as 50,000 NTU) were 

measured downstream of the burned area. The total loading 

! ! !

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105°28’ 105°26’ 105°24’ 105°22’ 105°20’ 105°18’

40°04’

40°03’

40°02’

40°01’

40°00’

Base map from U.S. Geological Survey National

Hydrologic Dataset

Fourmile Canyon

Fire Burn Area

Boulder

Boulder

Fourmile Creek 

Watershed

FCCR FCLG FCWM FCLM

FCBC

Creek

Gold

Run

Creek

Fourmile

0 2 KILOMETERS

0 1 MILE

Figure 2. Area of Fourmile Canyon burn and water-quality sample 

sites. 

1/1/20112/1/20113/1/20114/1/20115/1/20116/1/20117/1/20118/1/20119/1/2011

10/1/2011

0

10

20

30

40

0

10

20

30

40

A

B

EXPLANATION

Daily discharge in 2011

Discharge, in

cubic feet per second

Daily precipitation,

in inches

Discharge, in

cubic meters per second

Daily precipitation,

in millimeters

9/1/2010

10/1/201011/1/201012/1/2010

Sep 6-10

Fire

Historical mean daily discharge

1.0

0.8

0.6

0.4

0.2

0.0

1.2

0.8

0.4

0.0

Figure 3. A, Mean daily discharge in 2011 and historical mean daily 

discharge, Fourmile Creek (at site FCBC). B, Daily precipitation. Data 

from Murphy and others, 2012.
(defined as concentration times discharge) of nutrients and DOC 

transferred from burned areas to the stream was 1 to 2 orders of 

magnitude as great as loading from unburned areas. This large 

increase in sediment, carbon, and nutrient loading from burned 

watersheds has the potential to profoundly influence downstream water treatment processes, water-supply reservoirs, and 

aquatic ecosystems. Metal concentrations increased but did not 

exceed primary drinking-water standards. Iron and manganese 

exceeded secondary drinking-water standards during storm 

events, and concentrations of manganese remained elevated 

for several weeks after the storms. Although the observed 

manganese concentrations (as much as 0.49 mg L–1) do not pose 

a health risk, those increased concentrations can pose watertreatment problems and can foster the growth of microorganisms in reservoirs and distribution systems. 

On the basis of one year of research, low-intensity rain 

events and spring snowmelt runoff produced minor increases in 

some water quality indicators. High-intensity thunderstorms 10 

months after the wildfire, however, caused substantial change in 

water quality. Continued monitoring of Fourmile Creek would 

help hydrologists evaluate how water quality changes as the 

burned watershed recovers.

Figure 4. Stream discharge at 5-minute intervals and selected water quality characteristics in 

2010–2011 measured in Fourmile Creek, Colorado, at monitoring stations FCCR, FCLM, and FCBC.

2

4

6

8

30

60

0.0

0.5

1.0

1.5

9

12

0

10

20

30

500

1,000

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

10

20

30

40

100

1,000

10,000

DOC, in

milligrams per liter

Drinking-water

treatment threshold

B

Nitrate, in

milligrams per liter

C

Base flow

Thunderstorms

Spring

snowmelt

Base flow

Stream discharge, in

cubic feet per second

A

Drinking-water

treatment threshold

Turbidity, in

nephelometric units

D

40

6

Upstream from burn Downstream from burn

 FCCR FCLM

 FCBC

EXPLANATION
References Cited

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, v. 45, p. 

461–472.

McCleskey, R.B., Writer, J.H., and Murphy, S.F., in press, Water 

chemistry data for surface waters affected by the Fourmile 

Canyon wildfire, Colorado, 2010–2011: U.S. Geological 

Survey Open-File Report 2012–1104. 

Moody, J.A., and Martin, D.A., 2001, Initial hydrologic and 

geomorphic response following a wildfire in the Colorado 

Front Range: Earth Surface Processes and Landforms, v. 26, 

p.1049–1070.

Murphy, S.F., McCleskey, R.B., and Writer, J.W., 2012, Effects 

of flow regimes on stream turbidity and suspended solids 

after wildfire, Colorado Front Range, in Wildfire and water 

quality—Processes, impacts, and challenges, Conference 

in Banff, Canada, June 2012, Proceedings: Wallingford, 

Oxfordshire, U.K., International Association of Hydrological 

Sciences publication 354, p. 51–58.

Rhoades, C.C., Entwistle, Deborah, and Butler, Dana, 2011, 

The influence of wildfire extent and severity on streamwater 

chemistry, sediment and temperature following the Hayman 

fire, Colorado: International Journal of Wildland Fire, v. 20, 

p. 430–442.

Ruddy, B.C., Stevens, M.R., Verdin, K.L., and Elliott, J.G., 

2010, Probability and volume of potential post-wildfire debris 

flows in the 2010 Fourmile burn area, Boulder County, Colorado: US Geological Survey Open-File Report 2010–1244. 

5 p., accessed January 2012 at http://www.udfcd.org/FWP/

F2P2_Reports/FMCfire/USGS/OF10-1244.pdf. 

U.S. Geological Survey, 2011, National Water Information System (NWISWeb): U.S. Geological Survey database accessed 

15 December 2011 at http://waterdata.usgs.gov.

Westerling, A.L., Hidalgo, H.G., Cayan, D.R., and Swetnam, 

T.W., 2006, Warming and earlier spring increase Western 

U.S. forest wildfire activity: Science v. 313, p. 940–943.

Writer, J.H., McCleskey, R.B., and Murphy, S.F., 2012, Effects 

of wildfire on source-water quality and aquatic ecosystems, 

Colorado Front Range, in Wildfire and water quality—

Processes, impacts, and challenges, Conference in Banff, 

Canada, June 2012, Proceedings: Wallingford, Oxfordshire, 

U.K., International Association of Hydrological Sciences 

publication 354, p. 117–122.

For Further Information

Contact Jeffrey H. Writer (jwriter@usgs.gov; https://profile.usgs.gov/jwriter/) or Sheila F. Murphy (sfmurphy@usgs.

gov; https://profile.usgs.gov/sfmurphy).

Implications for Drinking-Water Treatment

Drinking-water utilities strive to provide safe drinking water for their communities. Management practices at a 

given utility commonly are based on past experience at individual treatment plants. Unfortunately, the unpredictable 

nature of wildfire makes it challenging to develop treatment-plant-specific strategies for treating source water degraded 

by the effects of wildfire. Findings from this study are likely applicable to other Colorado Front Range watersheds. Highintensity rainfall events in steep, burned watersheds are likely to move large amounts of suspended and dissolved material into downstream water supplies. The following problems may result:

Increased sediment loading of water-supply reservoirs, shortened reservoir lifetime, and increased maintenance 

costs;

Increased nutrient loading of reservoirs, which may promote algal blooms and associated disagreeable taste and 

odor;

Increased turbidity (cloudiness caused by suspended material) or increased iron and manganese concentrations, 

which may increase chemical treatment requirements and produce larger volumes of sludge, both of which would 

increase operating costs;

Increased dissolved organic carbon concentrations, which during disinfection may help form unwanted by-products 

(for instance, regulated carcinogens such as chloroform and trihalomethanes). 

During high-intensity storms after a wildfire, sediment, nutrients, and organic carbon may be delivered to drinkingwater plants or water-supply reservoirs at concentrations higher than observed previously, and those high concentrations 

may last many years. Source-water-quality problems lasted more than 5 years after the 2002 Hayman fire west of Denver, 

Colorado (Rhoades and others, 2011) and nearly a decade after a wildfire in the Canadian Rockies (Emelko and others, 

2011). Monitoring of source water downstream of burned watersheds may allow water managers to minimize objectionable effects, as by temporarily diverting some water or changing source water.