U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5039

Prepared in cooperation with the Town of Breckenridge, Colorado

Estimated Probabilities and Volumes of Postwildfire Debris 

Flows, a Prewildfire Evaluation for the Upper Blue River 

Watershed, Summit County, Colorado
Front Cover:

Debris-flow terminal lobe at the outlet of a tributary of Indiana Creek, Summit County, Colorado.

Photograph by John G. Elliott, U.S. Geological Survey, August 18, 2009.
Estimated Probabilities and Volumes of 

Postwildfire Debris Flows, a Prewildfire 

Evaluation for the Upper Blue River 

Watershed, Summit County, Colorado

By John G. Elliott, Jennifer L. Flynn, Clifford R. Bossong, and Stephen J. Char

Prepared in cooperation with the Town of Breckenridge, Colorado

Scientific Investigations Report 2011–5039

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Elliott, J.G., Flynn, J.L., Bossong, C.R., and Char, S.J., 2011, Estimated probabilities and volumes of postwildfire debris 

flows, a prewildfire evaluation for the upper Blue River watershed, Summit County, Colorado: U.S. Geological Survey 

Scientific Investigations Report 2011-5039, 22 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Upper Blue River Watershed Study Area ..................................................................................................3

Debris-Flow Regression Models.................................................................................................................4

Debris-Flow Probability........................................................................................................................4

Debris-Flow Volume .............................................................................................................................4

Input Data for Debris-Flow Models and Assumptions ...................................................................4

Verification of Debris-Flow Model Results.......................................................................................9

Estimated Probabilities and Volumes of Postwildfire Debris Flows....................................................10

Upper Blue River Watershed Debris-Flow Probabilities..............................................................10

Upper Blue River Watershed Debris-Flow Volumes.....................................................................15

Combined Relative Debris-Flow Hazard Ranking..........................................................................15

Summary and Conclusions.........................................................................................................................20

Acknowledgments.......................................................................................................................................21

References Cited..........................................................................................................................................21

Figures

1. Shaded relief map of the upper Blue River watershed upstream from Goose 

Pasture Tarn showing topography, primary watersheds and subwatersheds, 

forested area, and locations of photographs in figures 2, 3, and 4 ......................................2

2. Debris-flow marginal levee near the outlet of Fredonia Gulch...........................................11

3. Debris-flow terminal lobe at the outlet of subwatershed ind10, a tributary of 

Indiana Creek ..............................................................................................................................11

4. Debris-flow scoured channel near the outlet of subwatershed pen01, a 

tributary of Pennsylvania Creek ...............................................................................................11

5. Primary watersheds and subwatersheds showing estimated debris-flow 

probabilities in response to the 2-year-recurrence, 1-hour-duration rainfall ..................12

6. Primary watersheds and subwatersheds showing estimated debris-flow 

probabilities in response to the 10-year-recurrence, 1-hour-duration rainfall ................13

7. Primary watersheds and subwatersheds showing estimated debris-flow 

probabilities in response to the 25-year-recurrence, 1-hour-duration rainfall ................14

8. Primary watersheds and subwatersheds showing estimated debris-flow 

volumes in response to the 2-year-recurrence, 1-hour-duration rainfall .........................16

9. Primary watersheds and subwatersheds showing estimated debris-flow 

volumes in response to the 10-year-recurrence, 1-hour-duration rainfall .......................17

10. Primary watersheds and subwatersheds showing estimated debris-flow 

volumes in response to the 25-year-recurrence, 1-hour-duration rainfall .......................18

11. Subwatersheds showing the combined relative debris-flow hazard ranking 

in response to the 25-year-recurrence, 1-hour-duration rainfall .......................................19
iv

Tables

1. Debris-flow model input variables and output for primary watersheds .............................5

2. Debris-flow model input variables and output for subwatersheds within 

and adjacent to primary watersheds ........................................................................................6

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

hectare (ha) 2.471 acre

hectare (ha) 0.003861 square mile (mi2

) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 0.0008107 acre-foot (acre-ft)

cubic hectometer (hm3

) 810.7 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

Mass

megagram (Mg) 1.102 ton, short (2,000 lb)

megagram per year (Mg/yr) 1.102 ton per year (ton/yr)

Pressure

newton per square meter (N/m2

) 0.02088 pound per square foot (lb/ft2

) 

Density

gram per cubic centimeter (g/cm3

) 62.4220 pound per cubic foot (lb/ft3) 

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 
Abstract 

Debris flows resulting from rainfall on recently burned, 

rugged, forested areas create potential hazards to life, property, 

infrastructure, and water resources. The location, extent, and 

severity of wildfire and the subsequent rainfall intensity and 

duration cannot be known in advance. However, hypothetical 

scenarios based on empirical debris-flow models are useful planning tools for conceptualizing potential postwildfire 

effects. A prewildfire study to determine the potential for 

postwildfire debris flows in the upper Blue River watershed in 

Summit County, Colorado, was conducted in 2009 by the U.S. 

Geological Survey, in cooperation with the Town of Breckenridge, to provide Breckenridge with a relative measure of 

which subwatersheds might constitute the most serious debrisflow hazards. 

Potential postwildfire debris-flow probabilities and 

volumes for nine primary watersheds tributary to the upper 

Blue River and 50 subwatersheds located within and adjacent 

to the primary watersheds were estimated by using empirical 

debris-flow models. An assumption in the debris-flow models 

was that a moderate to severe wildfire affected 100 percent of 

the forest and shrub stands within the area. Three postwildfire 

precipitation scenarios were used to represent a range of likely 

precipitation scenarios that could occur shortly after a wildfire: a 2-year recurrence, 1-hour-duration rainfall; a 10-year 

recurrence, 1-hour-duration rainfall; and a 25-year recurrence, 

1-hour-duration rainfall. All of these precipitation scenarios 

resulted in debris flows from the hypothetically burned 

watersheds.

Subwatersheds with the lowest postwildfire debris-flow 

probabilities tended to have large areas of alpine and subalpine 

vegetation or sparse forest cover that would be minimally 

affected by wildfire. Subwatersheds with the highest probabilities tended to be steep, heavily forested, and relatively small 

in drainage area. Subwatersheds with the smallest estimated 

postwildfire debris-flow volumes tended to have small drainage areas, relatively little forest cover, less rugged topography, 

or were located in alpine and subalpine areas. Subwatersheds 

Estimated Probabilities and Volumes of Postwildfire 

Debris Flows, a Prewildfire Evaluation for the Upper Blue 

River Watershed, Summit County, Colorado

By John G. Elliott, Jennifer L. Flynn, Clifford R. Bossong, and Stephen J. Char 

with the highest estimated debris-flow volumes were those 

with the largest drainage areas. 

The subwatersheds with the greatest potential postwildfire and postprecipitation hazards are those with both high 

probabilities of debris-flow occurrence and large estimated 

volumes of debris-flow material. The high probabilities of 

postwildfire debris flows, the associated large estimated 

debris-flow volumes, and the densely populated areas along 

the creeks and near the outlets of the primary watersheds indicate that Indiana, Pennsylvania, and Spruce Creeks are associated with a relatively high combined debris-flow hazard. 

Introduction

One of the most potentially devastating postwildfire 

hazards is a debris flow (Cannon, 2001; Cannon and others, 

1998). Debris flows are fast-moving, high-density slurries of 

water, sediment, and debris that can have enormous destructive power (Costa and Jarrett, 1981; Hungr and others, 1984; 

Pierson and Costa, 1987; Costa, 1988). They are frequently 

triggered by intense rainfall or rapid snowmelt on steep 

hillsides covered with erodible material (Griffiths and others, 1996; Gartner and others, 2008). Although debris flows 

are a common geomorphic process in some areas without 

the influence of fire, a wildfire can transform conditions in a 

watershed with no recent history of debris flows into one that 

poses a substantial hazard. Debris flows resulting from rainfall 

on recently burned, rugged, forested areas create potential 

hazards to life, property, infrastructure, and water resources. 

Researchers are developing new techniques to assess the hazards posed by debris flows after wildfires (Cannon and others, 

2010). These techniques can be used in a prewildfire analysis 

to estimate hazards before wildfires occur (Stevens and others, 

2008). 

The upper Blue River watershed above Goose Pasture 

Tarn (fig. 1) is a 110-km2

 (42-mi2) area in southern Summit 

County, Colorado, that serves as the sole source of municipal water for the Town of Breckenridge and is an important 

component of the Colorado Springs municipal water supply 
! ! ! Blue River Indiana Creek Spruce Creek McCullough Gulch Pennsylvania Creek Crystal Creek Bemrose Creek Monte Cristo Creek Monte Cristo Creek Blue River ind00 spr00 mon00 mcc00 pen00 bem00 un200 un100 Location of Figure 3 Goose Pasture Tarn Location of Figure 2 Location of Figure 4 UV9 Denver Breckenridge SUMMIT COUNTY 0 0.5 1 2 MILES 0 0.5 1 2 KILOMETERS EXPLANATION Primary wate

SubwatersheForested are

! Photograph l

Base from U.S. Geological Survey digital data, 1:100,000 Shaded relief from U.S. Geological Survey digital data, 1:24,000 Universal Transverse Mercator Projection, Zone 13, North American Datum 1983 COLORADO TENMILE RANGE CONTINENTAL DIVIDE pen00 Primary wate

106°W 106°W 106°07'30"W 106°07'30"W 39°24'15"N 39°24'15"N

Figure 1. Shaded relief map of the upper Blue River watershed upstream from Goose Pasture Tarn showing topography, primary watershsubwatersheds, forested area, and locations of photographs in figures 2, 3, and 4.
Upper Blue River Watershed Study Area 3

(Elliott and others, 2007). Goose Pasture Tarn, a 951,000-m3

(771-acre-ft) reservoir constructed in 1965, is the municipal 

water-storage facility for Breckenridge and is vulnerable to 

potential postwildfire debris-flow sedimentation damage. 

The upper Blue River wildland/urban interface, an 

area where homes are interspersed with forested wildlands, 

includes the Towns of Breckenridge and Blue River, the 

Breckenridge Ski Area, private land in unincorporated areas, 

and important transportation, utility, and water-system infrastructure. Public land in the area upstream from Goose Pasture 

Tarn is part of Arapaho National Forest, which is managed by 

the U.S. Department of Agriculture Forest Service. 

Colorado experienced severe drought conditions in the 

late 20th and early 21st centuries (Kuhn, 2005), and drought, 

when combined with the accumulation of forest fuel, can lead 

to increased wildfire activity. Widespread Colorado wildfires 

in 2002 were associated with a prolonged period of below 

average spring and summer precipitation, high temperatures, 

and low humidity (Pielke and others, 2005). In 2009, the U.S. 

Geological Survey (USGS), in cooperation with the Town of 

Breckenridge, initiated a prewildfire study to determine the 

potential for postwildfire debris flows in the upper Blue River 

watershed. 

The objective of this study is to estimate the probability 

of postwildfire debris flows and to estimate the approximate 

volumes of debris flows delivered from specific watersheds in 

the upper Blue River watershed (upstream from Goose Pasture 

Tarn) in order to provide a relative measure of which watersheds might constitute the most serious postwildfire debrisflow hazards. Although the location, percentage of burned 

area, severity of wildfire, and storm intensity and duration 

after a wildfire cannot be known in advance, hypothetical or 

design scenarios, such as those used in this report, are useful planning tools for conceptualizing potential postwildfire 

effects. 

This report provides estimates of postwildfire debris-flow 

probabilities and volumes that could be produced shortly after 

an assumed moderate- to high-severity wildfire followed by 

1-hour-duration rainfall of varying recurrence probabilities. It 

was assumed that the hypothetical wildfire affected all forested 

areas in nine primary watersheds that are tributaries of the 

upper Blue River and 50 subwatersheds within and adjacent 

to these primary watersheds (fig. 1). This report also contains 

the results of onsite debris-flow verification assessments from 

14 subwatersheds in the upper Blue River watershed made 

in 2009. Using information provided in this report, land, 

water-supply, and municipal managers can consider where to 

concentrate planning for preparedness, mitigation, fuels treatment, and resource allocation in advance of the occurrence of 

wildfires. Also, in the event of a large wildfire, this information will help managers identify the watersheds and subwatersheds with the greatest postwildfire debris-flow hazards. 

Upper Blue River Watershed Study 

Area 

This study was conducted in the Blue River watershed 

upstream from Goose Pasture Tarn, hereinafter referred to 

as the “upper Blue River watershed.” Goose Pasture Tarn is 

a 951,000-m3 (771-acre-ft) manmade reservoir on the Blue 

River located in a glaciated, mountain valley in Summit 

County, Colorado, approximately 2.25 km (1.4 mi) south of 

the town of Breckenridge (fig. 1) (Elliott and others, 2007). 

The Blue River watershed upstream from the reservoir has a 

drainage area of approximately 110 km2 (42 mi2) (fig. 1). The 

watershed is bounded on the west by peaks of the Tenmile 

Range with summits greater than 3,960 m (13,000 ft), including 4,348 m (14,265 ft) Quandary Peak, and is bounded on the 

east by 3,660- and 3,960-m (12,000- and 13,000-ft) summits 

of the Continental Divide. Mountains in the Tenmile Range 

are composed mostly of Precambrian biotitic gneiss, schist, 

and migmatite with some areas of Tertiary and Cretaceous 

intrusive rocks and Pennsylvanian sedimentary rocks. The 

mountains on the eastern side of the Blue River watershed are 

a mixture of Permian and Pennsylvanian sedimentary formations, Cretaceous and Jurassic sedimentary formations, and 

Tertiary and Cretaceous intrusives (Tweto, 1979). 

The mountains above approximately 3,500 m (11,500 ft) 

are vegetated by alpine tundra; below that elevation the terrain 

is vegetated by subalpine coniferous forest. The Blue River 

upstream from Goose Pasture Tarn has a relatively low gradient and meanders through wetlands, beaver dams, and willow 

stands. Tributary streams to the Blue River are relatively steep 

and bouldery (Elliott and others, 2007), and debris-flow deposits are present in some of these tributary valleys throughout 

the study area.

Goose Pasture Tarn is the principal domestic waterstorage facility for the Town of Breckenridge and, since the 

reservoir was constructed in 1965, deltaic deposits have 

accumulated at the outlets of two perennial streams—Blue 

River and Indiana Creek—that provide most of the inflow and 

sediment to the reservoir (Elliott and others, 2007). The Blue 

River is a low-gradient braided channel and transports gravelto silt-size sediment. Indiana Creek is a steep-gradient channel 

that transports boulder- to silt-size sediment. Both deltas are 

composed predominantly of gravel, sand, and silt, but silt 

has been deposited throughout the reservoir. Aerial photographic analysis indicated both deltas grew rapidly during 

time intervals that included larger than average annual flood 

peaks on the Blue River. The largest recorded flood peaks at 

the streamflow-gaging station on the Blue River below Goose 

Pasture Tarn occurred in years when the mountain snowpack 

contained a high water content late in the spring (Elliott and 

others, 2007). Of significance to this study was the observation 
4 Estimated Probabilities and Volumes of Postwildfire Debris Flows for the Upper Blue River Watershed, Colorado

that the June 18, 1995, flood delivered a large amount of 

sediment to Goose Pasture Tarn from both the Blue River and 

Indiana Creek (Gary Roberts, Town of Breckenridge, oral 

commun., July 1, 2003).

Approximately 58,600 m3 (47.5 acre-ft) of sediment has 

accumulated in Goose Pasture Tarn and in the Blue River and 

Indiana Creek deltas, or an average of 1,470 m3 (1.19 acre-ft) 

per year. Elliott and others (2007) determined that if the 

future average sediment delivery rate to Goose Pasture Tarn 

is comparable to the 40 years prior to their study, the reservoir 

has many years of usefulness remaining. However, changes 

in the watershed related to climate, land use, and especially 

large-scale, severe forest fires could increase sediment loads 

by orders of magnitude and greatly shorten the useful life of 

the reservoir. 

Debris-Flow Regression Models 

Equations developed by Cannon and others (2010) were 

used to estimate the probability of debris-flow occurrence and 

estimated volumes of debris flows if fires of moderate to high 

severity consume all forest- and shrub-covered areas in the 

nine primary watersheds that are tributary to the upper Blue 

River (table 1) and in 50 subwatersheds within and adjacent 

to these larger primary watersheds (table 2), hereinafter 

referred to collectively as “watersheds.” The probability and 

volume equations are based on results from extensive studies 

of postwildfire debris flows that occurred in recently burned 

watersheds in the intermountain Western United States. 

The nine primary watersheds in this study range in 

size from 1.57 to 21.76 km2 (0.61 to 8.40 mi2), and the 50 

subwatersheds range in size from 0.05 to 11.84 km2 (0.02 to 

4.57 mi2). The nine primary watersheds include the combined 

areas of multiple modeled subwatersheds within them as 

well as other areas within the primary watershed, such as 

large, laterally planar hillslopes (fig. 1). Debris-flow probabilities and volumes estimated for each primary watershed are 

presented in table 1 separately from probabilities and volumes 

estimated for subwatersheds within those primary watersheds 

(table 2). 

Debris-Flow Probability 

The regression equation of debris-flow probability is 

based on empirical data described by Cannon and others 

(2010, model A). The equation is:

 P = e x /(1 + e x), (1)

where P is the probability of debris-flow occurrence in fractional form, and 

 x = –0.7 + 0.03(%SG30) – 1.6(R) + 0.06(%AB) + 

 0.07(I) + 0.2(%C) – 0.4(LL),

where %SG30 is the percentage of the watershed area with 

slopes equal to or greater than 30 percent; R is watershed ruggedness, the change in watershed elevation (meters) divided 

by the square root of the watershed area (square meters) 

(Melton, 1965); %AB is the percentage of watershed area 

burned at moderate to high severity; I is average storm intensity (in millimeters per hour); %C is the clay content of the 

soil (in percent); and LL is the liquid limit of the soil (percentage of soil moisture by weight), which is the water content at 

which a soil changes from a plastic to a liquid state. 

The debris-flow probability model was developed 

using logistic regression (Hosmer and Lemeshow, 2000) of 

a database of postwildfire debris flows collected throughout 

the intermountain west. Logistic regression calculates 

McFadden’s rho-squared, which is similar to the r-squared of 

linear regression (SPSS, Inc., 2000), but rho-squared tends 

to be smaller than r-squared and ranges from 0 to 1.0. Values 

between 0.20 and 0.40 indicate significant correlation (SPSS, 

Inc., 2000). McFadden’s rho-squared calculated for the 

Cannon and others (2010) debris-flow probability Model A, 

used in this report, is 0.35.

Debris-Flow Volume 

The regression equation developed by Cannon and others (2010, equation 2) was used to estimate a mean volume 

of debris-flow material deposited at the outlet of a recently 

burned watershed in the upper Blue River watershed. The 

equation is:

 ln V = 7.2 + 0.6(ln SG30) + 0.7(AB)0.5+ 0.2(T)0.5 + 0.3, (2)

where, V is the debris-flow volume (including water, sediment, 

and debris) in cubic meters; SG30 is the area of watershed 

with slopes equal to or greater than 30 percent (square 

kilometers); AB is the watershed area burned at moderate to 

high severity (square kilometers), which is a subset of the 

forested area; T is the total storm rainfall (millimeters); and 0.3 

is a bias correction that changes the predicted estimate from a 

median to a mean value (Helsel and Hirsch, 2002). The debrisflow volume equation has an r2

 of 0.83 and a standard error 

of 0.90. In model validation, the volume equation predicted 

87 percent of the debris-flow volumes within the 95-percent 

prediction interval; all reported volumes were within one order 

of magnitude of predicted volumes (Cannon and others, 2010).

Input Data for Debris-Flow Models and 

Assumptions 

Input data for the evaluations in the upper Blue River 

watershed were obtained from a variety of sources. Fifty-nine 

primary watersheds and subwatersheds (tables 1 and 2) were 

delineated from 10-m digital elevation maps (DEMs) using the 

GIS Weasel, a Geographic Information System tool developed 
Table 1. Debris-flow model input variables and output for primary watersheds.[RI, recurrence interval; km2, square kilometer; m/m, meter per meter; m3, cubic meter; ind, Indiana Creek; pen, Pennsylvania Creek; spr, Sp

McCullough Gulch; mon, Monte Cristo Creek; bem, Bemrose Creek; un, unnamed stream; fred, Fredonia Gulch; <1, less than 1 percent pro

INPUT OUTPUT

Watershed 

code

Watershed 

area

(km2)

Ruggedness

(m/m)

Area of 

moderateand 

high-burn 

severity

(km2)

Percentage 

of watershed 

with 

moderateand highburn 

severity

(percent)

Area with 

slopes 

greater 

than or 

equal to 

30 percent

(km2)

Percentage 

of watershed 

with 

slopes 

greater than 

or equal to 

30 percent

(percent)

Soil 

clay 

content

(percent)

Soil 

liquid 

limit

(percent)

2-year RI, 

1-hour 

rainfall 

debris-flow 

probability

(percent)

2-year RI, 

1-hour 

rainfall 

debris-flow 

volume

(m3)

10-year 

RI, 1-hour 

rainfall 

debris-flow 

probability

(percent)

10-year RI, 

1-hour rainfall debrisflow 

volume

(m3)

Primary watersheds1ind00 21.76 0.25 12.82 58.9 11.42 52.5 11.7 24.3 12 227,600 24 295,400 pen00 11.18 0.28 4.86 43.4 5.74 51.3 12.0 24.6 4 57,400 10 74,500 spr00 16.30 0.29 4.73 29.0 9.73 59.7 12.0 24.5 2 77,200 6 100,200 mcc00 12.29 0.35 2.25 18.3 8.29 67.5 12.1 24.5 <1 43,800 4 56,800 mon00 15.30 1.34 1.98 13.0 10.04 65.6 12.2 24.7 <1 46,000 <1 59,700 bem00 9.84 9.08 4.13 42.0 4.25 43.3 13.0 25.9 <1 42,600 <1 55,300 un200 3.97 94.70 1.30 32.7 3.17 79.8 12.2 24.7 <1 19,100 <1 24,800 un100 2.05 9.07 1.04 51.0 1.74 85.2 11.6 24.2 <1 12,300 <1 15,900 fred00 1.57 0.65 0.78 49.4 1.31 83.7 12.1 24.5 9 9,400 20 12,200 

1 Primary watersheds include nested subwatersheds; however, the primary watersheds were modeled as single watersheds.
Table 2. Debris-flow model input variables and output for subwatersheds within and adjacent to primary watersheds. [RI, recurrence interval; km2, square kilometer; m/m, meter per meter; m3, cubic meter; ind, Indiana Creek; pen, Pennsylvania Creek; spr, Spruce Creek; mcc, McCullough Gul

mon, Monte Cristo Creek; bem, Bemrose Creek; un, unnamed stream; <1, less than 1 percent probability] INPUT OUTPUT Sub- water- shed code Sub- water- shed area (km2) Rug- gedness (m/m) Area of medium- and high- burn severity (km2) Percentage of sub- watershed with medium- and high-burn severity (percent) Area with slopes greater than or equal to 30 percent (km2) Percentage of subwater- shed with slopes greater than or equal to 30 percent (percent) Soil clay content (percent) Soil liquid limit (percent) 2-year RI, 1-hour rainfall debris- flow prob- ability (percent) 2-year RI, 1-hour rainfall debris- flow volume (m3) 10-year RI, 1-hour rainfall debris- flow prob- ability (percent) 10-year RI, 1-hour rainfall debris- flow volume (m3) 25-year RI, 1-hour rainfall debris- flow prob- ability (percent) 25-year RI, 1-hour rainfall debris- flow volume (m3) 

Indiana Creek primary watershed ind01 0.15 0.73 0.15 99.3 0.08 51.3 10.6 23.5 42 1,200 64 1,600 77 1,800 

ind02 0.12 0.87 0.12 99.2 0.08 62.0 10.6 23.5 44 1,200 66 1,500 78 1,800 

ind03 0.08 0.91 0.07 97.3 0.06 81.6 10.6 23.5 54 1,000 75 1,300 84 1,500 

ind04 0.05 1.00 0.05 98.0 0.03 68.7 10.6 23.5 42 700 64 900 77 1,000 

ind05 1.21 0.39 1.15 94.5 0.57 46.6 10.6 23.5 44 6,500 67 8,400 79 9,700 

ind06 0.63 0.51 0.63 99.2 0.11 16.7 11.1 23.8 26 1,900 46 2,500 62 2,900 

ind07 0.55 0.36 0.55 100.0 0.21 38.2 10.6 23.5 48 2,800 70 3,700 81 4,200 

ind08 0.40 0.63 0.40 98.5 0.15 37.2 11.3 24.0 33 2,100 55 2,800 70 3,200 

ind09 0.56 0.53 0.55 98.6 0.40 71.5 10.6 23.5 63 4,200 81 5,400 89 6,300 

ind10 2.65 0.60 1.37 51.8 1.86 70.1 11.4 24.0 8 14,200 17 18,400 28 21,300 

ind11 0.18 0.54 0.18 100.0 0.08 46.6 10.6 23.5 47 1,300 69 1,700 80 2,000 

ind12 0.12 0.93 0.12 97.5 0.09 72.8 11.2 23.9 46 1,300 68 1,600 80 1,900 

ind13 2.13 0.68 0.79 37.1 1.37 64.3 11.8 24.3 2 9,700 6 12,600 10 14,500 ind14 1.52 0.75 0.32 20.9 0.87 57.1 12.2 24.6 <1 5,900 2 7,600 3 8,800 

ind15 2.29 0.33 1.37 59.8 1.54 67.2 12.4 24.8 15 12,700 30 16,400 45 19,000 

ind16 0.15 0.68 0.14 95.3 0.09 58.3 12.2 24.6 40 1,300 62 1,700 76 1,900 

ind17 0.13 0.62 0.08 60.0 0.02 12.3 12.2 24.6 2 400 5 600 9 700 

ind18 4.43 0.35 0.85 19.1 1.94 43.8 12.2 24.7 <1 12,200 2 15,800 3 18,300 

Pennsylvania Creek primary watershed pen01 0.22 0.48 0.22 100.0 0.03 14.4 10.7 23.6 27 700 48 1,000 63 1,100 

pen02 0.45 0.50 0.45 100.0 0.26 57.7 11.4 24.0 55 3,100 76 4,000 85 4,600 

pen03 1.85 0.46 1.05 56.7 0.97 52.3 12.0 24.5 7 8,600 17 11,200 27 13,000 

pen04 0.09 0.81 0.09 100.0 0.07 77.2 10.9 23.7 58 1,100 78 1,400 87 1,600 

pen05 0.22 0.48 0.21 93.2 0.08 35.4 11.4 24.0 30 1,300 52 1,700 67 1,900 

pen06 0.40 0.66 0.08 19.6 0.19 48.6 12.2 24.6 <1 2,000 1 2,500 2 2,900 

pen07 0.34 0.63 0.02 6.9 0.15 44.6 12.2 24.6 <1 1,500 <1 2,000 1 2,300 
Table 2. Debris-flow model input variables and output for subwatersheds within and adjacent to primary watersheds.—Continued[RI, recurrence interval; km2, square kilometer; m/m, meter per meter; m3, cubic meter; ind, Indiana Creek; pen, Pennsylvania Creek; spr, Spruce Creek; mcc, McCullough Gul

mon, Monte Cristo Creek; bem, Bemrose Creek; un, unnamed stream; <1, less than 1 percent probability]

INPUT OUTPUT

Subwatershed 

code

Subwatershed 

area

(km2)

Ruggedness

(m/m)

Area 

of 

mediumand highburn 

severity

(km2)

Percentage 

of subwatershed 

with 

medium- and 

high-burn 

severity

(percent)

Area with 

slopes 

greater 

than or 

equal to 

30 percent

(km2)

Percentage of 

subwatershed with 

slopes 

greater than 

or equal to 

30 percent

(percent)

Soil 

clay 

content

(percent)

Soil 

liquid 

limit

(percent)

2-year 

RI, 

1-hour 

rainfall 

debrisflow probability

(percent)

2-year 

RI, 

1-hour 

rainfall 

debrisflow 

volume

(m3)

10-year 

RI, 

1-hour 

rainfall 

debrisflow probability

(percent)

10-year 

RI, 

1-hour 

rainfall 

debrisflow 

volume

(m3)

25-year 

RI, 

1-hour 

rainfall 

debrisflow 

probability

(percent)

25-year 

RI, 1-hour 

rainfall 

debrisflow 

volume

(m3)

Pennsylvania Creek primary watershedpen08 0.07 0.83 0.01 15.3 0.02 20.4 12.2 24.6 <1 400 <1 500 <1 600 

pen09 2.74 0.32 0.03 1.1 1.78 65.1 12.3 24.9 <1 6,900 1 8,900 2 10,300 Spruce Creek primary watershedspr01 0.09 0.60 0.09 100.0 0.03 33.9 10.6 23.5 35 700 57 900 71 1,000 spr02 0.08 0.64 0.08 100.0 0.04 54.2 10.6 23.5 49 800 70 1,000 81 1,200 spr03 0.10 0.66 0.10 100.0 0.04 43.1 10.6 23.5 40 800 62 1,000 75 1,200 spr04 5.31 0.44 0.51 9.6 3.46 65.2 12.1 24.5 <1 14,900 2 19,400 3 22,500 spr05 0.52 0.45 0.50 97.3 0.27 53.3 12.2 24.6 48 3,200 70 4,200 81 4,900 

spr06 5.97 0.37 0.38 6.4 3.81 63.7 12.2 24.6 <1 14,800 2 19,200 3 22,200 spr07 0.76 0.46 0.64 84.4 0.43 56.2 12.2 24.6 32 4,500 53 5,900 68 6,800 McCullough Gulch primary watershedmcc01 0.23 0.84 0.09 38.1 0.15 64.8 11.0 23.8 2 1,700 5 2,200 9 2,500 mcc02 0.58 0.90 0.27 47.0 0.36 62.2 11.6 24.2 3 3,400 7 4,400 12 5,100 mcc03 10.24 0.34 0.76 7.5 7.05 68.9 12.2 24.6 <1 25,600 2 33,200 4 38,500 Monte Cristo Creek primary watershedmon01 0.47 0.61 0.27 57.5 0.13 27.4 12.6 26.2 2 1,800 4 2,400 8 2,700 mon02 0.34 0.60 0.12 35.6 0.18 53.8 10.7 23.6 2 2,000 5 2,600 9 3,000 mon03 1.44 0.64 0.10 6.7 0.73 50.8 12.3 25.0 <1 4,400 <1 5,800 1 6,700 

mon04 11.84 0.30 0.62 5.3 8.71 73.5 12.2 24.7 <1 27,400 2 35,600 4 41,200 Bemrose Creek primary watershedbem01 0.16 0.53 0.12 77.6 0.01 5.1 12.2 24.6 6 300 13 400 21 500 

bem02 5.13 0.34 1.35 26.2 3.25 63.3 12.4 25.5 2 19,700 4 25,500 7 29,500 bem03 0.11 0.56 0.11 99.1 0.08 70.3 12.2 24.6 59 1,200 78 1,500 87 1,700 bem04 0.69 0.59 0.38 54.8 0.08 11.0 12.6 26.3 <1 1,400 2 1,800 4 2,100 bem05 2.15 0.32 0.92 42.9 0.34 15.7 15.0 27.5 <1 4,400 2 5,700 3 6,600 
Table 2. Debris-flow model input variables and output for subwatersheds within and adjacent to primary watersheds.—Continued [RI, recurrence interval; km2, square kilometer; m/m, meter per meter; m3, cubic meter; ind, Indiana Creek; pen, Pennsylvania Creek; spr, Spruce Creek; mcc, McCullough Gul

mon, Monte Cristo Creek; bem, Bemrose Creek; un, unnamed stream; <1, less than 1 percent probability] INPUT OUTPUT Sub- water- shed code Sub- water- shed area (km2) Rug- gedness (m/m) Area of medium- and high- burn severity (km2) Percentage of sub- watershed with medium- and high-burn severity (percent) Area with slopes greater than or equal to 30 percent (km2) Percentage of subwater- shed with slopes greater than or equal to 30 percent (percent) Soil clay content (percent) Soil liquid limit (percent) 2-year RI, 1-hour rainfall debris- flow prob- ability (percent) 2-year RI, 1-hour rainfall debris- flow volume (m3) 10-year RI, 1-hour rainfall debris- flow prob- ability (percent) 10-year RI, 1-hour rainfall debris- flow volume (m3) 25-year RI, 1-hour rainfall debris- flow prob- ability (percent) 25-year RI, 1-hour rainfall debris- flow volume (m3) 

Unnamed stream, primary watershed number 200 un201 2.79 0.49 0.77 27.7 2.49 89.2 12.3 24.8 4 13,800 9 17,900 15 20,700 un202 0.86 0.52 0.26 30.4 0.55 64.1 12.2 24.6 2 4,300 5 5,600 9 6,400 Unnamed stream, primary watershed number 100 un101 0.11 1.33 0.09 86.9 0.09 81.6 10.7 23.6 24 1,200 44 1,600 60 1,800 un102 1.67 0.70 0.73 44.1 1.51 90.7 11.9 24.4 8 10,000 17 13,000 27 15,100 
Debris-Flow Regression Models 9

by Viger and Leavesley (2007). Watershed sizes in the analysis 

ranged from 0.05 to 21.76 km2

 (0.02 to 8.40 mi2), which 

is consistent with the range of watershed areas used in the 

debris-flow models developed by Cannon and others (2010). 

The delineated watersheds cover 70 percent of the total upper 

Blue River watershed upstream from Goose Pasture Tarn. The 

remaining 30 percent largely consisted of non-channelized 

hillslopes and other areas where postwildfire erosion could 

occur, but where that erosion would be unlikely to take the 

form of a debris flow (fig. 1). 

Forested areas were used as a surrogate for areas of 

moderate- to high-burn severity (fig. 1), and it was assumed 

that all of the forest and shrub cover, which was defined from 

the National Land Cover Database (Homer and others, 2007), 

would burn at moderate- to high-burn severity within each of 

the watersheds. Although this assumption may characterize 

only extensive and high-burn severity wildfires, it provides a 

consistent basis for comparison of debris-flow hazards among 

watersheds in the upper Blue River watershed as well as 

providing a worst-case scenario for debris-flow probability 

and volume. 

High-burn severity is defined by Lindsey (2002) as 

the complete consumption of the forest litter and duff and 

combustion of all fine fuels in the canopy. A deep ash layer 

may be present on the forest floor, and the top layer of the 

mineral soil may be changed in color due to substantial soil 

heating where large-diameter fuels were consumed. Moderateburn severity is defined as the consumption of forest litter and 

duff in discontinuous patches. Leaves or needles, although 

scorched, may remain on trees. Foliage and twigs on the forest 

floor are consumed, and some heating of the mineral soils may 

occur if the soil organic layer is thin. 

Rainfall, in terms of both storm recurrence and 

precipitation duration, is an essential element in the generation 

of postwildfire debris flows. The debris flows studied by 

Cannon and others (2010) to develop equations 1 and 2 

were generated by short-duration convective rainstorms with 

recurrence intervals ranging from less than 2 years to as 

many as 10 years. Another researcher noted that the 25-year 

recurrence rainfall might be more representative of storms 

that generate other debris flows because a more frequently 

occurring storm might deliver too little rainfall runoff to 

sustain a debris flow, whereas a less frequently occurring 

storm might deliver too much rainfall runoff, creating a 

sediment-laden water flood rather than a debris flow (J.S. 

O’Brien, FLO Engineering, Inc., oral commun., 2002).

Postwildfire studies of the 2002 Hayman, Coal Seam, 

and Missionary Ridge burned areas estimated that burned 

watersheds were the most vulnerable to extensive erosion 

and potential debris flows for a 4- to 6-year period following 

those wildfires (Elliott and others, 2005), whereas Cannon and 

others (2010) found that most postwildfire debris-flow activity 

occurred within about 2 years after the wildfire. Therefore, a 

2-year recurrence rainfall is reasonably likely to occur while 

the burned area is most vulnerable to erosion, but such a 

storm might not represent a worst-case scenario. To represent 

weather conditions that possibly could result in more severe 

postwildfire erosion, debris-flow probabilities and volumes in 

response to the 10-year and 25-year recurrence rainfall events 

also were simulated for the upper Blue River watersheds. 

Three postwildfire precipitation scenarios and recurrence 

intervals were used for the postwildfire debris-flow analysis 

in the upper Blue River watershed. These scenarios were (1) 

a 2-year recurrence, 1-hour-duration rainfall total of 18.8 mm 

(0.74 in.); (2) a 10-year recurrence, 1-hour-duration rainfall 

total of 31.8 mm (1.25 in.); and (3) a 25-year recurrence, 

1-hour-duration rainfall total of 40.6 mm (1.60 in.). Henceforth, the precipitation scenarios will be referred to as the “2-, 

10-, and 25-year storms.”

Rainfall totals for the upper Blue River watershed were 

determined from the National Oceanic and Atmospheric 

Administration (NOAA) Atlas II for Colorado (Miller and 

others, 1973). A 1-hour rainfall duration was chosen for the 

scenarios because it was a relatively short-lived event. The 

total storm rainfall for each scenario was considered to occur 

uniformly over each watershed.

Other input variables for the debris-flow model were 

determined from a variety of sources. The watershed area and 

percentage of watershed area with 30 percent or greater slopes 

were determined using ArcMap with topography from 10-m 

DEMs. Raw data for soil properties were compiled from the 

State Soil Geographic (STATSGO) database (U.S. Department 

of Agriculture, 1991), which was processed by Schwartz and 

Alexander (1995) to obtain soil clay content and liquid limit. 

Soil properties were spatially averaged when more than one 

value occurred in a watershed.

Verification of Debris-Flow Model Results 

Preliminary estimates of debris-flow probability and volume were made with the model for reconnaissance purposes 

and onsite verification of model predictions. The presence of 

older debris-flow deposits or scoured channels was considered 

to be geomorphic evidence that debris flows had occurred at 

some time in the past, and that the debris-flow models of Cannon and others (2010) were appropriate for use in the upper 

Blue River watershed. The purpose of the reconnaissance was 

to verify the process only, and no attempt was made to determine what watershed condition (postwildfire or unburned) 

existed at the time the debris flow was initiated. 

The upper Blue River watershed above Goose Pasture 

Tarn initially was subdivided into 116 erosion runoff units 

(ERUs) that roughly corresponded with low stream-order 

watersheds, as well as hillslope areas and unchannelized 

hollows likely to produce storm runoff. The reconnaissance 

scenario model was run for each ERU with the assumptions 

that (1) all trees and shrubs had been burned with a moderate 

to high severity, followed by (2) a 1-hour rainfall that produced 25 mm of precipitation. This reconnaissance scenario 

represented a relatively rare wildfire (a moderate- to high-burn 

severity of the entire ERU) and a relatively common rainfall 
10 Estimated Probabilities and Volumes of Postwildfire Debris Flows for the Upper Blue River Watershed, Colorado

(roughly equivalent to a 7-year recurrence, 1-hour rainfall). 

Debris-flow probabilities for all ERUs ranged from less than 1 

to 68 percent. 

Fourteen of the 116 reconnaissance scenario ERUs 

with the greatest estimated debris-flow probabilities (greater 

than 40 percent) for the design storm were visited by USGS 

personnel in August 2009 and inspected for geomorphic 

evidence of previous debris-flow or flood activity. Most of 

these high-probability ERUs showed evidence of previous 

debris-flow activity including marginal levees (fig. 2), terminal lobes (fig. 3), debris-flow fans, or debris-flow scoured 

channels (fig. 4) (Costa, 1988; Pierson, 2005). Two ERUs for 

which relatively high debris-flow probabilities were estimated 

showed no debris-flow evidence. Three showed evidence of 

mass movement whose source and processes were multiple 

or inconclusive; for example, where lobe-shaped deposits of 

colluvium or reworked glacial till could have been formed by 

hillslope creep, landslide, earthflow, solifluction, rockfall, or 

combinations thereof (Keefer and Johnson, 1983). 

Debris-flow evidence (marginal levees and (or) a terminal lobe) also were observed in four ERUs with reconnaissance scenario probabilities that ranged from 4 to 14 percent. 

No attempt was made to correlate any observed debris-flow 

deposit with a specific storm characteristic or date, nor were 

debris-flow volumes estimated. However, where the reconnaissance scenario model predicted a high probability of debrisflow activity for the reference storm, geomorphic evidence 

generally was found and was corroborative. The geomorphic 

evidence of debris-flow activity was subtle in most ERUs and 

debris-flow deposits commonly were found in heavily forested 

locations, indicating that the most recent debris-flow activity 

was at least several decades or more old (fig. 2). It was not 

determined whether any of the observed debris flows in the 

reconnaissance ERUs were the result of previous wildfires; 

however, the field evidence was indicative that debris-flow 

processes had been active in these locations. 

Estimated Probabilities and Volumes of 

Postwildfire Debris Flows 

After the preliminary reconnaissance, ERUs were 

replaced with redrawn watershed boundaries to reflect 

geomorphic areas similar to those from which debris-flow 

equations 1 and 2 (Cannon and others, 2010) were derived. 

The redrawn watersheds were land-surface areas of runoff 

accumulation that had a distinctive outlet such as a confluence 

with a larger stream or an alluvial fan. The models for debrisflow probability and volume (eqs. 1 and 2) were rerun with the 

redrawn watershed boundaries and with the assumptions that 

100 percent of the forested area in each watershed was burned 

at moderate to high severity and that the burn was followed 

by the design rainfall within 4 to 6 years (Elliott and others, 

2005). 

Upper Blue River Watershed Debris-Flow 

Probabilities 

Results of the debris-flow probability modeling are 

shown as maps in figures 5, 6, and 7, and corresponding 

numerical values are presented in tables 1 and 2. Although 

each subwatershed is entirely shaded by a color representing 

the subwatershed debris-flow probability, the estimated 

probability is relevant for the contributing area upstream 

from the subwatershed outlet and not at every location within 

the subwatershed. Estimated debris-flow probabilities for 

the nine primary watersheds are shown as color-coded dots 

at the watershed outlet (where watershed boundary lines 

converge) to distinguish the debris-flow probability of the 

primary watershed from the probabilities of the individual 

subwatersheds nested within or adjacent to the primary 

watershed.

All of these precipitation scenarios resulted in debris 

flows from the hypothetically burned watersheds. The 

estimated probabilities for postwildfire debris flows in the 

50 subwatersheds in the upper Blue River watershed ranged 

from less than 1 to 63 percent in response to the 2-year 

storm (2-year recurrence, 1-hour-duration rainfall), less than 

1 to 81 percent in response to the 10-year storm (10-year 

recurrence, 1-hour-duration rainfall), and less than 1 to 89 

percent in response to the 25-year storm (25-year recurrence, 

1-hour-duration rainfall) (table 2). Subwatersheds with the 

lowest postwildfire debris-flow probabilities tended to have 

large areas of alpine and subalpine vegetation or other areas 

with sparse forest cover. Subwatersheds with the highest 

probabilities tended to be steep, heavily forested, and 

relatively small in drainage area. Twenty-two subwatersheds 

had a greater than 60 percent probability of producing a debris 

flow in response to the 25-year storm (fig. 7 and table 2). 

Cannon and others (2010) found that low-order tributaries 

with a mean area of 1.7 km2

 produced most of the debris 

flows in their study area. Many of the subwatersheds with 

the highest debris-flow probabilities in this study were in the 

north and northeastern part of the upper Blue River watershed, 

notably tributaries to Indiana, Pennsylvania, and Spruce 

Creeks (figs. 5-7). 

The nine primary watersheds were evaluated separately 

because they consisted of nested subwatersheds, and any 

potential debris flow reaching the primary watershed outlet 

could be the result of debris-flow contributions from the 

nested subwatersheds as well as runoff from other contributing 

land surfaces (for example, laterally planar hillslopes) within 

the primary watershed (figs. 5–7). Postwildfire debris-flow 

probabilities for the primary watersheds ranged from less than 

1 to 12 percent in response to the 2-year storm, less than 1 

to 24 percent in response to the 10-year storm, and less than 

1 to 38 percent in response to the 25-year storm (table 1). 

The Indiana Creek, Pennsylvania Creek, and Spruce Creek 

watersheds each had a 10 percent or greater probability of 

producing a debris flow at the watershed outlet in response 

to a 25-year storm (table 1). Indiana Creek had a 12-percent 
Estimated Probabilities and Volumes of Postwildfire Debris Flows 11

Figure 2. Debris-flow 

marginal levee near the outlet 

of Fredonia Gulch. View is 

looking upstream.

Figure 3. Debris-flow terminal lobe 

at the outlet of subwatershed ind10, 

a tributary of Indiana Creek. View is 

looking normal to flow direction.

Figure 4. Debris-flow scoured 

channel near the outlet of 

subwatershed pen01, a tributary 

of Pennsylvania Creek. View is 

looking upstream.
ind03 ind05 ind06 ind08 ind09 ind10 ind11 ind12 ind13 ind14 ind17 ind15 ind16 ind02 ind01 ind04 ind07 pen09 pen01 pen02 pen03 pen06 pen07 pen05 pen08 pen04 spr05 spr04 spr03 spr01 spr07 spr02 mcc03 mcc02 mcc01 mon04 mon03 mon02 mon01 bem01 bem02 bem03 bem04 bem05 un201 un202 un101 un102 spr06 Blue River Indiana Creek Spruce Creek McCullough Gulch Pennsylvania Creek Crystal Creek Bemrose Creek Monte Cristo Creek Monte Cristo Creek EXPLANATION Estimated debris-flow probability at primary watershed outlet, in percent Estimat

proboutl

0–20 21–40 41–60 61–80 81–100 Primary watersheds 

0 0.5 1 2 MILES 0 0.5 1 2 KILOMETERS TENMILE RANGE CONTINENTAL DIVIDE bem01 

Base from U.S. Geological Survey digital data, 1:100,000 Universal Transverse Mercator Projection, Zone 13, North American Datum 1983 106°W 106°W 106°07’30”W 106°07’30”W 39°22’30”N 39°22’30”N

Figure 5. Primary watersheds and subwatersheds showing estimated debris-flow probabilities in response to the 2-year-recurrence, 

rainfall. 
ind03 ind05

ind06

ind08

ind09

ind10

ind11 ind12 ind13

ind14

ind17

ind15

ind16

ind02 ind01 ind04

ind07

pen09

pen01

pen02

pen03

pen06

pen07

pen05

pen08

pen04

spr05

spr04

spr03

spr01

spr07

spr02

mcc03

mcc02

mcc01

mon04

mon03

mon02

mon01

bem01

bem02

bem03

bem04

bem05

un202

un201

un101

un102

spr06

Blue

River

Indiana Creek Spruce Creek

McCullough Gulch

Pennsylvania

Creek

Crystal Creek

Bemrose Creek

Monte Cristo Creek Monte Cristo Creek

0–2021–4041–6061–8081–100

Primary watersheds

0 0.5 1 2 MILES

0 0.5 1 2 KILOMETERS

TENMILE RANGE

DIVIDE

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator Projection,Zone 13, North American Datum 1983

bem01 

106°W

106°W

106°07’30”W

106°07’30”W

39°22’30”N 39°22’30”N

CONTINENTAL EXPLANATION Estimated debris-flow probability at primary watershed outlet, in percent Estimated 

probaboutlet, 

Figure 6. Primary watersheds and subwatersheds showing estimated debris-flow probabilities in response to the 10-year-recurrence

rainfall. 
ind03 ind05 ind06 ind08 ind09 ind10 ind11 ind12 ind13 ind14 ind17 ind15 ind16 ind02 ind01 ind04 ind07 pen09 pen01 pen02 pen03 pen06 pen07 pen05 pen08 pen04 spr05 spr04 spr03 spr01 spr07 spr02 mcc03 mcc02 mcc01 mon04 mon03 mon02 mon01 bem01 bem02 bem03 bem04 bem05 un202 un201 un101 un102 spr06 Blue River Indiana Creek Spruce Creek McCullough Gulch Pennsylvania Creek Crystal Creek Bemrose Creek Monte Cristo Creek Monte Cristo Creek 0–20 21–40 41–60 61–80 81–100 

0 0.5 1 2 MILES 0 0.5 1 2 KILOMETERS TENMILE RANGE DIVIDE Base from U.S. Geological Survey digital data, 1:100,000 Universal Transverse Mercator Projection, Zone 13, North American Datum 1983 106°W 106°W 106°07’30”W 106°07’30”W 39°22’30”N 39°22’30”N CONTINENTAL Primary watersheds bem01 

EXPLANATION Estimated debris-flow probability at primary watershed outlet, in percent Estimate

probaoutle

Figure 7. Primary watersheds and subwatersheds showing estimated debris-flow probabilities in response to the 25-year-recurrence, 1-horainfall. 
Estimated Probabilities and Volumes of Postwildfire Debris Flows 15

probability of producing a debris flow in response to as little 

as a 2-year storm if the entire forested part of the watershed 

was moderately to severely burned. 

It was possible for a large primary watershed to have a 

near-to-zero percent probability of a debris flow reaching the 

watershed outlet even though debris flows were possible in 

some subwatersheds within the primary watershed because of 

limited transport potential downstream from the subwatershed 

outlet or because of the relatively small size of the debris-flow 

contributing area within the primary watershed. Cannon and 

others (2010) found that debris flows were not observed at the 

outlets of watersheds greater than about 30 km2

 (12 mi2) in 

area. 

Upper Blue River Watershed Debris-Flow 

Volumes 

Results of the debris-flow volume models are shown as 

maps in figures 8, 9, and 10, and corresponding numerical 

values are presented in tables 1 and 2. Although each 

subwatershed is entirely shaded by a color representing the 

subwatershed debris-flow volume, the estimated volume is 

applicable only for a debris flow delivering sediment to a point 

near the subwatershed outlet and not at every location within 

the subwatershed. Estimated debris-flow volumes for the 

nine primary watersheds are shown as color-coded dots at the 

watershed outlet (where watershed boundary lines converge).

The estimated volumes for potential postwildfire debris 

flows in the 50 subwatersheds in the upper Blue River 

watershed ranged from about 300 to 27,400 m3

 in response to 

the 2-year storm, 400 to 35,600 m3

 in response to the 10-year 

storm, and 500 to 41,200 m3

 in response to the 25-year storm 

(table 2). Subwatersheds with the smallest postwildfire debrisflow volumes tended to have small drainage areas, relatively 

little forest cover, and less rugged topography, or were in 

alpine and subalpine zones. Subwatersheds with the highest 

debris-flow volumes were those with the largest drainage 

areas. Fourteen subwatersheds had estimated debris-flow 

volumes greater than 10,000 m3

 in response to a 25-year storm 

(fig. 10 and table 2). 

As with the probability estimates, debris-flow volume 

estimates for the nine primary watersheds are presented 

separately because they consisted of nested subwatersheds 

(figs. 8-10). Postwildfire debris-flow volume estimates for the 

primary watersheds ranged from about 9,400 to 227,600 m3

 in 

response to the 2-year storm, 12,200 to 295,400 m3

 in response 

to the 10-year storm, and 14,100 to 342,000 m3

 in response 

to the 25-year storm (table 1). Indiana Creek, Pennsylvania 

Creek, and Spruce Creek had estimated debris-flow volumes 

greater than 86,000 m3

 in response to a 25-year storm (fig. 10 

and table 1). 

Although watersheds burned at moderate to high severity 

in the study area potentially can produce large volumes of 

debris-flow material (water, sediment, and other debris), 

determining where that material could be deposited below the 

watershed outlet is beyond the scope of this study. Wide and 

relatively low-gradient stream valleys such as the Blue River 

and Indiana Creek potentially could intercept and capture 

some debris-flow material produced in tributaries before the 

material reaches Goose Pasture Tarn. Pre-existing debrisflow deposits of unknown source or trigger mechanisms were 

observed near the outlet of Indiana Creek a short distance 

upstream from the inflow to Goose Pasture Tarn (Elliott and 

others, 2007). 

Combined Relative Debris-Flow Hazard Ranking 

The watersheds with the greatest potential postwildfire 

and postprecipitation debris-flow hazards are those with 

both high probabilities of debris-flow occurrence and large 

estimated volumes of material (Cannon and others, 2010). 

Results from the 25-year storm debris-flow probability and 

volume equations were merged to produce a combined 

relative debris-flow hazard ranking for the 50 subwatersheds 

in the upper Blue River watershed (fig. 11) to provide an 

overall indicator of the relative hazards associated with each 

subwatershed. 

For each subwatershed, the debris-flow probability 

rank, with 1 associated with the largest probability (table 2), 

was added to the debris-flow volume rank, with 1 associated 

with the greatest volume (table 2), to derive a preliminary 

combined rank sum. The preliminary combined rank sums 

for the 50 subwatersheds ranged from 22 (highest combined 

hazard) to 101 (lowest combined hazard). The preliminary 

combined rank sums for each subwatershed were renumbered 

with 1 assigned to the subwatershed with the highest 

combined hazard, 2 assigned to the subwatershed with the 

second highest combined hazard, and so forth through 31 for 

the lowest combined hazard in the 50th-ranked subwatershed. 

There were numerous preliminary combined rank sum ties, 

and all tied values were given the same combined relative 

debris-flow hazard ranking. Therefore the final combined 

relative debris-flow hazard rankings ranged from 1 through 31, 

not 1 through 50 (table 2). 

The 10 subwatersheds with the greatest combined 

relative debris-flow hazard rankings are ind05, ind07, 

ind09, ind10, ind15 in the Indiana Creek watershed; pen02 

in the Pennsylvania Creek watershed; spr05 and spr07 in 

the Spruce Creek watershed; and un102 and un201 in the 

unnamed watersheds (fig. 11 and table 2). Combined relative 

debris-flow hazard rankings were not calculated for the nine 

primary watersheds because, as composite areas of multiple 

subwatersheds and unmodeled laterally planar hillslope 

areas, the inevitable direct comparison of primary watershed 

rankings with the subwatershed rankings would be misleading. 

Nevertheless, the high probabilities of postwildfire debris 

flows, the associated large debris-flow volumes, and the 

densely populated areas along the creeks and near the primary 

watershed outlets indicate that Indiana, Pennsylvania, and 
ind03 ind05 ind06 ind08 ind09 ind10 ind11 ind12 ind13 ind14 ind17 ind15 ind16 ind02 ind01 ind04 ind07 pen09 pen01 pen02 pen03 pen06 pen07 pen05 pen08 pen04 spr05 spr04 spr03 spr01 spr07 spr02 mcc03 mcc02 mcc01 mon04 mon03 mon02 mon01 bem01 bem02 bem03 bem04 bem05 un202 un201 un101 un102 spr06 Blue River Indiana Creek Spruce Creek McCullough Gulch Pennsylvania Creek Crystal Creek Bemrose Creek Monte Cristo Creek Monte Cristo Creek 303–1,500 1,501–2,500 2,501–5,000 5,001–10,000 >10,000 

0 0.5 1 2 MILES 0 0.5 1 2 KILOMETERS TENMILE RANGE DIVIDE Base from U.S. Geological Survey digital data, 1:100,000 Universal Transverse Mercator Projection, Zone 13, North American Datum 1983 106°W 106°W 106°07’30”W 106°07’30”W 39°22’30”N 39°22’30”N CONTINENTAL Primary watersheds b

EXPLANATION Estimated debris-flow volumes at primary watershed outlet, in cubic meters Estim

vo

Figure 8. Primary watersheds and subwatersheds showing estimated debris-flow volumes in response to the 2-year-recurrence, 1-h

rainfall. 
ind03 ind05

ind06

ind08

ind09

ind10

ind11 ind12

ind13

ind14

ind17

ind15

ind16

ind02 ind01 ind04

ind07

pen09

pen01

pen02

pen03

pen06

pen07

pen05

pen08

pen04

spr05

spr04

spr03

spr01

spr07

spr02

mcc03

mcc02

mcc01

mon04

mon03

mon02

mon01

bem01

bem02 bem03

bem04

bem05

un202

un201

un101

un102

spr06

Blue

River

Indiana Creek Spruce Creek

McCullough Gulch

Pennsylvania

Creek

Crystal Creek

Bemrose Creek

Monte Cristo Creek Monte Cristo Creek

393–1,5001,501–2,5002,501–5,0005,001–10,000>10,000

0 0.5 1 2 MILES0 0.5 1 2 KILOMETERS

TENMILE RANGE

DIVIDE

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator Projection,Zone 13, North American Datum 1983

106°W

106°W

106°07’30”W

106°07’30”W

39°22’30”N 39°22’30”N

CONTINENTAL

Primary watersheds bem

EXPLANATION

Estimated debris-flow 

volumes at primarywatershed outlet, incubic meters

Estima

volout

Figure 9. Primary watersheds and subwatersheds showing estimated debris-flow volumes in response to the 10-year-recurrence, 1

rainfall.
ind03 ind05 ind06 ind08 ind09 ind10 ind11 ind12 ind13 ind14 ind17 ind15 ind16 ind02 ind01 ind04 ind07 pen09 pen01 pen02 pen03 pen06 pen07 pen05 pen08 pen04 spr05 spr04 spr03 spr01 spr07 spr02 mcc03 mcc02 mcc01 mon04 mon03 mon02 mon01 bem01 bem02 bem03 bem04 bem05 un202 un201 un101 un102 spr06 Blue River Indiana Creek Spruce Creek McCullough Gulch Pennsylvania Creek Crystal Creek Bemrose Creek Monte Cristo Creek Monte Cristo Creek 455–1,500 1,501–2,500 2,501–5,000 5,001–10,000 >10,000 

0 0.5 1 2 MILES 0 0.5 1 2 KILOMETERS TENMILE RANGE DIVIDE Base from U.S. Geological Survey digital data, 1:100,000 Universal Transverse Mercator Projection, Zone 13, North American Datum 1983 106°W 106°W 106°07’30”W 106°07’30”W 39°22’30”N 39°22’30”N CONTINENTAL Primary watersheds bem01 

EXPLANATION Estimated debris-flow volumes at primary watershed outlet, in cubic meters Estimat

voluoutle

Figure 10. Primary watersheds and subwatersheds showing estimated debris-flow volumes in response to the 25-year-recurrence, 1

rainfall. 
Spruce Creek

ind03 ind05

ind06

ind08

ind09

ind10

ind11 ind12 ind13

ind14

ind17

ind15

ind16

ind02 ind01 ind04

ind07

pen09

pen01

pen02

pen03

pen06

pen07

pen05

pen08

pen04

spr05

spr04

spr03

spr01

spr07

spr02

mcc03

mcc02

mcc01

mon04

mon03

mon02

mon01

bem01

bem02

bem03

bem04

bem05

un202

un201

un101

un102

spr06

Blue

River

Indiana Creek

McCullough Gulch

Pennsylvania

Creek

Crystal Creek

Bemrose Creek

Monte Cristo Creek Monte Cristo Creek

EXPLAN

Combined relativ ranking at subw

1–89–1213–1819–2425–31

Subwate

0 0.5 1 2 MILES

0 0.5 1 2 KILOMETERS

Base from U.S. Geological Survey digital data, 1:100,000Universal Transverse Mercator Projection,Zone 13, North American Datum 1983

106°W

106°W

106°07’30”W

106°07’30”W

39°22’30”N 39°22’30”N

TENMILE RANGE

DIVIDE

CONTINENTAL

Primary w

bem01 Subwate

Figure 11. Subwatersheds showing the combined relative debris-flow hazard ranking in response to the 25-year-recurrence, 1-hour-d

rainfall.
20 Estimated Probabilities and Volumes of Postwildfire Debris Flows for the Upper Blue River Watershed, Colorado

Spruce Creeks are associated with a high combined relative 

debris-flow hazard. 

Summary and Conclusions 

Debris flows resulting from rainfall on recently burned, 

rugged, forested areas create potential hazards to life, 

property, infrastructure, and water resources. In 2009, the 

U.S. Geological Survey, in cooperation with the Town of 

Breckenridge, initiated a prewildfire study to determine the 

potential for postwildfire debris flows in the upper Blue River 

watershed, Summit County, Colorado. The objective was to 

estimate the probability of postwildfire debris flows and to 

estimate the approximate volumes of debris flows originating 

in nine primary watersheds that are tributaries of the Blue 

River, as well as from 50 subwatersheds within and adjacent 

to the primary watersheds in order to provide the Town of 

Breckenridge with a relative measure of which watersheds 

might constitute the most serious postwildfire debris-flow 

hazards. 

This report describes estimates of postwildfire 

probabilities of debris-flow occurrence and estimated volumes 

of debris flow that could be produced in the upper Blue River 

watershed under an assumed moderate- to high-burn severity 

wildfire of all forested areas followed by 1-hour-duration 

rainfall of varying recurrence probabilities. Using information 

provided in this report, land and water-supply managers can 

consider where to concentrate planning for preparedness, 

mitigation, fuels treatments, and resource allocation in 

anticipation of wildfire in the upper Blue River watershed. 

Also, in the event of a large wildfire, this information will 

help managers identify specific watersheds with the greatest 

postwildfire debris-flow hazard. 

Potential postwildfire debris-flow probabilities and 

volumes were based on empirical equations. Probability and 

volume estimates were made for nine primary watersheds and 

50 subwatersheds located within and adjacent to the primary 

watersheds. The nine primary watersheds ranged from 1.57 to 

21.76 km2

 (0.61 to 8.40 mi2), and the 50 subwatersheds ranged 

in drainage area from 0.05 to 11.84 km2

 (0.02 to 4.57 mi2). An 

assumption in the models was that a moderate to severe burn 

affected 100 percent of the forest and shrub stands within the 

watershed. 

Three postwildfire precipitation scenarios were used 

to represent a range of likely precipitation scenarios that 

could occur shortly after a wildfire: (1) a 2-year recurrence, 

1-hour-duration rainfall total of 18.8 mm (0.74 in.); (2) a 

10-year recurrence, 1-hour-duration rainfall total of 31.8 mm 

(1.25 in.); and (3) a 25-year recurrence, 1-hour-duration 

rainfall total of 40.6 mm (1.60 in.). 

All of these precipitation scenarios resulted in debris 

flows from the hypothetically burned watersheds. The 

estimated probabilities for postwildfire debris flows in the 

50 subwatersheds in the upper Blue River watershed ranged 

from less than 1 to 63 percent in response to the 2-year storm, 

less than 1 to 81 percent in response to the 10-year storm, and 

less than 1 to 89 percent in response to the 25-year storm. 

Postwildfire debris-flow probabilities for the nine primary 

watersheds ranged from less than 1 to 12 percent in response 

to the 2-year storm, less than 1 to 24 percent in response to 

the 10-year storm, and less than 1 to 38 percent in response to 

the 25-year storm. Subwatersheds with the lowest postwildfire 

debris-flow probabilities tended to have large areas of alpine 

and subalpine vegetation or sparse forest cover that would 

be minimally affected by wildfire. Subwatersheds with the 

highest probabilities tended to be steep, heavily forested, and 

relatively small in drainage area. Twenty-two subwatersheds 

had a greater than 60 percent probability of producing a debris 

flow in response to a 25-year, 1-hour-duration storm. Many 

of the subwatersheds with the highest debris-flow probability 

in this study were in the north and northeastern part of the 

upper Blue River watershed, notably tributaries to Indiana, 

Pennsylvania, and Spruce Creeks. 

The estimated volumes for potential postwildfire debris 

flows in the 50 subwatersheds in the upper Blue River 

watershed ranged from 300 to 27,400 m3

 in response to the 

2-year storm, 400 to 35,600 m3

 in response to the 10-year 

storm, and 500 to 41,200 m3

 in response to the 25-year storm. 

Subwatersheds for which the smallest postwildfire debrisflow volumes were estimated tended to be those with small 

drainage areas, relatively little forest cover, and less rugged 

topography, or were located in alpine and subalpine areas. 

Subwatersheds with the highest estimated debris-flow volumes 

were those with the largest drainage areas. Fourteen of the 50 

subwatersheds had estimated debris-flow volumes greater than 

10,000 m3

 in response to a 25-year, 1-hour-duration storm. 

Postwildfire debris-flow volume estimates for the nine primary 

watersheds ranged from 9,400 to 227,600 m3

 in response to 

the 2-year storm, 12,200 to 295,400 m3

 in response to the 

10-year storm, and 14,100 to 342,000 m3

 in response to the 

25-year storm. Of the nine primary watersheds, Indiana Creek, 

Pennsylvania Creek, and Spruce Creek had estimated debrisflow volumes greater than 86,000 m3

 in response to a 25-year, 

1-hour-duration storm. 

The watersheds with the greatest potential postwildfire 

and post-precipitation hazards are those with both high 

probabilities of debris-flow occurrence and large estimated 

volumes of material. The 10 subwatersheds with the greatest 

combined relative debris-flow hazards are ind05, ind07, 

ind09, ind10, ind15 in the Indiana Creek watershed; pen02 

in the Pennsylvania Creek watershed; spr05 and spr07 in 

the Spruce Creek watershed; and un102 and un201 in the 

unnamed watersheds. The high probabilities of postwildfire 

debris flows, the associated large debris-flow volumes, and the 

densely populated areas along the creeks and near the outlets 

of the primary watersheds indicate that Indiana, Pennsylvania, 

and Spruce Creeks have a relatively high combined debrisflow hazard. 

The location, percentage of burned area, severity of 

wildfire, and subsequent storm intensity and duration cannot 

be known in advance; however, hypothetical scenarios, such 
References Cited 21

as those used in this report, are useful planning tools for 

conceptualizing potential postwildfire effects. Although the 

estimated probabilities of debris-flow occurrences are high 

and the estimated debris-flow volumes are large, this study did 

not consider whether the hypothetical debris flows will reach 

watershed outlets that are proximal to important infrastructure. 

The models used in this study were developed only to estimate 

postwildfire debris-flow characteristics at the watershed 

outlet. Substantial flooding and other fluvial erosion processes 

that could cause substantial damage also can occur under 

postwildfire conditions. 

Acknowledgments

Roland Viger, of the USGS, assisted with watershed 

delineation by using the Geographic Information System 

Weasel tool. Keelin R. Schaffrath, of the USGS, assisted 

with field verification of previous debris-flow deposits in 

the upper Blue River watershed. Barbara C. Ruddy and 

Andrew E. LaMotte, of the USGS, reviewed the debris-flow 

model metadata. Kenneth R. Odom and Susan H. Cannon, 

of the USGS, performed technical reviews of the report. All 

photographs were taken by John G. Elliott, USGS.

References Cited

Cannon, S.H., 2001, Debris-flow generation from recently 

burned drainage basins: Environmental and Engineering 

Geoscience, v. 7 no. 4, p. 321–341. 

Cannon, S.H., Powers, P.S., and Savage, W.Z., 1998, Firerelated hyperconcentrated and debris flows on Storm King 

Mountain, Glenwood Springs, Colorado, USA: Environmental Geology, v. 34, no. 2–3, p. 210–218.

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

A.H., and Parrett, C., 2010, Predicting the probability and 

volume of postwildfire debris flows in the intermountain 

western United States: Geological Society of America Bulletin, v. 122, p. 127–144. 

Costa, J.E., 1988, Rheologic, geomorphic, and sedimentologic 

differentiation of water floods, hyperconcentrated flows, 

and debris flows, in Flood geomorphology: New York, John 

Wiley and Sons, p. 113–122. 

Costa, J.E., and Jarrett, R.D., 1981, Debris flows in small 

mountain stream channels of Colorado and their hydrologic 

impacts: Bulletin of the Association of Engineering 

Geologists, v. XVIII, no. 3, p. 309–322. 

Elliott, J.G., Char, S.J., Linhart, S.M., Stevens, V.C., and 

O’Neill, G.B., 2007, Sedimentation in Goose-Pasture 

Tarn, 1965–2005: U.S. Geological Survey Scientific 

Investigations Report 2006–5265, 45 p.

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

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

Catherine, Wagner, Jason, Char, S.J., Bauer, M.A., and 

Wilds, S.R., 2005, Analysis and mapping of post-wildfire 

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., Cannon, S.H., Santi, P.M., and Dewolfe, V.G., 

2008, Empirical models to predict the volumes of debris 

flows generated by recently burned basins in the western 

U.S.: Geomorphology, v. 96, p. 339–354, doi: 10.1016/j.

geomorph.2007.02.033.

Griffiths, P.G., Webb, R.H., and Melis, T.S., 1996, Initiation 

and frequency of debris flows in Grand Canyon, Arizona: 

U.S. Geological Survey Open-File Report 96–491, 35 p. 

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water Resources Investigations, book 4, chap. A3, 

510 p., http://pubs.usgs.gov/twri/twri4a3/html/pdf_new.html

(accessed February 26, 2010).

Homer, Colin, Dewitz, Jon, Fry, Joyce, Coan, M., Hossain, 

N., Larson, C., Herold, N., McKerrow, A., VanDriel, J.K., 

and Wickham, J., 2007, Completion of the 2001 National 

Land Cover Database for the conterminous United States: 

Photogrammetric Engineering and Remote Sensing, v. 73, 

no. 4, p. 337–341.

Hosmer, D.W., and Lemeshow, Stanley, 2000, Applied logistic 

regression, 2d ed.: New York, John Wiley & Sons, Inc., 

375 p.

Hungr, O., Morgan, G.C., and Kellerhals, R., 1984, 

Quantitative analysis of debris-torrent hazards for design of 

remedial measures: Canadian Geotechnical Journal, v. 21, 

p. 663–677. 

Keefer, D.K., and Johnson, A.M., 1983, Earth flows—

Morphology, mobilization, and movement: U.S. Geological 

Survey Professional Paper 1264, 56 p.

Kuhn, Gerhard, 2005, Historical perspective of statewide 

streamflows during the 2002 and 1977 droughts in 

Colorado: U.S. Geological Survey Scientific Investigations 

Report 2005–5174, 84 p.

Lindsey, Rebecca, 2002, Satellites aid burn area rehabilitation: 

National Aeronautic and Space Administration online 

report, various pagination, available online at http://

earthobservatory.nasa.gov/Study/BAER/ (accessed February 

15, 2007).

Melton, M.A., 1965, The geomorphic and paleoclimate 

significance of alluvial deposits in southern Arizona: 

Journal of Geology, v. 73, p. 1–38.
22 Estimated Probabilities and Volumes of Postwildfire Debris Flows for the Upper Blue River Watershed, Colorado

Miller, J.F., Frederick, R.H., and Tracey, R.J., 1973, 

Precipitation-frequency atlas of the Western United States, 

v. 3—Colorado: Silver Spring, Md., U.S. Department 

of Commerce, National Oceanic and Atmospheric 

Administration, National Weather Service. 

Pielke, R.A., Sr., Doeskin, N., Bliss, O., Green, T., Chaffin, 

C., Salas, J.D., Woodhouse, C.A., Lukas, J., and Wolter, 

K., 2005, Drought 2002 in Colorado—An unprecedented 

drought or a routine drought?: Pure and Applied 

Geophysics, v. 162, p. 1455–1479. 

Pierson, T.P., 2005, Distinguishing between debris flows 

and floods from field evidence in small watersheds: U.S. 

Geological Survey Fact Sheet 2004–3142, 4 p.

Pierson, T.C., and Costa, J.E., 1987, A rheological classification of subaerial sediment-water flows: Geological Society 

of America, Reviews in Engineering Geology, v. VII, 12 p. 

Schwartz, G.E., and Alexander, R.B., 1995, State Soil Geographic (STATSGO) data base for the conterminous United 

States: U.S. Geological Survey Open-File Report 95–449. 

SPSS, Inc., 2000, SYSTAT 10, Statistics I—Software documentation: Chicago, SPSS, Inc., 663 p. 

Stevens, M.R., Bossong, C.R., Litke, D.W., Viger, R.J., 

Rupert, M.G., and Char, S.J., 2008, Estimated probability of 

post-wildfire debris-flow occurrence and estimated volume 

of debris flows from a pre-fire analysis in the Three Lakes 

Watershed, Grand County, Colorado: U.S. Geological Survey Scientific Investigations Map 3009, 1 sheet.

Tweto, Ogden, 1979, Geologic map of Colorado: U.S. 

Geological Survey Map.

U.S. Department of Agriculture, National Resources Conservation Service, National Soil Survey Center, 1991, State 

Soil Geographic (STATSGO) database: Data use information, Miscellaneous Publication Number 1492, 110 p. 

(Revised July 1994.)

Viger, R.J., and Leavesley, G.H., 2007, The GIS Weasel user’s 

manual: U.S. Geological Survey Techniques and Methods, 

book 6, chap. B4, 201 p., available online at http://pubs.

usgs.gov/tm/2007/06B04/index.html (accessed February 9, 

2010). 
Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/
Elliott and others—Estimated Probabilities and Volumes of Postwildfire Debris Flows for the Upper Blue River Watershed, Colorado—SIR 2011–5039