Prepared in cooperation with Hinsdale County, Colorado

Postwildfire Debris-Flow Hazard Assessment of the Area 

Burned by the 2013 West Fork Fire Complex, Southwestern 

Colorado

By Kristine L. Verdin, Jean A. Dupree, and Michael R. Stevens

Open-File Report 2013–1259

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

SUZETTE M. KIMBALL, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013 

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Suggested citation:

Verdin, K.L., Dupree, J.A, and Stevens, M.R., Postwildfire debris-flow hazard assessment of the area burned by 

the 2013 West Fork Fire Complex, southwestern Colorado: U.S. Geological Survey Open-File Report 2013–1259, 

30 p., 3 plates, http://pubs.usgs.gov/of/2013/1259. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.

Cover photograph: Hope Creek, looking upstream, about 0.40 kilometers upstream from forest road

crossing. (photograph by Michael Stevens, U.S. Geological Survey, September 2013)
iii

Contents

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

Introduction.................................................................................................................................................................... 2

Methods Used To Estimate Debris-Flow Hazards ......................................................................................................... 2

Probability, Volume, and Combined Hazard Ranking of Potential Debris Flows............................................................ 5

Postwildfire Field Visit and Prewildfire Aerial Photography ........................................................................................ 6

Use and Limitations of the Maps................................................................................................................................ 7

References Cited........................................................................................................................................................... 7

Figures 

Figure 1. Location map with burn severity and locations of photographs in figures 2 through 19............................... 9 

Figure 2. Aerial prewildfire photograph (2011) of lower reach of Little Squaw Creek ................................................10

Figure 3. Aerial prewildfire photograph (2011) of upper Trout Creek ........................................................................11

Figure 4. Aerial prewildfire photograph (2011) of upper Goose Creek ......................................................................12

Figure 5. Aerial prewildfire photograph (2011) of area between the mouths of Texas Creek and Workman Creek 

along the Rio Grande ......................................................................................................................................13

Figure 6. Debris-flow or alluvial fan and levees in the Hope Creek watershed..........................................................14

Figure 7. Debris-flow or alluvial fan and debris-flow or alluvial levees in the Hope Creek watershed .......................15

Figure 8. Leading edge of debris-flow or alluvial fan eroded by Hope Creek ............................................................16

Figure 9. Erosion of slope colluvium on slope adjacent to Hope Creek.....................................................................17

Figure 10. Hope Creek photograph showing culvert and road crossing ....................................................................18

Figure 11. Road cut through debris-flow or alluvial fan deposit at the mouth of Lake Creek at U.S. Highway 160 ...19

Figure 12. Little Squaw Creek cutting through debris-flow or alluvial fan at the mouth at the Rio Grande ................20

Figure 13. Relict dry channel through debris-flow or alluvial fan just east of active channel of Little Squaw Creek 

at the mouth at the Rio Grande .......................................................................................................................21

Figure 14. Debris-flow or alluvial levee on fan feature at mouth of Little Squaw Creek at the Rio Grande................22

Figure 15. Trees possibly deformed by debris-flow or alluvial material on Little Squaw Creek fan at the Rio 

Grande Narrow channels with small drainage areas on steep slopes above the Rio Grande .........................23

Figure 17. Erosion channels at Rio Grande downstream from Little Squaw Creek...................................................24

Figure 18. Alluvial fan at inlet to Lake Humphreys on Goose Creek .........................................................................25

Figure 19. Siphon for hydropower generation at Lake Humphreys ...........................................................................26

Table

Table 1. Estimated debris flow probabilities, volumes and combined hazard rankings for the 2013 West Fork 

Fire Complex in southwestern Colorado..........................................................................................................28

Plates

Plate 1. Estimated probability of postwildfire debris flows in the area burned by the 2013 West Fork Fire 

Complex in southwestern Colorado............................................................................................................... link

Plate 2. Estimated volumes of postwildfire debris flows in the area burned by the 2013 West Fork Fire 

Complex in southwestern Colorado............................................................................................................... link

Plate 3. Map showing combined probability and volume relative hazard ranking of postwildfire debris flows 

in the area burned by the 2013 West Fork Fire Complex in southwestern Colorado..................................... link
iv

Conversion Factors

SI to Inch/Pound 

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.308 cubic yard (yd3) 

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

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).
Postwildfire Debris-Flow Hazard Assessment of the Area 

Burned by the 2013 West Fork Fire Complex, 

Southwestern Colorado

By Kristine L. Verdin, Jean A. Dupree, and Michael R. Stevens

Abstract

This report presents a preliminary emergency assessment of the debris-flow hazards from 

drainage basins burned by the 2013 West Fork Fire Complex near South Fork in southwestern 

Colorado. Empirical models derived from statistical evaluation of data collected from recently burned 

basins throughout the intermountain western United States were used to estimate the probability of 

debris-flow occurrence, potential volume of debris flows, and the combined debris-flow hazard ranking 

along the drainage network within and just downstream from the burned area, and to estimate the same 

for 54 drainage basins of interest within the perimeter of the burned area. Input data for the debris-flow 

models included topographic variables, soil characteristics, burn severity, and rainfall totals and 

intensities for a (1) 2-year-recurrence, 1-hour-duration rainfall, referred to as a 2-year storm; 

(2) 10-year-recurrence, 1-hour-duration rainfall, referred to as a 10-year storm; and (3) 25-yearrecurrence, 1-hour-duration rainfall, referred to as a 25-year storm. 

Estimated debris-flow probabilities at the pour points of the 54 drainage basins of interest ranged 

from less than 1 to 65 percent in response to the 2-year storm; from 1 to 77 percent in response to the 

10-year storm; and from 1 to 83 percent in response to the 25-year storm. Twelve of the 54 drainage 

basins of interest have a 30-percent probability or greater of producing a debris flow in response to the 

25-year storm. Estimated debris-flow volumes for all rainfalls modeled range from a low of 2,400 cubic 

meters to a high of greater than 100,000 cubic meters. Estimated debris-flow volumes increase with 

basin size and distance along the drainage network, but some smaller drainages also were predicted to 

produce substantial debris flows. One of the 54 drainage basins of interest had the highest combined 

hazard ranking, while 9 other basins had the second highest combined hazard ranking. Of these 

10 basins with the 2 highest combined hazard rankings, 7 basins had predicted debris-flow volumes 

exceeding 100,000 cubic meters, while 3 had predicted probabilities of debris flows exceeding 

60 percent. The 10 basins with high combined hazard ranking include 3 tributaries in the headwaters 

of Trout Creek, four tributaries to the West Fork San Juan River, Hope Creek draining toward a county 

road on the eastern edge of the burn, Lake Fork draining to U.S. Highway 160, and Leopard Creek on 

the northern edge of the burn. The probabilities and volumes for the modeled storms indicate a potential 

for debris-flow impacts on structures, reservoirs, roads, bridges, and culverts located within and 

immediately downstream from the burned area. U.S. Highway 160, on the eastern edge of the burn area, 

also is susceptible to impacts from debris flows. 
2

Introduction

The West Fork Fire Complex consisted of three lightning-caused wildfires (the West Fork, 

Windy Pass, and Papoose), which burned in the San Juan and Rio Grande National Forests and private 

property in June and July 2013. The West Fork Fire Complex covered approximately 438 square 

kilometers (km2

) (107,776 acres) of land near South Fork in southwestern Colorado (Colo.) and was 

situated at the headwaters and along the drainage basin of the main stem of the Rio Grande and many of 

its tributaries. Burn severity was moderate or high for 59 percent of the area within the burn perimeter 

(fig. 1). The area burned is at risk of postwildfire erosion, such as that caused by debris flows and flash 

floods. 

Debris flows have been documented after many fires in the western United States (Cannon and 

others, 2010) and can threaten lives, property, infrastructure, aquatic habitats, and water supplies. 

Wildfires can denude hillslopes of vegetation and change soil properties that affect watershed hydrology 

and sediment-transport processes. Even small postwildfire rainstorms can increase overland runoff that 

erodes soil, rock, ash, and vegetative debris from hillslopes (Cannon and others, 2008). This increased 

runoff concentrates in stream channels and entrains sediment that can lead to the generation of 

destructive debris flows. Debris-flow hazards are most significant in the 3 years (yr) following wildfires 

(Susan Cannon, U.S. Geological Survey, written commun., 2010). 

The purpose of this report, prepared in cooperation with Hinsdale County, Colorado, is to 

present a preliminary emergency assessment of the debris-flow hazards from basins burned by the 2013 

West Fork Fire Complex. Estimates of the predicted probability of debris-flow occurrence and volume 

of debris along the drainage network throughout the entire area are provided, as well as estimates for 

drainage basins above 54 selected basin outlets in response to 3 design storms: (1) 2-year-recurrence, 

1-hour duration rainfall, referred to as a 2-year storm (a 50-percent chance of occurrence in any given 

year); (2) 10-year-recurrence, 1-hour-duration rainfall, referred to as a 10-year storm (a 10-percent 

chance of occurrence in any given year); and (3) 25-year-recurrence, 1-hour-duration rainfall, referred 

to as a 25-year storm (a 4-percent chance of occurrence in any given year). 

Methods Used To Estimate Debris-Flow Hazards

A set of empirical equations (models) documented in Cannon and others (2010) and derived 

from statistical evaluation of data collected from recently burned basins throughout the intermountain 

western United States were used to estimate the probability of debris-flow occurrence and volumes of 

debris flows along the drainage network and for 54 selected drainage basins of interest. The regression 

equation of debris-flow probability (eq. 1) is as follows: 

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

where

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

e x is the exponential function where e represents the mathematical constant 2.718. 

Equation 2 is used to calculate x:

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

+ 0.07(I) + 0.2(%C) – 0.4(LL), (2)

where

%SG30 is the percentage of the drainage-basin area with slope equal to or greater than 

30 percent; 
3

R is drainage-basin ruggedness: the change in drainage-basin elevation (meters) divided 

by the square root of the drainage-basin area (square meters) (Melton, 1965); 

%AB is the percentage of drainage-basin area burned at moderate to high severity; 

I is average storm intensity (calculated by dividing total storm rainfall [Perica and 

others, 2013] by the storm duration, in millimeters per hour); 

%C is clay content of the soil (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 plastic to liquid behavior (Das, 1983). 

Cannon and others (2010) also developed an empirical model that can be used to estimate the 

volume of debris flow that would likely be produced from recently burned drainage basins: 

Ln (V) = 7.2 + 0.6(Ln SG30) + 0.7(AB)

0.5 + 0.2(T)0.5 + 0.3, (3) 

where

V is the debris-flow volume, including water, sediment, and debris (cubic meters); 

SG30 is the area of a drainage basin with slopes equal to or greater than 30 percent 

(square kilometers); 

AB is the drainage-basin area burned at moderate to high severity (square kilometers); 

T is the total storm rainfall (millimeters); and 

0.3 is a bias-correction factor that changes the predicted estimate from a median to a 

mean value (Helsel and Hirsch, 2002). 

Values for both probability and volume were obtained along drainage networks using the 

continuous parameterization technique (Verdin and Greenlee, 2003; Verdin and Worstell, 2008). With 

this technique, estimates of debris-flow probability and volume (Cannon and others, 2010) were 

obtained for every 10-meter (m) pixel along the drainage network (plates 1 and 2) as a function of 

conditions in the drainage basin at elevations higher than each 10-m pixel. The technique used here 

allows for a synoptic view of conditions throughout the entire study area, which can be used to identify 

specific 10-m cells or stream reaches that might be vulnerable to debris flows; the technique also aids in 

locating sites for installation of precipitation and streamgages and in identifying potential erosionmitigation sites. 

The base layer upon which the continuous-parameterization layers are built is the 1/3-arc-second 

National Elevation Dataset (Gesch and others, 2002). This digital elevation model (DEM) was projected 

into a Colorado-appropriate projection system (UTM, Zone 13) and processed using standard DEMconditioning tools in ArcGIS (ESRI, 2009). Once the overland flow structure was derived using the 

DEM (in the form of a flow-direction matrix), the independent variables driving the probability and 

volume equations were evaluated for every grid cell within the extent of the DEM using the continuousparameterization approach. 

The independent variables necessary for evaluation of the probability and volume equations are 

of four types:

1. Topographic variables: The slope variables (%SG30, eq. 2 and SG30, eq. 3) along with the 

ruggedness (R, eq. 2) were all derived from the 1/3-arc-second DEM (Gesch and others, 2002) 

using standard geographic information system (GIS) processing techniques; 

2. Soil variables: Two soil variables are used in the probability equation (eqs. 1 and 2). Both the 

percent clay and liquid limit of the soil were taken from the State Soil Geographic (STATSGO) 

database (National Soil Survey Center, 1991);
4

3. Burned area variables: The final burn severity dataset was obtained in the form of a shapefile 

from the Rio Grande Watershed Emergency Action Coordination team (RWEACT). Polygons 

within this dataset with a CLASS attribute of “High Burned” or “Moderate Burned” were 

selected to produce the mask of areas burned at moderate to high severity. This mask was used 

to create the required burn variables (%AB, eq. 2 and AB, eq 3); 

4. Precipitation variables: Three storm events were modeled: (a) 2-year-recurrence, 1-hour duration 

rainfall, referred to as a 2-year storm (a 50-percent chance of occurrence in any given year); 

(b) 10-year-recurrence, 1-hour-duration rainfall, referred to as a 10-year storm (a 10-percent 

chance of occurrence in any given year); and (c) 25-year-recurrence, 1-hour-duration rainfall, 

referred to as a 25-year storm (a 4-percent chance of occurrence in any given year). These design 

events were defined using NOAA Atlas 14 (Perica and others, 2013) and its associated 

Precipitation Frequency (PF) Data Server. The PF Data Server allows users to download the 

spatially varying precipitation frequency estimates for entire states. Using these digital data

allows the design storms to vary spatially, reflecting the orographic effects of the mountainous 

terrain and the natural variation in precipitation across the large burn area. The extremes of the 

design storms are (1) 2-year storm precipitation ranging from 14 to 24 millimeters (mm) across 

the burn area with an average precipitation of 19 mm; (2) 10-year storm ranging from 21 to 

34 mm across the burn area with an average precipitation of 29 mm; and (3) 25-year storm 

ranging from 27 to 45 mm across the burn area with an average precipitation of 35 mm. The 

higher precipitation values correspond to orographic highs with the lower precipitation values 

occurring along the Rio Grande valley.

Once the surfaces of the independent variables were developed, the probability and volume 

equations were solved using map algebra for each grid cell along the drainage network, thus deriving 

the probability and volume surfaces. 

Debris-flow hazards along a stream channel and at the pour point of a selected basin also can be 

represented by a combined relative debris-flow hazard ranking that is based on a combination of both 

probability of occurrence and volume (Cannon and others, 2010). For this assessment, the estimated 

values of debris-flow probability and volume were categorized into five ranked classes (following the 

class break-out shown on plates 1 and 2), and these class ranks were averaged to calculate the combined 

probability and volume relative hazard ranking (plate 3). This combined hazard ranking identifies a 

possible range of response from drainage basins considering both the probability of occurrence of a 

debris flow and the predicted volume should a debris flow occur (Cannon and others, 2010). For 

example, the most hazardous drainage basins will have both the highest probabilities of occurrence and 

the largest estimated volumes of material. Slightly less hazardous would be drainage basins modeled 

with a combination of either low probabilities and larger volume estimates or high probabilities and 

smaller volume estimates. Since five ranked classes were used, the maximum possible combined hazard 

ranking is 5. This would occur if both the probability and volume values fell into the highest category 

(probability in excess of 60 percent and volume in excess of 100,000 cubic meters [m3

]) or one of the 

probability and volume were in the maximum category and the other was in the second to the highest 

category (for an averaged value of 4.5, which is rounded to 5 for this report).

Following calculation of debris-flow probabilities, volumes, and combined relative debris-flow 

hazard rankings continuously along the drainage networks, the 54 basins of interest were delineated and 

numbered to aid in discussion of model results. These delineated drainage basins were selected to meet 

two criteria which were used in the original development of the probability and volume equations: (1) 

the size of the contributing area and (2) the size of the area burned at moderate to high severity upstream 
5

from the pour point was within the range of the data used in the development of the probability and 

volumes equations (0.01 to ≈30 square kilometer (km2) for contributing area and 0.01 to ≈15 km2 for 

moderate/high burned area) (Joe Gartner, U.S. Geological Survey, written commun., 2013). Debris-flow 

probabilities and volumes were extracted from the probability and volume surface for these locations

and combined hazard rankings developed. These values are shown in plates 1, 2, and 3 and are 

summarized in table 1. Probability, volume, or combined hazard ranking for the numbered drainage 

basins represents the value at the basin outlet. Probability or hazard ranking may be higher or lower and 

volume rankings may be smaller for subbasins or points within the delineated drainage basins, as 

indicated by the stream segment analysis within the drainage basins. 

Probability, Volume, and Combined Hazard Ranking of Potential Debris Flows

In response to the 2-year storm, one basin affected by the burn (basin 46; plate 1, table 1) was 

identified as having a probability of debris-flow occurrence greater than or equal to 60 percent, and an 

additional three basins (19, 21, and 47) had probabilities between 30 and 45 percent. All four of these 

basins are small, ranging in size from 0.3 km2

 to 1.9 km2. Estimated volumes of debris flows for these 

four basins ranged from 2,400 to 12,000 cubic meters (m3

). One of these basins (46), a tributary to West 

Fork San Juan River, has a combined hazard ranking of 4 (the second highest modeled hazard ranking) 

while the remaining basins (19, 21, and 47) have combined hazard rankings of 2 or 3. 

The 10-year storm resulted in the same basin (46; plate 1, table 1) with a probability of debris 

flow in excess of 60 percent, corresponding volume estimate of 11,000 m3, and combined hazard 

ranking of 4. Seven additional basins (16, 19, 21, 34, 39, 47, and 53) had probabilities of debris flow 

between 30 and 59 percent. These seven basins were modeled as producing volumes from 2,900 to 

greater than 100,000 m3. Two of these eight basins (Decker Creek [39] and a tributary to the West Fork 

San Juan River [46]) had combined hazard rankings of 4 while the other five basins had a combined 

hazard ranking of 3. 

The 25-year storm shows additional basins with higher probabilities of debris flows than would 

result from the 2-year and 10-year storms (plate 1, table 1). Higher debris-flow volumes also are likely 

to be produced in response to this storm. Four basins (two tributaries to Trout Creek [19 and 21] and 

two tributaries to West Fork San Juan [46 and 47]) show a probability of debris flow in excess of 

60 percent. An additional 8 basins (7, 15, 16, 32, 34, 39, 41, and 53) have probabilities between 30 and 

59 percent. These 8 basins were modeled as producing volumes from 8,700 to greater than 100,000 m3

. 

For the 25-year storm, only 10 out of the 54 basins of interest are modeled as expected to produce 

debris-flow volumes less than about 10,000 m3

. One basin (Decker Creek [39]) is modeled as having the 

maximum combined hazard ranking of 5. This basin has a modeled probability of 45 percent and an 

expected volume greater than 100,000 m3

. An additional nine basins (17, 18, 19, 33, 41, 44, 46, 47, and 

48) are modeled as having a combined hazard ranking of 4. These nine basins have probabilities from 

16 to 83 percent in response to the 25-year storm and corresponding volumes from 12,000 m3 to greater 

than 100,000 m3. Six of these nine basins (17, 18, 33, 41, 44, and 48) had modeled volumes in excess of 

100,000 m3. 

The only basin with a combined hazard ranking of 5 is Decker Creek (39), which drains to the 

eastern edge of the burn and contributes flow to a small reservoir on the eastern edge of the burn. This 

basin, with a modeled probability in response to the 25-year storm of 45 percent and a modeled volume 

of greater than 100,000 m3 could contribute large volumes of material to the small reservoir in the event 

of a debris flow. Of the basins with a combined hazard rankings of 4 in response to the 25-year storm, 

three (East Trout Creek [17], West Trout Creek [18], and Trout Creek tributary 1 [19]) are located in 

headwaters of Trout Creek, a tributary to the Rio Grande. Four others (44, 46, 47, and 48) are tributaries 
6

to the West Fork San Juan River. Leopard Creek (33) draining north out of the burned area and Hope 

Creek (41) draining east to a county road near U.S. Highway 160 also have combined hazard ranking of 

4. Decker Creek (39) and Hope Creek (41), along with basin 40 (Lake Fork, combined hazard ranking 

of 3) could contribute debris flows which could impact U.S. Highway 160 on the eastern edge of the 

West Fork wildfire burn area (plate 3). Additionally, even small debris flows that affect structures and 

reservoirs at the basin outlets could cause damage. 

Postwildfire Field Visit and Prewildfire Aerial Photography

A field visit to the area of the West Fork Fire Complex burn area on September 4, 2013, and 

inspection of prewildfire aerial photography indicated that debris flows, floods, or both, have produced 

debris-flow and alluvial fans in and adjacent to the burn area (figs. 2–19). Locations for each 

photograph in figures 2 through 19 are shown on figure 1. Whether these debris-flow or alluvial fans are 

caused or enhanced by increased runoff because of wildfires is unknown and beyond the scope of this 

report. However, the presence of these features throughout the area burned by the West Fork Fire 

Complex suggest that the region is at least minimally susceptible to potentially hazardous flooding or 

debris flows. 

Caution may be necessary in or near the burn area during storms or high snowmelt regardless of 

the predicted debris-flow probability estimates in this report. The empirical models used in this report 

were constructed with variables important to debris-flow prediction only and are not designed to predict 

floods. Burn areas commonly produce floods during snowmelt and rainstorms that have hyperconcentrated sediment concentrations, which can be as damaging as debris flows, and may be more 

likely to travel to the mouth of large drainage basins than debris flows. 

Areas where field reconnaissance was not possible were evaluated using prewildfire aerial 

photography and are shown in figures 2–5 and identified on figure 1. Figure 2 shows fan structures at 

the mouth of both Squaw and Little Squaw Creeks at the Rio Grande. Figure 3 shows a reach of upper 

Trout Creek and basin 19 and sediment erosion areas, which has an estimated debris-flow probability of 

64 percent and debris-flow volume of 14,000 cubic meters in response to a 25-yr recurrence storm. Two 

fan structures, which may be alluvial or debris-flow deposits also are shown on figure 3. Figure 4 shows 

a reach of Goose Creek upstream from Lake Humphreys (plates 1–3) that contains erosion source areas 

for the fan deposits at the mouths of small tributary drainages. Figure 5 shows several fan deposits at the 

mouths of various watersheds between the mouths of Texas and Workman Creeks. Again, there is no 

direct evidence that wildfire was involved in the events that created these fan deposits. 

One of the sites visited in the field was at Hope Creek (figs. 6–10), located about 3 miles east of 

U.S. Highway 160 where the forest road crosses Hope Creek. Figures 6–8 show evidence of debris-flow 

or alluvial deposits along Hope Creek including a fan structure and levees along the stream. Figure 9 

shows erosion on a side-slope along Hope Creek that occurred in response to summer rains in 2013 after 

the West Fork Complex wildfire. Figure 10 shows the culvert at the forest road crossing Hope Creek. 

Culverts and bridges may become blocked during high streamflows and could back up water and cause 

failure of road embankments. The next drainage basin to the north is Lake Creek; figure 11 shows a 

debris-flow or alluvial fan cut by U.S. Highway 160 at the mouth at South Fork Rio Grande (fig. 1, 

plates 1–3).

Squaw Creek and Little Squaw Creek flow into the Rio Grande just downstream from Rio 

Grande Reservoir near Creede, Colorado. During a site visit to the mouth of Little Squaw Creek at the 

Rio Grande (fig. 1, plates 1–3), evidence of a large debris-flow or alluvial fan was identified (fig. 2). 

Unsorted deposits with large cobbles, boulders, and deformed trees on the surface of the fan are shown 

in figures 12–15 and small erosion channels were observed throughout the burned and unburned 
7

watershed in the study area. Figures 16 and 17 (location in fig. 1) provide examples of these channels 

along the Rio Grande downstream for a few miles below Rio Grande Reservoir (plates 1–3). The narrow 

channels with small drainage areas on steep slopes above the Rio Grande may be affected by 

avalanches, dry ravel, alluvial (water dominated) erosion, and debris-flow processes (fig. 16). The 

channels, while apparently active, do not contain large accumulations of debris or significant fans. 

Goose Creek is a large basin that drains north to the Rio Grande near Wagon Wheel Gap and 

was partially burned in the West Fork Fire Complex (figs. 1 and 4, plates 1–3). The upper part of the 

drainage basin was burned, particularly the area upstream from Lake Humphreys (fig. 18), a reservoir 

formed by a large concrete arch dam with hydropower generation capacity (fig. 19). Figure 18 shows an 

alluvial fan at the Goose Creek inlet to Lake Humphreys (fig. 1). The burned slope in the background 

(top right) is basin 32 (debris-flow probability of 30 percent and debris-flow volume of 8,700 cubic 

meters in response to the 25-year storm), which could flow into the inlet area if triggered. Figure 19 is 

a photograph of the siphon for hydropower generation at Lake Humphreys that would likely be affected 

by floating debris flowing into the lake from upper Goose Creek. 

Use and Limitations of the Maps 

This assessment provides estimates of debris-flow probability and volume for drainage basins 

burned by the West Fork Fire Complex in response to three design storms based on predictive models 

developed from data from burned areas throughout the western United States. Larger, less-frequent 

storms are more likely to produce much larger debris flows. Because individual storms may not affect 

the entire area at any given time, debris flows may not be produced from all basins during storms. The 

estimates are most significant in the 3 years following the wildfire (Susan Cannon, U.S. Geological 

Survey, written commun., 2010). 

Plates 1–3 may be used to prioritize areas where emergency-flood warnings or erosion 

mitigation may be needed prior to rainstorms within these basins, their outlets, or areas downstream 

from these basins. This assessment evaluates only potential postwildfire debris flows (Cannon and 

others, 2007). Substantial hazards from flash floods without debris flow may persist for many years 

after a fire. 

This work is preliminary and is subject to revision and is being provided because of the need for 

timely “best science” information. The assessment is provided on the condition that neither the U.S. 

Geological Survey nor the United States Government may be held liable for any damages resulting from 

the authorized or unauthorized use of the assessment. 

References Cited

Cannon, S.H., Gartner, J.E., and Michael, J.A., 2007, Methods for the emergency assessment of debrisflow hazards from basins burned by the fires of 2007, southern California: U.S. Geological Survey 

Open-File Report 2007–1384, 10 p. (Also available at http://pubs.usgs.gov/of/2007/1384/, accessed 

September 2013.) 

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. 

Cannon, S.H., Gartner, J.E., Wilson, R.C., and Laber, J.L., 2008, Storm rainfall conditions for floods 

and debris flows from recently burned areas in southwestern Colorado and southern California: 

Geomorphology, v. 96, p. 250–269, doi:10.1019/j.geomorph.2008.03.019. 

Das, B.M., 1983, Advanced soil mechanics: New York, McGraw-Hill, 511 p. 
8

ESRI, 2009, ArcGIS v. 9.3: Redlands, Calif., ESRI. 

Gesch, D., Oimoen, M., Greenlee, S., Nelson, C., Steuck, M., and Tyler, D., 2002, The national 

elevation dataset: Photogrammetric Engineering and Remote Sensing, v. 68, no. 1, p. 5–11. 

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in water resources: New York, Elsevier 

Studies in Environmental Science, v. 49, 529 p. 

Melton, M.A., 1965, The geomorphic and paleoclimate significance of alluvial deposits in southern 

Arizona: Journal of Geology, v. 73, p. 1–38. 

National Soil Survey Center, 1991, State Soil Geographic (STATSGO) database—Data use 

information: U.S. Department of Agriculture, National Resources Conservation Service, 

Miscellaneous Publication 1492, 110 p. [Revised July, 1994] 

Perica, S., Martin, D., Pavlovic, S., Roy, I., St. Laurent, M., Trypaluk, C., Unruh, D., Yekta, M., and 

Bannin, G., Precipitation-frequency atlas of the United States, Volume 8 Version 2.0: Midwestern 

states (Colorado, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Oklahoma, 

South Dakota, Wisconsin): Silver Spring, Md., U.S. Department of Commerce, National Oceanic and 

Atmospheric Administration, National Weather Service, accessed September 2013, at 

http://www.nws.noaa.gov/oh/hdsc/PF_documents/Atlas14_Volume8.pdf. 

Verdin, K.L., and Greenlee, S., 2003, Continuous parameterization using EDNA, in 2003 ESRI User’s 

Conference, San Diego, Calif., July 7–11, 2003, Proceedings: Redlands, Calif., ESRI, accessed 

September 2013, at http://gis.esri.com/library/userconf/proc03/p0617.pdf. 

Verdin, K.L., and Worstell, B., 2008, A fully distributed implementation of mean annual streamflow 

regional regression equations: Journal of the American Water Resources Association, v. 44, p. 1537–

1547, doi: 10.1111/j.1752-1688.2008.00258.x 
9

Figure 1. Location map with burn severity and locations of photographs in figures 2 through 19.
10

Figure 2. Aerial prewildfire photograph (NAIP Imagery, U.S. Department of Agriculture, Farm Service Agency, 

2011) of lower reach of Little Squaw Creek and Squaw Creek (fig. 1) which flows from bottom to top of the 

image (south to north) into the Rio Grande west of Creede, Colo. Erosive areas are prominent in the lower part 

of the photograph, and a debris-flow fan is evident at the mouths of the watersheds at the Rio Grande.
11

Figure 3. Aerial prewildfire photograph (NAIP Imagery, U.S. Department of Agriculture, Farm Service Agency, 

2011) of upper Trout Creek (basin 19, fig. 1) which flows from bottom to top of the image (south to north) and 

then discharges into the Rio Grande west of Creede, Colo. Sediment erosion areas are prominent adjacent to 

beetle-killed spruce which burned in the West Fork Complex fire (Papoose burn). 
12

Figure 4. Aerial prewildfire photograph (NAIP Imagery, U.S. Department of Agriculture, Farm Service Agency, 

2011) Goose Creek (fig. 1) which flows from bottom to top of the image (south to north) and eventually 

discharges into the Rio Grande east of Creede, Colo. Sediment erosion areas (located near basin 28 on 

plate 1) are prominent adjacent to beetle-killed spruce which burned in the West Fork Complex fire 

(West Fork burn). 
13

Figure 5. Aerial prewildfire photograph (NAIP Imagery, U.S. Department of Agriculture, Farm Service Agency, 

2011) of area between the mouths of Texas Creek and Workman Creek along the Rio Grande (fig. 1). Fan 

structures of debris-flow, alluvial, or mixed origin are visible at the mouths of several creeks. 
14

Figure 6. Debris-flow or alluvial fan and levees in the Hope Creek watershed (basin 41), view upstream (fig. 1). 

(photograph by Michael Stevens)
15

Figure 7. Debris-flow or alluvial fan and debris-flow or alluvial levees in the Hope Creek watershed (basin 41) 

(fig. 1), taken a short distance upstream from figure 6. (photograph by Michael Stevens)
16

Figure 8. Leading edge of debris-flow or alluvial fan eroded by Hope Creek (fig.1). (photograph by Michael 

Stevens) 
17

Figure 9. Erosion of slope colluvium from late summer postfire rainfall in 2013 on slope adjacent to Hope Creek 

(fig.1). (photograph by Michael Stevens)
18

Figure 10. Hope Creek (fig. 1) could transport debris that could obstruct the culvert at the forest road crossing and 

cause backup of water and possible failure of the road embankment. (photograph by Michael Stevens)
19

Figure 11. Road cut through debris-flow or alluvial fan deposit at the mouth of Lake Creek at U.S. Highway 160 

(fig. 1). (photograph by Michael Stevens)
20

Figure 12. Little Squaw Creek (basin 4, plates 1-3) cutting through debris-flow or alluvial fan at the mouth at the 

Rio Grande (fig. 1). (photograph by Michael Stevens)
21

Figure 13. Relict dry channel through debris-flow or alluvial fan just east of active channel of Little Squaw Creek at 

the mouth at the Rio Grande (fig. 1). View downstream toward Rio Grande. (photograph by Michael Stevens)
22

Figure 14. Debris-flow or alluvial levee on fan feature at mouth of Little Squaw Creek at the Rio Grande (fig. 1) 

(Note rock resting on tree trunk). (photograph by Michael Stevens)
23

Figure 15. Trees possibly deformed by debris-flow or alluvial material on Little Squaw Creek fan at the Rio Grande 

(fig. 1). (photograph by Michael Stevens)
24

Figure 16. Narrow channels (fig. 1) with small drainage areas on steep slopes above the Rio Grande (flows right to 

left) that may be affected by avalanche, dry ravel, alluvial (water dominated) erosion, and debris-flow 

processes. (photograph by Michael Stevens)
25

Figure 17. Erosion channels at base of steep slope at Rio Grande downstream from Little Squaw Creek (fig. 1), 

while apparently active, do not have large accumulations of debris or significant fans, possibly indicating 

relatively small potential debris-flow volumes. (photograph by Michael Stevens)
26

Figure 18. Alluvial fan at inlet to Lake Humphreys on Goose Creek (fig. 1). Burned slope in the background 

(top right) is basin 32 (probability of 30 percent and volume of 8,700 cubic meters with the 25-year recurrence 

1-hour rainfall) which could flow into the inlet area if triggered. (photograph by Michael Stevens)
27

Figure 19. Siphon for hydropower generation at Lake Humphreys (fig. 1) could be affected by floating debris 

flowing into the lake from upper Goose Creek. (photograph by Michael Stevens)
28

Table 1. Estimated debris-flow probabilities, volumes, and combined hazard rankings for the 2013 West Fork Fire Complex in southwestern 

Colorado.

[dd, decimal degrees; mm, millimeters; km2, square kilometers; %, percent, m3, cubic meters; <, less than; >, greater than]

Basin 

identifier Description

Basin Pour Point

2-year/1-hour precipitation 10-year/1-hour precipitation 25-year/1-hour precipitation

14–24 mm (average 19 mm) 21–34 mm (average 29 mm) 27–45 mm (average 35 mm)

Latitude

(dd)

Longitude

(dd)

Drainage 

area

(km2) 

Area burned 

at moderate 

or high 

severity

(km2) 

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

1 House Canyon 37.7889 –107.1832 10.7 2.4 1 30,000 2 2 38,000 2 3 43,000 2

2 Road Canyon 37.7874 –107.1732 25.3 3.9 1 73,000 3 1 91,000 3 2 >100,000 3

3 Ingalls Gulch 37.7794 –107.1477 2.7 0.8 1 10,000 2 3 13,000 2 4 15,000 2

4 Little Squaw Creek 37.6879 –107.2007 30.0 2.4 <1 67,000 3 1 85,000 3 1 98,000 3

5 Rio Grande tributary 1 37.7299 –107.2266 0.9 0.7 4 3,100 1 7 3,900 1 12 4,500 1

6 Papoose Creek 37.7362 –107.1940 8.3 3.7 2 33,000 2 4 42,000 2 7 48,000 2

7 Rio Grande tributary 2 37.7374 –107.1872 1.3 1.1 14 6,600 2 27 8,500 2 39 9,800 3

8 Texas Creek 37.7444 –107.1560 30.0 6.3 <1 73,000 3 1 94,000 3 1 >100,000 3

9 Texas Creek 

tributary

37.7558 –107.1437 1.9 1.0 2 5,800 2 3 7,400 2 6 8,500 2

10 Rio Grande tributary 3 37.7553 –107.1182 1.3 1.1 9 5,600 2 19 7,200 2 28 8,300 2 

11 Rio Grande tributary 4 37.7460 –107.1124 1.6 1.1 7 7,400 2 14 9,500 2 22 11,000 2 

12 Rio Grande tributary 5 37.7409 –107.1079 1.1 0.7 7 6,100 2 14 7,800 2 22 8,900 2 

13 Fern Creek 37.7384 –107.0892 8.4 5.7 9 51,000 3 18 66,000 3 27 75,000 3 

14 Woodfern Creek 37.7354 –107.0813 10.1 5.1 4 62,000 3 9 79,000 3 15 90,000 3 

15 Workman Creek 37.7284 –107.0756 4.7 4.3 15 22,000 2 27 28,000 3 38 32,000 3 

16 Cliff Creek 37.7071 –107.0500 5.0 5.0 19 20,000 2 32 25,000 3 43 28,000 3

17 East Trout Creek 37.6360 –107.0938 29.5 13.2 8 >100,000 3 14 >100,000 3 21 >100,000 4

18 West Trout Creek 37.6448 –107.0914 28.3 13.2 6 >100,000 3 14 >100,000 3 21 >100,000 4

19 Trout Creek 

tributary 1

37.6498 –107.0889 1.3 1.2 34 9,700 3 52 12,000 3 64 14,000 4

20 Copper Creek 37.6582 –107.0804 7.7 1.9 1 23,000 2 1 29,000 2 2 32,000 2 

21 Trout Creek 

tributary 2

37.6654 –107.0712 0.3 0.3 35 2,400 2 52 2,900 3 63 3,300 3

22 Trout Creek 

tributary 3

37.6655 –107.0711 1.2 0.4 1 5,400 2 1 6,700 2 2 7,500 2

23 Jumper Creek 37.6720 –107.0602 8.4 3.0 2 37,000 2 4 47,000 2 6 54,000 3 

24 Trout Creek 

tributary 4

37.6789 –107.0347 3.3 2.0 2 8,300 2 4 10,000 2 6 11,000 2
29

Table 1. Estimated debris-flow probabilities, volumes, and combined hazard rankings for the 2013 West Fork Fire Complex in southwestern 

Colorado.—Continued

[dd, decimal degrees; mm, millimeters; km2, square kilometers; %, percent, m3, cubic meters; <, less than; >, greater than]

Basin 

identifier Description

Basin Pour Point

2-year/1-hour precipitation 10-year/1-hour precipitation 25-year/1-hour precipitation

14–24 mm (average 19 mm) 21–34 mm (average 29 mm) 27–45 mm (average 35 mm)

Latitude

(dd)

Longitude

(dd)

Drainage 

area

(km2) 

Area burned 

at moderate 

or high 

severity

(km2) 

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

25 Middle Creek 37.6485 –107.0358 11.1 5.3 2 40,000 2 3 48,000 2 5 53,000 3

26 Goose Creek 1 37.6016 –106.8959 29.6 9.0 3 >100,000 3 5 >100,000 3 8 >100,000 3

27 Goose Creek 

tributary 1

37.6050 –106.8961 4.0 0.5 <1 12,000 2 1 15,000 2 1 16,000 2

28 Goose Creek 

tributary 2

37.6124 –106.8963 1.4 0.2 <1 6,000 2 1 7,200 2 1 8,100 2

29 Goose Creek 

tributary 3

37.6197 –106.8901 1.6 0.4 1 7,700 2 2 9,200 2 3 10,000 2

30 Goose Creek 

tributary 4

37.6312 –106.8780 6.9 1.5 1 23,000 2 1 27,000 2 2 31,000 2

31 Goose Creek 

tributary 5

37.6387 –106.8731 2.5 1.7 5 11,000 2 9 13,000 2 14 14,000 2

32 Goose Creek 

tributary 6

37.6686 –106.8569 1.3 1.0 15 6,600 2 23 7,800 2 30 8,700 3

33 Leopard Creek 37.6782 –106.8232 23.7 15.0 6 >100,000 3 11 >100,000 3 16 >100,000 4 

34 Leopard Creek 

tributary

37.6817 –106.8000 2.7 2.3 19 14,000 2 31 17,000 3 43 19,000 3

35 Elk Creek 37.6952 –106.7408 10.4 4.7 3 53,000 3 5 64,000 3 8 72,000 3 

36 Raspberry Gulch 37.6804 –106.7401 6.1 4.3 5 23,000 2 10 28,000 2 15 32,000 3 

37 Trout Creek (south) 

tributary

37.6395 –106.7519 12.1 5.2 2 46,000 2 3 56,000 3 5 63,000 3

38 Trout Creek (south) 37.6311 –106.7620 6.2 4.6 7 24,000 2 14 29,000 2 20 32,000 3 

39 Decker Creek 37.6131 –106.7608 10.7 9.1 21 84,000 3 34 >100,000 4 45 >100,000 5

40 Lake Fork 37.5626 –106.7687 28.5 14.0 3 >100,000 3 6 >100,000 3 10 >100,000 3 

41 Hope Creek 37.5473 –106.7876 27.6 15.0 14 >100,000 3 24 >100,000 4 34 >100,000 4 

42 Cimarron Creek 37.5144 –106.9578 29.5 6.2 3 >100,000 3 7 >100,000 3 10 >100,000 3 

43 Cimarron Creek 

tributary

37.5142 –106.9579 5.0 1.3 4 23,000 2 8 27,000 2 12 30,000 2

44 West Fork San Juan 

River 1

37.5129 –106.9550 15.0 3.8 10 81,000 3 18 98,000 3 25 >100,000 4

45 West Fork San Juan 

River tributary 1

37.5082 –106.9467 1.3 0.1 3 5,800 2 6 7,000 2 8 7,700 2
30

Table 1. Estimated debris-flow probabilities, volumes, and combined hazard rankings for the 2013 West Fork Fire Complex in southwestern 

Colorado.—Continued

[dd, decimal degrees; mm, millimeters; km2, square kilometers; %, percent, m3, cubic meters; <, less than; >, greater than]

Basin 

identifier Description

Basin Pour Point

2-year/1-hour precipitation 10-year/1-hour precipitation 25-year/1-hour precipitation

14–24 mm (average 19 mm) 21–34 mm (average 29 mm) 27–45 mm (average 35 mm)

Latitude

(dd)

Longitude

(dd)

Drainage 

area

(km2) 

Area burned 

at moderate 

or high 

severity

(km2) 

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

Probability

(%)

Volume

(m3) 

Combined 

hazard 

ranking

46 West Fork San Juan 

River tributary 2

37.4950 –106.9352 1.2 0.9 65 9,100 4 77 11,000 4 83 12,000 4

47 West Fork San Juan 

River tributary 3

37.4932 –106.9354 1.9 1.1 40 12,000 3 55 15,000 3 65 16,000 4

48 Beaver Creek 37.5244 –106.9118 30.0 9.4 6 >100,000 3 11 >100,000 3 17 >100,000 4

49 Beaver Creek 

tributary

37.5118 –106.9233 2.7 0.7 4 13,000 2 7 15,000 2 10 17,000 2

50 West Fork San Juan 

River tributary 4

37.4867 –106.9303 2.1 0.3 2 8,500 2 4 10,000 2 5 11,000 2

51 West Fork San Juan 

River tributary 5

37.4849 –106.9302 3.1 0.7 5 15,000 2 9 18,000 2 13 20,000 2

52 Burro Creek 37.4724 –106.9272 7.0 1.0 1 24,000 2 2 29,000 2 3 32,000 2

53 West Fork San Juan 

River tributary 6

37.4719 –106.9263 1.2 0.6 19 8,300 2 30 9,900 2 38 11,000 3

54 Treasure Creek 37.4265 –106.8227 3.2 0.5 1 12,000 2 2 15,000 2 3 16,000 2