U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2013–1108

Prepared in cooperation with U.S. Department of Agriculture Forest Service, 

Lincoln National Forest

Postwildfire Debris-Flow Hazard Assessment of the 

Area Burned by the 2012 Little Bear Fire, South-Central 

New Mexico
Front cover:

Top, Severely burned hillslope, Little Bear Fire, 2012, south-central New Mexico.

Bottom, Sediment deposition at the mouth of Spring Canyon following the Little Bear Fire, 2012, southcentral New Mexico.

Back cover:

Top, Example of steep terrain in the area burned by the Little Bear Fire, 2012, south-central New 

Mexico.

Middle and bottom, Severely burned hillslope with fall vegetation regrowth, near Monjeau Peak, 

following the Little Bear Fire, 2012, south-central New Mexico.
Postwildfire Debris-Flow Hazard 

Assessment of the Area Burned by the 2012 

Little Bear Fire, South-Central New Mexico

By Anne C. Tillery and Anne Marie Matherne

Prepared in cooperation with U.S. Department of Agriculture Forest Service, 

Lincoln National Forest

Open-File Report 2013–1108

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

Suggested citation:

Tillery, A.C., and Matherne, A.M., 2013, Postwildfire debris-flow hazard assessment of the area burned by the 

2012 Little Bear Fire, south-central New Mexico: U.S. Geological Survey Open-File Report 2013–1108, 15 p., 3 pls., 

http://pubs.usgs.gov/of/2013/1108/. 

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iii

Acknowledgments

Barbara Ruddy and Dennis Staley of the U.S. Geological Survey performed technical reviews of 

the report. 

v

Contents

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

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

Physical Setting of the Little Bear Fire Burn Area ..................................................................................2

Methods Used To Estimate Debris-Flow Hazards ...................................................................................4

Model Application .........................................................................................................................................5

Debris-Flow Hazard Assessment ...............................................................................................................8

Debris-Flow Probability Estimates ....................................................................................................8

Debris-Flow Volume Estimates ..........................................................................................................9

Combined Relative Debris-Flow Hazard Rankings .......................................................................11

Limitations of Hazard Assessments ................................................................................................11

Summary ......................................................................................................................................................12

References Cited .........................................................................................................................................13

Plates

(Available online at http://pubs.usgs.gov/of/2013/1108/.)

1. Estimated probability of postwildfire debris flows in the area burned by the 2012 

Little Bear Fire, south-central New Mexico

2. Estimated volume of postwildfire debris flows in the area burned by the 2012 

Little Bear Fire, south-central New Mexico

3. Combined probability and volume relative hazard ranking of postwildfire debris 

flows in the area burned by the 2012 Little Bear Fire, south-central New Mexico

Figures

1. Map showing Little Bear Fire burn severity, as established by the Burned Area 

Emergency Response (BAER) Team, June 28, 2012. Modified from data supplied 

by the U.S. Department of Agriculture Forest Service, Little Bear Fire BAER Team ........3

2. Photograph showing sediment deposition at the mouth of Spring Canyon following 

the Little Bear Fire, south-central New Mexico, summer 2012. Rio Bonito is in the 

foreground ....................................................................................................................................9

3. Photograph showing North Fork Eagle Creek near Alto, New Mexico (08387550), 

streamgage before and after the Little Bear Fire in 2012. A, View from below 

weir, 2007 (photograph by Patrick Swift). B, View from above weir, 2013 ........................10

Table

1. Estimated debris-flow probabilities and volumes for the 2012 Little Bear Fire, southcentral New Mexico ....................................................................................................................6
vi

Conversion Factors 

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

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile (mi2)

square meter (m2) 10.76 square foot (ft2) 

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

Flow rate

millimeter per hour (mm/h) 0.03937 inch per hour (in/h)

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 2012 Little Bear Fire, South-Central 

New Mexico

By Anne C. Tillery and Anne Marie Matherne

Abstract 

A preliminary hazard assessment was developed of the 

debris-flow potential from 56 drainage basins burned by 

the Little Bear Fire in south-central New Mexico in June 

2012. The Little Bear Fire burned approximately 179 square 

kilometers (km2) (44,330 acres), including about 143 km2

(35,300 acres) of National Forest System lands of the Lincoln 

National Forest. Within the Lincoln National Forest, about 

72 km2 (17,664 acres) of the White Mountain Wilderness 

were burned. The burn area also included about 34 km2

(8,500 acres) of private lands. Burn severity was high or 

moderate on 53 percent of the burn area. The area burned is at 

risk of substantial postwildfire erosion, such as that caused by 

debris flows and flash floods. 

A postwildfire debris-flow hazard assessment of the area 

burned by the Little Bear Fire was performed by the U.S. 

Geological Survey in cooperation with the U.S. Department 

of Agriculture Forest Service, Lincoln National Forest. A set 

of two empirical hazard-assessment models developed by 

using data from recently burned drainage basins throughout 

the intermountain Western United States was used to estimate 

the probability of debris-flow occurrence and volume of debris 

flows along the burn area drainage network and for selected 

drainage basins within the burn area. The models incorporate 

measures of areal burn extent and severity, topography, soils, 

and storm rainfall intensity to estimate the probability and 

volume of debris flows following the fire. Relative hazard 

rankings of postwildfire debris flows were produced by 

summing the estimated probability and volume ranking to 

illustrate those areas with the highest potential occurrence of 

debris flows with the largest volumes. 

The probability that a drainage basin could produce 

debris flows and the volume of a possible debris flow at 

the basin outlet were estimated for three design storms: 

(1) a 2-year-recurrence, 30-minute-duration rainfall of 27 

millimeters (mm) (a 50 percent chance of occurrence in any 

given year); (2) a 10-year-recurrence, 30-minute-duration 

rainfall of 42 mm (a 10 percent chance of occurrence in any 

given year); and (3) a 25-year-recurrence, 30-minute-duration 

rainfall of 51 mm (a 4 percent chance of occurrence in any 

given year). Thirty-nine percent of the 56 drainage basins 

modeled have a high (greater than 80 percent) probability 

of debris flows in response to the 2-year design storm; 80 

percent of the modeled drainage basins have a high probability 

of debris flows in response to the 25-year design storm. For 

debris-flow volume, 7 percent of the modeled drainage basins 

have an estimated debris-flow volume greater than 100,000 

cubic meters (m3) in response to the 2-year design storm; 9 

percent of the drainage basins are included in the greater than 

100,000 m3 category for both the 10-year and the 25-year 

design storms. Drainage basins in the greater than 100,000 m3 

volume category also received the highest combined hazard 

ranking. 

The maps presented herein may be used to prioritize 

areas where emergency erosion mitigation or other protective 

measures may be needed prior to rainstorms within these 

drainage basins, their outlets, or areas downstream from these 

drainage basins within the 2- to 3-year period of vulnerability. 

This work is preliminary and is subject to revision. The 

assessment herein is provided on the condition that neither 

the U.S. Geological Survey nor the U.S. Government may be 

held liable for any damages resulting from the authorized or 

unauthorized use of the assessment.

Introduction

In June 2012, the Little Bear Fire burned approximately 

179 square kilometers (km2) (44,330 acres) in south-central 

New Mexico, including about 143 km2 (35,300 acres) of 

National Forest System lands of the Lincoln National Forest 

(pl. 1). Within the Lincoln National Forest, about 72 km2 

(17,664 acres) of the White Mountain Wilderness were 

burned, including 22.7 kilometers (14.1 miles) of Outstanding 

National Resource Waters within the upper and middle Rio 

Bonito drainage basin (Snyder and others, 2012). Outstanding 

National Resource Waters have been designated by the New 

Mexico Water Quality Control Commission to receive special 

protection against degradation under New Mexico water 

quality standards and the Federal Clean Water Act (New 

Mexico Environment Department, 2013). Affected waters 

with the Outstanding National Resource Waters designation 

included parts of Bonito Creek, Argentina Creek, Little Bonito 
2 Postwildfire Debris-Flow Hazard Assessment of the Area Burned by the 2012 Little Bear Fire, South-Central New Mexico

Creek, Aspen Creek, Turkey Creek, Roadmaker Creek, and 

Mills Creek (pl. 1). The burn area also included about 34 km2 

(8,500 acres) of private lands. A total of 254 structures were 

destroyed, of which 242 were residences (Payne and Beach, 

2012; Snyder and others, 2012).

Debris flows have been documented after many fires in 

the Western United States (Cannon and others, 2007, 2010; 

DeGraff and others, 2011). Rainfall on burn areas can result 

in transport and deposition of large volumes of sediment, both 

within and downchannel from burn areas. The rapid transport 

of large amounts of material makes debris flows particularly 

dangerous. In addition, debris flows following a wildfire can 

occur in places where flooding or sedimentation has not been 

observed in the past and can be generated in response to shortduration and low-recurrence-interval rainfall (Cannon and 

others, 2007, 2010; DeGraff and others, 2011). 

The hydrologic response of drainage basins intensifies 

under postwildfire conditions of decreased vegetation cover 

and altered soil properties. Wildfires can consume rainfallintercepting canopy, litter, and duff (Moody and Martin, 

2001a, 2001b; Cannon and Gartner, 2005). Water-repellent 

qualities in some soils can be increased or induced by the 

intense heat of a wildfire (DeBano, 1981; Doerr and others, 

2000; Letey, 2001; Woods and others, 2006), and increased 

overland flow and erosion can occur as a result (Wells, 1987; 

Moody and Martin, 2001a, 2001b). Fine ash, which may 

expand when wetted, can block soil pore spaces and further 

reduce infiltration of water (Romkens and others, 1990; 

Woods and others, 2006). After a wildfire, the drainage basin 

response to rainfall events shifts, in general terms, from 

infiltration dominated to runoff dominated (Cannon and 

others, 2010). Because of reduced soil infiltration, rainfall 

on wildfire burn scars can run off almost immediately as 

overland flow. This runoff in low-order channels can erode 

surficial materials, producing a flow that is rich in ash, soil, 

boulders, and dislodged vegetation. As additional sediment 

is entrained, sediment-laden flow in channels can transition 

into debris flows that can affect lives, property, infrastructure, 

aquatic habitats, and water supplies (Cannon and Gartner, 

2005). Debris flows are most frequent within 2–3 years after 

wildfires, when vegetative cover is absent or reduced and 

abundant materials are available for erosion and transport 

(Cannon and Gartner, 2005; Cannon and others, 2010). 

Variability in climatic conditions following a wildfire, such as 

an extended period that is wetter or dryer than normal, could 

result in variability in this period of vulnerability.

The area burned by the Little Bear Fire is at risk of 

substantial postwildfire erosion, such as that caused by 

debris flows and flash floods. A postwildfire debris-flow 

hazard assessment of the area burned by the Little Bear Fire 

was performed by the U.S. Geological Survey (USGS) in 

cooperation with the U.S. Department of Agriculture (USDA) 

Forest Service, Lincoln National Forest. The purpose of 

this report is to present a preliminary hazard assessment of 

the debris-flow potential for drainage basins burned by the 

2012 Little Bear Fire. Similar assessments of postwildfire 

debris-flow potential have been developed for earlier largescale New Mexico wildfires, including the Track (Tillery and 

others, 2011a), Las Conchas (Tillery and others, 2011b), and 

Whitewater-Baldy Complex Fires (Tillery and others, 2012). 

Physical Setting of the Little Bear Fire 

Burn Area

The Little Bear Fire started on the northeast flank of 

Sierra Blanca Peak and burned northeast across the Rio Bonito 

drainage basin and other drainage basins in the mountains 

adjacent to Alto, Villa Madonna, Angus, and several other 

small communities (pl. 1). Burn severity was high or moderate 

on 53 percent of the burn area (fig. 1). The terrain within the 

burn area is composed of very steep and rugged mountains 

and interspersed valleys with vegetation ranging from pinyonjuniper (Pinus and Juniperus spp.) scrub at an elevation of 

about 2,000 meters (m) (6,500 feet [ft]) to mixed conifer 

and alpine grasslands at an elevation higher than 3,600 m 

(11,900 ft). Numerous steep and short drainages converge 

into larger streams with the potential to transport substantial 

water and debris during postfire rain events (Snyder and 

others, 2012). High-severity burn areas in the White Mountain 

Wilderness were identified in the headwaters of Big Bear 

Canyon, Dark Betsy Canyon, and South Fork Rio Bonito, 

among others (fig. 1). These drainage basins drain into the Rio 

Bonito, which drains into Bonito Lake. Bonito Lake provides 

60 percent of the municipal water for the City of Alamogordo 

and Holloman Air Force Base, about 64 km (40 mi) southwest 

of Sierra Blanca Peak (Snyder and others, 2012). High- to 

moderate-severity burn conditions also occurred in parts of 

Little Creek drainage basin, in the vicinity of the community 

of Villa Madonna, and in parts of North Fork Eagle Creek 

drainage basin, which includes the Village of Ruidoso 

municipal wells and a group of historic USDA Forest Service 

cabins (Matherne and others, 2010; Snyder and others, 2012). 

Alto Reservoir, located on Eagle Creek downstream from 

the burn area, provides a portion of the Village of Ruidoso 

municipal water supply. The Village of Ruidoso lies about 

10 km (6 mi) south of Alto Reservoir. Drainage basins along 

Highway 532 above Alto Reservoir and parts of Magado 

Creek drainage basin in the northeastern part of the burn area 

were also affected (Snyder and others, 2012). 

Geologically, the area is composed primarily of igneous 

intrusive and volcanic rocks of Tertiary age (Thompson, 

1972), including tuff deposits, and volcaniclastic sedimentary 

and lahar deposits (Rawling, 2011). More recent sedimentary 

and secondary tuff deposits are found in the eastern part of 

the burn area (Green and Jones, 1997; Weldon, 2012). Soils 

within the burn area are composed primarily of residuum 

derived from the underlying volcanic or sedimentary sources, 

colluvium, and alluvium. Soils within the burn area are highly 

variable, ranging from shallow to deep, fine to loamy, and 

skeletal to nonskeletal in nature and occur on slopes ranging 

from 0 to 80 percent (Hill and others, 2012). 
Figure 1. Little Bear Fire burn severity, as established by the Burned Area Emergency Response (BAER) Team, June 28, 2012. Modified from data suppof Agriculture Forest Service, Little Bear Fire BAER Team.

South Fork Eagle Creek

Angus

North

Fork Eagle Creek

VillaMadonna

Rio Bonito

South Fork Rio

Bonito

Magado Creek

Angus

Canyon

B

North Fork Rio Ru idoso

105°50'W 105°45'W 105°40'W

33°30'N33°25'N

A

r

i

z

o

n

a

Location of the

Little Bear Fire

0 0.5 1 0 0.8 1.6 

EXPLANALittle Bea

Very LowModHigh
4 Postwildfire Debris-Flow Hazard Assessment of the Area Burned by the 2012 Little Bear Fire, South-Central New Mexico

Methods Used To Estimate Debris-Flow 

Hazards 

For this preliminary hazard assessment, a set of two 

empirical hazard-assessment models was used to estimate the 

probability of debris flow and to estimate debris-flow volume. 

The model outputs were combined to produce a relative 

ranking of debris-flow hazard along drainage networks and for 

selected drainage basins in response to three design storms in 

the Little Bear Fire burn area. The model for predicting debrisflow probability was developed by Cannon and others (2010) 

by using logistic multiple-regression analyses of data from 388 

drainage basins in 15 burn areas in the intermountain Western 

United States. Conditions in each basin were quantified by 

using readily obtained measures of burn extent and severity, 

hillslope gradient, soil properties, and storm rainfall. Statistical 

analyses were used to identify the variables that most strongly 

influenced debris-flow occurrence and to build the predictive 

model. Equation 1 is used to calculate debris-flow probability 

(Cannon and others, 2010): 

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 percent of the drainage basin area with 

slopes equal to or greater than 30 percent; 

R is drainage basin ruggedness, the change 

in drainage basin elevation (in meters) 

divided by the square root of the drainage 

basin area (in square meters) (Melton, 

1965);

%AB is the percentage of drainage basin area 

burned at moderate and high severities; 

I is average storm intensity (the total storm 

rainfall divided by the storm duration, in 

millimeters per hour);

%C is the percentage of the clay content of the 

soil; and

LL is the liquid limit of the soil (the percent 

of soil moisture by weight at which soil 

begins to have the properties of a liquid).

A second statistical model was used to estimate the 

volume of material that could issue from the basin outlet 

of a recently burned drainage basin in response to a given 

magnitude storm. This model was developed by using multiple 

linear-regression analyses of data compiled from 56 debrisflow-producing drainage basins burned by eight fires (Cannon 

and others, 2010). Debris-flow volume measurements were 

derived from records of the amount of material removed from 

sediment-retention basins and from field measurements of 

the amount of material eroded from the main channels within 

a burned drainage basin. Statistical analyses were used to 

identify the variables that most strongly influenced debrisflow volume. The model provides estimates of the volume 

of material that may pass through a drainage-basin outlet in 

response to a single rain event. The model has the following 

form: 

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

+ 0.7(AB)

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

where 

Ln is the natural log function;

V is the debris-flow volume (in cubic meters);

SG30 is the drainage basin area with slopes equal 

to or greater than 30 percent (in square 

kilometers);

AB is the drainage basin area burned at moderate 

and high severities (in square kilometers);

T is the total storm rainfall (in 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; 

Cannon and others, 2010). 

Values for both probability and volume were 

obtained along drainage networks by 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-m pixel along the drainage 

network (pls. 1 and 2) as a function of conditions in the 

drainage basin upstream from each pixel. This technique 

was developed as an alternative to basin-characterization 

approaches used in the past (for example, Cannon and 

others, 2010), which require definition of outlets and their 

corresponding drainage basins at the beginning of the 

analysis. The technique used here allows for a synoptic view 

of conditions throughout the study area, which can be used to 

identify specific 10-m pixels or stream reaches within a basin 

that might pose a higher risk of debris flows; the technique 

also aids in sampling design and monitoring-site selection.

The base layer upon which the continuousparameterization layers were built is the 1/3-arc-second 

National Elevation Dataset (Gesch and others, 2002). This 

digital elevation model (DEM) was transformed into a 

projection system appropriate to the study area (Universal 

Transverse Mercator, Zone 13) and processed by using 

standard DEM-conditioning tools in ArcGIS (Environmental 

Systems Research Institute, Inc., 2009) and RiverTools (Rivix, 

LLC, 2012). The DEM was used to derive the overland 
Model Application 5

flow structure in the form of a flow-direction matrix in eight 

directions. The flow-direction matrix and a weighted flow 

accumulation algorithm were then used to derive values of 

the independent spatial variables driving the probability and 

volume equations for every grid cell within the extent of the 

DEM. 

Because of orographic effects of the mountainous 

terrain and the size of the burn area, actual storm rainfall 

totals and intensities will vary in both space and time. For 

this study, however, the maximum rainfall of the design 

storm was assumed to be uniform over the entire burn area, 

providing the most conservative estimate of the probability 

and volumes of potential debris flows. Values for all of the 

other independent variables driving the debris-flow probability 

and volume equations were obtained by using the continuousparameterization approach. The independent-variable values 

can be represented as forming continuous surfaces over the 

burn area. Once the surfaces of the independent variables were 

developed, the probability and volume equations were solved 

by using map algebra for each grid cell along the drainage 

network, thus deriving the probability and volume surfaces. 

Along the drainage network, drainage basins were delineated 

so that the area of the basin at the farthest downstream pixel 

modeled was within the size range for which the models were 

developed, 0.01–103 km2 for the probability model and 0.01–

27.9 km2 for the volume model (Cannon and others, 2010). 

Identification of the probability or volume of a debris flow at 

locations within the study area can be obtained by querying 

the derived surfaces. 

Debris-flow hazards from a given basin can also 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 relatively ranked classes, 

and these classes were added together to calculate the 

combined probability and volume relative hazard ranking 

(pl. 3). This combined ranking identifies a possible range of 

responses from drainage basins with the highest probabilities 

of producing debris flows with the largest volumes to drainage 

basins with the lowest probabilities of producing debris 

flows with the smallest volumes (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.

Model Application

Debris-flow probabilities and volumes were calculated 

along the drainage networks for the Little Bear Fire burn area 

by using the two empirical models. Within the area analyzed, 

56 drainage basins were delineated and numbered to aid 

in discussion of model results (pl. 1 and table 1). Smaller 

side drainage basins were not delineated and numbered, but 

these drainage basins were included in the stream segment 

analysis. Probability, volume, or hazard ranking for the 

numbered drainage basins represents the value at the basin 

outlet. Probability or hazard rankings 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. The 

drainage basins were delineated by analyzing elevation data 

derived from 10-m DEMs (U.S. Geological Survey, 2011) 

with geographic information system (GIS) hydrological tools. 

Debris-flow probability and volume were estimated for every 

10-m pixel along mapped drainage networks of the burn area 

as a function of conditions in the modeled drainage basin 

above each pixel (Verdin and Greenlee, 2003; Verdin and 

Worstell, 2008). Drainage basins were delineated so that the 

area of the basin at the farthest downstream pixel modeled was 

within the size range for which the models were developed, 

0.01–103 km2 for the probability model and 0.01–27.9 km2 for 

the volume model (Cannon and others, 2010). Drainage basins 

with a total area exceeding the range of the volume model or 

for which more detailed information about tributary drainage 

basins was desired, such as Rio Bonito and North Fork Eagle 

Creek, were subdivided into side tributaries. Stream reaches 

draining the large drainage basins are highlighted on plates 1 

through 3 as “drainages within burn areas that can be affected 

by the combined effects of debris flows generated from side 

tributaries.” For large drainage basins falling within the size 

range of the probability model but outside the size range of the 

volume model, probability estimates are indicated along the 

streamline by segment, but no basin number is assigned for the 

combined basin, and no relative hazard ranking is calculated. 

Areas for drainage basins analyzed for volume and relative 

hazard ranking averaged 2.7 km2 and ranged from 0.4 km2 to 

15.1 km2. 

Measures of the physical properties of soils within 

each basin were obtained from the State Soil Geographic 

(STATSGO) database (Schwartz and Alexander, 1995). If 

more than one soil unit was mapped within a given basin, 

a spatially weighted average of the soil variable values 

was calculated. The clay content and liquid limit of soils 

in drainage basins burned by the Little Bear Fire covered a 

narrow range, about 28–35 percent for clay content and about 

31–36 percent for the liquid limit. 

The Burned Area Emergency Response (BAER) Image 

Support Team of the USGS Earth Resources Observation and 

Science (EROS) Center and the USDA Forest Service Remote 

Sensing Applications Center provided a map of Burned Area 

Reflectance Classification (BARC), which was used as an 

indicator of the distribution of burn severity within the fire 

perimeter (U.S. Department of Agriculture Forest Service, 

written commun., 2012). The BARC map for the Little Bear 

Fire indicates that the moderate- and high-severity burn areas 

totaled about 95 km2 (23,500 acres), 53 percent of the total 

burn area. The high-severity burn area includes South Fork 
Table 1. Estimated debris-flow probabilities and volumes for the 2012 Little Bear Fire, south-central New Mexico.—Continued [mm, millimeters; k2, square kilometers; %, percent; m3, cubic meters; <, less than; >, greater than] Selected basin Description Percent of basin with slope greater than 30% Drainage area (km2) 2-year-recurrence, 30-minute-duration rainfall 10-year-recurrence, 30-minute-duration rainfall 

30-

27 mm 42 mm 

Prob- ability (%) Volume (m3) Combined hazard ranking Prob- ability (%) Volume (m3) Combined hazard ranking Prob- ability (%) 

1 Cherry Creek 57.2 2.22 41 17,489 3 86 25,469 4 96 2 Grapevine Canyon 66.7 0.53 <20 <5,000 1 31 6,511 2 61 3 Unnamed tributary to Magado Creek 28.3 1.44 <20 5,416 1 <20 7,888 1 39 4 Unnamed tributary to Magado Creek 33.1 2.54 20 14,373 2 68 20,932 3 88 5 Ferguson Canyon 31.4 1.18 35 7,555 2 83 11,003 4 94 6 Unnamed tributary to Magado Creek 16.5 1.61 62 7,814 3 93 11,380 4 98 7 Unnamed tributary to Magado Creek 2.9 1.11 24 <5,000 2 74 <5,000 3 91 8 Crocket Canyon 5.4 4.10 26 9,304 2 75 13,549 3 92 9 Unnamed tributary to Magado Creek 1.7 2.09 <20 <5,000 1 <20 <5,000 1 33 

10 Unnamed tributary to Bitter Canyon 0.2 1.45 <20 <5,000 1 <20 <5,000 1 38 

11 Little Bonito Creek 58.3 1.88 <20 9,671 1 43 14,084 3 73 

12 Upper Bonito Creek/Aspen Canyon 79.7 7.59 <20 32,698 2 61 47,619 3 85 13 Little Bear Canyon 90.3 4.18 33 31,985 2 81 46,580 4 94 14 Unnamed tributary to Bonito Creek 85.1 0.59 <20 5,757 1 60 8,384 2 84 15 Argentina Canyon 70.2 4.01 <20 16,451 2 50 23,958 3 78 16 Turkey Canyon 82.8 4.76 <20 19,793 2 60 28,825 3 84 17 Bear Creek 86.6 7.92 86 >100,000 5 98 >100,000 5 99 18 Tanbark Canyon 79.3 6.65 <20 33,147 2 64 48,273 3 87 19 Unnamed tributary to Bonito Creek 88.4 0.70 98 10,282 4 100 14,974 4 100 20 George Washington Canyon 85.8 1.12 93 14,584 4 99 21,239 4 100 21 Dark Betsy Canyon 86.6 2.77 98 41,197 4 100 59,997 4 100 22 South Fork Rio Bonito 93.6 15.10 98 >100,000 5 100 >100,000 5 100 23 Bluefront Canyon 87.5 2.55 96 35,523 4 100 51,733 4 100 24 Wallsmith Canyon 87.6 2.22 98 32,559 4 100 47,416 4 100 25 Roadmaker Canyon 88.5 2.37 97 33,847 4 100 49,293 4 100 26 Littleton Canyon 78.0 3.99 86 46,859 4 98 68,242 4 99 27 Anan Canyon 73.8 6.80 96 >100,000 5 99 >100,000 5 100 28 Beal Canyon 92.8 1.32 95 18,580 4 99 27,059 4 100 29 Spring Canyon 87.7 1.45 97 20,547 4 100 29,923 4 100 

Table 1. Estimated debris-flow probabilities and volumes for the 2012 Little Bear Fire, south-central New Mexico.[mm, millimeters; km2, square kilometers; %, percent; m3, cubic meters; <, less than; >, greater than]
Table 1. Estimated debris-flow probabilities and volumes for the 2012 Little Bear Fire, south-central New Mexico.—Continued[mm, millimeters; k2, square kilometers; %, percent; m3, cubic meters; <, less than; >, greater than]

Selected 

basin Description

Percent 

of basin 

with 

slope 

greater 

than 30%

Drainage 

area 

(km2)

2-year-recurrence, 

30-minute-duration rainfall

10-year-recurrence, 

30-minute-duration rainfall

30-

27 mm 42 mm 

Probability 

(%)

Volume 

(m3)

Combined 

hazard 

ranking

Probability 

(%)

Volume 

(m3)

Combined 

hazard 

ranking

Probability 

(%)

30 Mineral Farms Canyon 84.2 1.91 86 22,134 4 98 32,234 4 99 31 Schoolhouse Canyon 75.1 1.35 94 17,358 4 99 25,278 4 100 32 Nolten Canyon 86.3 1.18 96 16,101 4 100 23,448 4 100 33 Philadelphia Canyon 57.6 7.84 87 >100,000 5 98 >100,000 5 100 34 Jackson Canyon 77.3 1.45 91 17,698 4 99 25,774 4 100 35 Mills Canyon 81.2 4.69 97 78,221 4 100 >100,000 5 100 36 Crocket Canyon 37.6 1.51 73 11,882 3 96 17,304 4 99 37 Peters Canyon 47.3 2.17 62 18,456 3 94 26,878 4 98 38 Angus Canyon 45.9 0.75 24 6,028 2 73 8,778 3 91 39 Unnamed tributary to Bonito Creek 54.2 0.61 41 5,867 2 86 8,544 3 96 40 Unnamed tributary to Bonito Creek 37.5 1.14 <20 6,723 1 47 9,791 2 76 41 Unnamed tributary to Bonito Creek 25.9 1.02 <20 <5,000 1 <20 5,865 1 42 42 Unnamed tributary to Bonito Creek 14.8 0.41 <20 <5,000 1 <20 <5,000 1 36 43 Unnamed tributary to Little Creek 0.4 1.71 <20 <5,000 1 26 <5,000 1 55 44 Vickers Canyon 39.7 1.18 73 9,708 3 96 14,139 4 99 45 Little Creek 61.7 3.44 71 36,172 3 96 52,679 4 99 46 Phillips Canyon 48.5 1.95 <20 13,139 2 55 19,134 3 82 47 Flume Canyon 66.4 1.42 43 13,905 3 87 20,251 4 96 48 Hubbard Canyon 62.3 1.91 28 16,437 2 77 23,938 3 92 49 Upper Eagle Creek 94.5 4.46 80 53,067 3 97 77,284 4 99 50 Unnamed tributary to Eagle Creek 92.3 1.20 96 16,951 4 100 24,686 4 100 51 Telephone Canyon 90.7 1.00 97 14,365 4 100 20,921 4 100 52 Johnson Canyon 80.5 1.15 57 12,606 3 92 18,359 4 98 53 Carlton Canyon 83.8 1.84 <20 13,979 2 48 20,358 3 77 54 Unnamed tributary to South Fork Eagle 

Creek

88.0 1.04 88 13,443 4 98 19,577 4 100 

55 Unnamed tributary to North Fork Rio 

Ruidoso

87.1 1.19 76 13,838 3 97 20,153 4 99 

56 Upper North Fork Rio Ruidoso 70.4 5.79 39 45,014 2 85 65,555 4 95 
8 Postwildfire Debris-Flow Hazard Assessment of the Area Burned by the 2012 Little Bear Fire, South-Central New Mexico

Rio Bonito and parts of tributaries north and south of Rio 

Bonito from Big Bear and George Washington Canyons and 

east to approximately Mills Creek and Philadelphia Canyon. 

Also included are parts of North Fork Eagle Creek, Little 

Creek, and North Fork Rio Ruidoso in the southern burn area 

and Magado Creek in the northeastern burn area (pl. 1 and 

fig. 1). The drainage basins selected for delineation included 

the moderate- and high-severity burn areas where slope, 

drainage basin configuration, and topographic setting indicated 

susceptibility to debris-flow processes or areas where 

proximity to development indicated a vulnerability to hazards 

presented by debris flows.

Postwildfire debris flows in the intermountain Western 

United States often occur in response to short-duration, 

high-intensity rainfall events. Cannon and others (2008) 

found that most debris flows occur in response to storms 

with short recurrence intervals (from 2 to 10 years), and 

Kean and others (2011) demonstrated that intense rain in 

periods of less than 30 minutes generated postwildfire debris 

flows. To characterize the effects of these rainfall conditions, 

the probability that a drainage basin could produce debris 

flows and the volume of a possible debris flow at the basin 

outlet were estimated for three design storms: (1) a 2-yearrecurrence, 30-minute-duration rainfall of 27 millimeters 

(mm) (1 inch) (a 50 percent chance of occurrence in any given 

year); (2) a 10-year-recurrence, 30-minute-duration rainfall of 

42 mm (1.7 inches) (a 10 percent chance of occurrence in any 

given year); and (3) a 25-year-recurrence, 30-minute-duration 

rainfall of 51 mm (2 inches) (a 4 percent chance of occurrence 

in any given year). Precipitation data were from Bonnin and 

others (2006), based on extrapolated isohyetal values for a 

midelevation location within the burn area. 

Results for the three design storms are presented in 

table 1. Results for the 2-year-recurrence rainfall (presented in 

pls. 1 through 3) were chosen for the map presentation because 

the 2-year event exhibited the best differentiation among the 

drainage basins and best highlighted the highest probability 

response drainage basins. For the 10- and 25-year-recurrence 

rainfalls, a greater percentage of the drainage basins fall into 

the higher probability intervals. An uneven distribution and 

intensity of rainfall over the burn area may result in varying 

response probabilities in varying parts of the burn area.

Debris-Flow Hazard Assessment

The hazards of debris flows from drainage basins 

burned by the Little Bear Fire were assessed by estimating 

the probability of occurrence, by estimating the volume of 

potential debris flows, and by combining the probability and 

volume into a relative hazard ranking. 

Debris-Flow Probability Estimates 

In response to the 2-year-recurrence rainfall, modeling 

indicated that 22 drainage basins (17, 19–35, 49–51, and 54) 

have high probabilities of debris-flow occurrence (greater 

than or equal to 80 percent) (pl. 1 and table 1). For the 

10-year-recurrence rainfall, an additional 13 drainage basins 

(1, 5, 6, 13, 36, 37, 39, 44, 45, 47, 52, 55, and 56) have high 

probabilities of debris-flow occurrence, and for the 25-yearrecurrence rainfall, an additional 10 drainage basins (4, 7, 

8, 12, 14, 16, 18, 38, 46, and 48), representing together 80 

percent of the total, have high probabilities of debris-flow 

occurrence (table 1). 

Basins with a high probability of debris-flow occurrence 

in response to the 2-year-recurrence rainfall were primarily 

located in the central part of the burn area, including 

tributaries to South Fork Rio Bonito (22); Big Bear (17), Dark 

Betsy (21), Wallsmith (24) and Roadmaker (25) Canyons 

above Bonito Lake; and tributary canyons north and south 

of Rio Bonito from Bonito Lake to Mills Creek (35) and 

Philadelphia Canyon (33) (pl. 1). Selected tributaries on 

North Fork Eagle Creek were also modeled as having high 

probabilities of debris-flow occurrence in response to the 

2-year-recurrence rainfall (pl. 1). Debris flows generated from 

burned drainage basins may directly affect Bonito Lake and 

buildings and infrastructure along Rio Bonito, as well as parts 

of North Fork Eagle Creek and the Villa Madonna area.

Stream segment analysis identifies stream segments 

within a basin with a higher or lower probability of debrisflow occurrence than predicted at the basin outlet. For 

example, both South Fork Rio Bonito (22) and Big Bear 

Canyon (17) have a high probability of debris flow throughout 

much of the drainage basins but contain stream segments 

with a lower probability of debris flows (pl. 1). Philadelphia 

Canyon (33) was modeled as having primarily a low 

probability of debris flows in the upper reaches but increasing 

to an 80–90 percent debris-flow probability at the basin outlet. 

In contrast, North Fork Eagle Creek (49–53) and Little Creek 

(45) contain high-probability segments within parts of the 

upper reaches but probabilities decrease abruptly for stream 

segments near the basin outlets. Stream segment analysis 

along North Fork Eagle Creek indicates a high probability 

of debris flows from the outlet of Upper Eagle Creek (49) to 

below the confluence with Carlton Canyon (53), where the 

probability decreases to 60–79 percent.

The location of observed postwildfire debris flows 

supports the probability analysis. On June 22, 2012, 

precipitation produced debris flows and mud flows within 

the burn area (Weldon, 2012). About 13 mm (0.5 inches) 

of precipitation fell in 1 hour, an approximately 1-yearrecurrence rainfall (Bonnin and others, 2006), with more than 

25 mm (1 inch) of total precipitation observed. Evidence of 

debris flows was identified at three locations in response to 

the June 22 storm, at Tanbark (18), George Washington (20), 

and Anan (27) Canyons (K. Weldon, U.S. Department of 

Agriculture Forest Service, written commun., 2012) (pl. 1). 

Debris flows and large amounts of sediment movement 

were also noted at other locations throughout the summer, 

although the amount and intensity of rainfall associated with 

this sediment movement are not known. These locations 

include the mouth of Spring Canyon (29) (pl. 1 and fig. 2) 
Debris-Flow Hazard Assessment 9

(J. Snyder, U.S. Department of Agriculture Forest Service, 

written commun., 2012) and North Fork Eagle Creek, where 

transported sediment impinged on cabins and buried a USGS 

streamgage (pl. 1 and fig. 3). George Washington and Spring 

Canyons and the upper parts of North Fork Eagle Creek (49 

and 50) were all modeled as having greater than 80 percent 

probability of debris flows in response to the 2-year-recurrence 

rainfall throughout the drainage basins. The stream segment 

where there was debris-flow evidence in Anan Canyon (27) 

was modeled with 60–79 percent probability of debris flows in 

response to the 2-year-recurrence rainfall (pl. 1 and table 1). 

The upper parts of Tanbark Canyon (18) are outside of the 

burn perimeter, and much of the area inside the burn perimeter 

within Tanbark Canyon was classified as low-severity burn. 

Parts along the eastern edge of Tanbark Canyon basin, where 

debris-flow deposits were identified, however, were classified 

as moderate- to high-severity burn, and the stream segment 

analysis modeled these side tributaries as having a 40–79 

percent probability of debris-flow occurrence. The debris-flow 

analysis and location of debris-flow deposits within Tanbark 

and Anan Canyons demonstrate the utility of the continuousparameterization technique in identifying and highlighting 

high-probability response areas within a basin with variable 

burn intensity.

High debris-flow probabilities reflect the combined 

effects of drainage basins being nearly completely burned at 

high and moderate severities and having steep slopes. The 

localized nature of the debris flows reported in response to 

the June 22 storm reflects, in part, the localized nature of the 

summer storm cells compared to the assumption of uniform 

precipitation over the entire burn area, the conservative 

assumption on which the model was based.

Debris-Flow Volume Estimates

The debris-flow volumes estimated in this assessment 

are independent of the estimated debris-flow probabilities. 

As a result, drainage basins with high predicted debris-flow 

probabilities represent varying degrees of hazard to areas 

downstream, depending on the predicted volume of material 

mobilized in a debris flow. Estimated debris-flow volumes can 

vary by stream segment, as indicated by changes in stream 

segment color along a drainage network (pl. 2). The stream 

segment analysis estimates the volume of debris-flow material 

mobilized above a given analysis point but does not account 

for potential within-basin deposition of mobilized material. 

Basin color on plate 2 indicates the estimated debris-flow 

volume at the basin outlet. Estimated debris-flow volumes 

ranged from less than 5,000 cubic meters (m3) to greater than 

100,000 m3 for the modeled storms (table 1). Drainage basins 

with estimated debris-flow volumes greater than 100,000 

m3 included Bear Creek (17), South Fork Rio Bonito (22), 

Anan Canyon (27), and Philadelphia Canyon (33) for the 

2-year-recurrence rainfall (pl. 2) and an additional basin, 

Figure 2. Sediment deposition at the mouth of Spring Canyon following the Little Bear Fire, south-central New Mexico, summer 2012. 

Rio Bonito is in the foreground.
10 Postwildfire Debris-Flow Hazard Assessment of the Area Burned by the 2012 Little Bear Fire, South-Central New Mexico

Figure 3. North Fork Eagle Creek near Alto, New Mexico (08387550), streamgage before and after the Little Bear Fire in 2012. A, View 

from below weir, 2007 (photograph by Patrick Swift). B, View from above weir, 2013.

A

B
Debris-Flow Hazard Assessment 11

Mills Canyon (35), for the 10-year and 25-year-recurrence 

rainfalls. Drainage basins with estimated debris-flow volumes 

greater than 50,000 m3 for the 2-year-recurrence rainfall also 

included Mills Canyon (35) and Upper Eagle Creek (49), 

with additional drainage basins Dark Betsy Canyon (21), 

Bluefront Canyon (23), Littleton Canyon (26), Little Creek 

(45), and Upper North Fork Rio Ruidoso (46) (pl. 2), for the 

10-year-recurrence rainfall and Upper Bonito Creek/Aspen 

Canyon (12), Little Bear Canyon (13), Tanbark Canyon 

(18), Wallsmith Canyon (24), and Roadmaker Canyon (25) 

for the 25-year-recurrence rainfall. Stream segment analysis 

indicated that South Fork Rio Bonito (22) has a high estimated 

debris-flow volume along much of the main stem, with lesser 

volumes estimated from the upper reaches and side channels. 

In Bear Creek (17), Anan Canyon (27), and Philadelphia 

Canyon (33), the highest estimated debris-flow volume is 

confined to the lower reach of the drainage basins, with 

decreasing estimated volumes headward in the drainage 

basins and in the side channels. North Fork Eagle Creek has 

high estimates of debris-flow volume in the lower reaches 

of Upper Eagle Creek (49), but the highest volumes were 

estimated for North Fork Eagle Creek below Telegraph 

Canyon (51). It is not known if the estimated volumes of 

material are sufficient to dam watercourses or cause flooding, 

which could affect resources downstream from the evaluated 

drainage basins. 

Combined Relative Debris-Flow Hazard 

Rankings

Combined probability and volume relative hazard 

rankings of postwildfire debris flows were produced by 

summing the estimated probability and volume ranking to 

illustrate those areas with the highest potential occurrence of 

debris flows with the largest volumes. Rankings are shown for 

the 2-year-recurrence rainfall (pl. 3). The highest combined 

hazard ranking is predicted for four drainage basins, Bear 

Creek (17), South Fork Rio Bonito (22), Anan Canyon (27), 

and Philadelphia Canyon (33) for the 2-year-recurrence 

rainfall (table 1). With the second highest combined hazard 

ranking, an additional 17 drainage basins (19–21, 23–26, 

28–32, 34, 35, 50, 51, and 54) are included. For the 10-yearrecurrence rainfall, those same four drainage basins plus 

Mills Canyon (35) were modeled with the highest combined 

hazard ranking. Including the second highest combined hazard 

ranking category adds drainage basins 1, 5, 6, 13, 19–21, 

23–26, 28–32, 34, 36, 37, 44, 45, 47, 49–52, and 54–56. For 

the 25-year-recurrence rainfall, an additional 10 drainage 

basins (4, 8, 12, 14, 16, 18, 38, 39, 46, and 48), representing 

together 79 percent of the total, were modeled with the two 

highest combined hazard rankings (table 1). 

Stream segment analysis indicated a relative hazard 

ranking at the two highest categories over most of the 

central Rio Bonito drainage basin. Little Bear Canyon (12) 

is generally ranked at a medium hazard but has segments of 

higher hazard ranking in the upper basin. Stream segments in 

Magado Creek drainage basin (1–9) were modeled over the 

range of established rankings, with high rankings in the upper 

reaches of drainage basins 4–7. Within the southern part of the 

burn area, high relative hazard rankings were found in parts 

of most drainage basins, including Little Creek (45), North 

Fork Eagle Creek (49–52), and North Fork Rio Ruidoso (56). 

North Fork Eagle Creek from Telegraph Canyon (51) to below 

Carlton Canyon (53) is the only segment in the southern burn 

area modeled with the highest relative hazard ranking.

Limitations of Hazard Assessments 

This assessment provides estimates of debris-flow 

probability, volume, and combined relative hazard ranking 

in response to 30-minute-duration design storms with 2-, 

10-, and 25-year-recurrence probabilities for drainage basins 

burned by the Little Bear Fire. The term %SG30, the percent 

of the drainage basin area with slopes equal to or greater 

than 30 percent, is a factor in both the probability model and 

the volume model. In 70 percent of the 56 drainage basins 

modeled, more than 50 percent of the basin area has a slope 

greater than 30 percent (table 1). The percentage of basin area 

with a greater than 30 percent slope ranges from 0.2 to 94.5 

percent, with a median of 75.1 percent. The drainage basin 

with the median value is Schoolhouse Canyon (31), and the 

drainage basin with the highest value is Upper Eagle Creek 

(49). Because of the high percentage of area with steep slopes 

in many of the delineated drainage basins within the burn 

area, the slope factor is an important contributor to both the 

probability estimates and the volume estimates of debris flows 

within the Little Bear Fire burn area. 

The probability of debris flows increases with increasing 

recurrence interval (lower probability of occurrence but higher 

intensity) design storm. The probability of debris flows in 

response to the 2-year design storm is high (greater than 80 

percent) for 39 percent of the 56 drainage basins modeled; the 

probability of debris flows in response to the 25-year design 

storm is high for 80 percent of the modeled drainage basins. 

For debris-flow volume, estimated debris-flow volume is 

greater than 100,000 m3 in response to the 2-year design storm 

in 7 percent of the modeled drainage basins; 9 percent of the 

drainage basins are included in the greater than 100,000 m3 

category for both the 10-year and the 25-year design storms. 

Drainage basins in the greater than 100,000 m3 volume 

category were also modeled with the highest combined hazard 

ranking. The small increase in the number of drainage basins 

with the highest combined hazard ranking with increasing 

storm intensity reflects the fact that, while a large percentage 

of drainage basins may potentially produce debris flows, the 

estimated volume of material associated with these events is 

limited. This statement does not imply that debris flows of any 

size do not have the potential to cause damage but provides a 

method for evaluating areas within the burn area with greater 

potential hazard from a given magnitude rain event. Larger, 

less frequent storms (for example, a 50-year-recurrence 
12 Postwildfire Debris-Flow Hazard Assessment of the Area Burned by the 2012 Little Bear Fire, South-Central New Mexico

rainfall) are likely to produce larger debris flows, and smaller, 

more frequent storms (for example, a 1-year-recurrence 

rainfall) could also trigger debris flows, but they would likely 

be smaller. Higher probabilities of debris flows than those 

shown on plate 1 may exist within any part of the drainage 

basins. Because not all rainstorms will be large enough to 

affect the entire burn area, debris flows may not be produced 

from all drainage basins during a given storm. 

It is important to note that the maps shown in plates 1, 

2, and 3 do not identify those areas that can be affected by 

debris flows as the material moves downstream from the 

basin outlets (Cannon and others, 2010). Additionally, further 

investigation is needed to assess the potential for debris flows 

to affect structures at, or downstream from, basin outlets and 

to increase the threat of flooding downstream by damaging or 

blocking bridges or flood-mitigation structures.

The variables included in the models and used in this 

assessment are considered to directly affect debris-flow 

generation in the intermountain Western United States. 

Conditions other than those used in the models (for example, 

the amount of sediment stored in a canyon) could also affect 

debris-flow production. Data necessary to evaluate such 

effects, however, are not readily available. 

The potential for debris-flow activity decreases with time 

as revegetation stabilizes hillslopes and the supply of erodible 

material decreases in the canyons. If dry conditions prevent 

sufficient regrowth of vegetation, however, this recovery 

period may be longer. In contrast, if rainfall events for the 

first year are frequent but mild, recovery and stabilization of 

soil with vegetation may occur rapidly and diminish debrisflow hazards the following year. After the Little Bear Fire, 

approximately 36 km2 (8,970 acres) of land with moderate- to 

high-severity burn were treated by seeding, 41 km2 (10,241 

acres) were treated by using seed with straw mulch, and about 

0.7 km2 on the Mescalero Apache Reservation were treated 

by mulching with straw (Gary Ziehe, U.S. Department of 

Agriculture Forest Service, oral commun., 2012). Mulching 

is expected to reduce the probability and volume of erosion 

in these areas compared to pretreatment levels (Groen and 

Woods, 2008; Robichaud and others, 2010). The debris-flow 

analysis is based on initial postwildfire conditions and does not 

account for potential mitigating effects of postfire treatments. 

The analysis does serve to highlight, especially by the stream 

segment analysis, those parts of a basin with an increased 

debris-flow hazard ranking based on physical characteristics. 

The assessment given herein is estimated to be applicable for 

2–3 years after the fire, depending on precipitation distribution 

(Cannon and others, 2010). 

The maps in this report may be used to prioritize areas 

where emergency erosion mitigation or other protective 

measures may be needed prior to rainstorms within these 

drainage basins, their outlets, or areas downstream from these 

drainage basins within the 2- to 3-year period of vulnerability 

following the Little Bear Fire. This assessment evaluates 

only postwildfire debris flows and does not consider hazards 

associated with flash floods; such hazards may remain for 

many years after a fire. 

This work is preliminary and is subject to revision. It is 

being provided because of the need for timely best science 

information. The assessment herein is provided on the 

condition that neither the USGS nor the U.S. Government may 

be held liable for any damages resulting from the authorized 

or unauthorized use of the assessment. 

Summary 

In June 2012, the Little Bear Fire burned approximately 

179 square kilometers (km2) (44,330 acres) in south-central 

New Mexico, including about 143 km2 (35,300 acres) of 

National Forest System lands of the Lincoln National Forest. 

Within the Lincoln National Forest, about 72 km2 (17,664 

acres) of the White Mountain Wilderness were burned. The 

burn area also included about 34 km2 (8,500 acres) of private 

lands. Burn severity was high or moderate on 53 percent of 

the burn area, which is characterized by very steep and rugged 

mountains and interspersed valleys with vegetation ranging 

from pinyon-juniper (Pinus and Juniperus spp.) scrub at an 

elevation of about 2,000 meters (6,500 feet) to mixed conifer 

forests and alpine grasslands at an elevation higher than 3,600 

meters (11,900 feet). 

High- to moderate-severity burn areas were identified 

in the headwaters of drainage basins that drain into the Rio 

Bonito which subsequently flows through Bonito Lake. Bonito 

Lake provides 60 percent of the municipal water for the 

City of Alamogordo and Holloman Air Force Base. High- to 

moderate-severity burn areas were also identified in parts of 

Little Creek drainage basin, in the vicinity of the community 

of Villa Madonna, and in parts of North Fork Eagle Creek 

drainage basin, which includes the Village of Ruidoso 

municipal wells and a group of historic U.S. Department of 

Agriculture (USDA) Forest Service cabins. Alto Reservoir, 

located on Eagle Creek downstream from the burn area, 

provides a portion of the Village of Ruidoso municipal water 

supply. Drainage basins along Highway 532 above Alto 

Reservoir and parts of Magado Creek drainage basin in the 

northeastern part of the burn area were also affected. The 

area burned by the Little Bear Fire is at risk of substantial 

postwildfire erosion, such as that caused by debris flows and 

flash floods. 

A postwildfire debris-flow hazard assessment of the area 

burned by the Little Bear Fire was performed by the U.S. 

Geological Survey in cooperation with the USDA Forest 

Service, Lincoln National Forest. A set of two empirical 

hazard-assessment models developed from data collected in 

recently burned drainage basins throughout the intermountain 

Western United States was used to estimate the probability 

of occurrence and volume of debris flows along the burn area 

drainage network and for selected drainage basins within the 

Little Bear Fire burn area in response to 30-minute-duration 

design storms of 2-, 10-, and 25-year-recurrence rainfall. The 

models incorporate measures of areal burn extent and severity, 

topography, soils, and storm rainfall intensity to estimate the 

probability and volume of debris flows following the fire. 
References Cited 13

The probability of debris flows in response to the 2-year 

design storm is high (greater than 80 percent) for 39 percent of 

the 56 drainage basins modeled; the probability of debris flows 

in response to the 25-year design storm is high for 80 percent 

of the modeled drainage basins. For debris-flow volume, 

estimated debris-flow volume is greater than 100,000 cubic 

meters (m3) in response to the 2-year design storm in 7 percent 

of the modeled drainage basins; 9 percent of the drainage 

basins are included in the greater than 100,000 m3 category 

for both the 10-year and the 25-year design storms. Drainage 

basins in the greater than 100,000 m3 volume category also 

were also modeled with the highest combined hazard ranking. 

The debris-flow analysis is based on initial postfire 

conditions and does not account for potential mitigating effects 

of postfire treatments such as mulching and seeding carried 

out by the USDA Forest Service following the Little Bear Fire. 

The analysis does serve to highlight, especially by the stream 

segment analysis, those parts of a basin with an increased 

debris-flow hazard ranking based on physical characteristics. 

The maps presented herein may be used to prioritize areas 

where emergency erosion mitigation or other protective 

measures may be needed prior to rainstorms within these 

drainage basins, their outlets, or areas downstream from these 

drainage basins within the 2- to 3-year period of vulnerability. 

Variability in climatic conditions following a wildfire, such as 

an extended period that is wetter or dryer than normal, could 

result in variability in this period of vulnerability.

This work is preliminary and is subject to revision. It is 

being provided because of the need for timely best science 

information. The assessment herein is provided on the 

condition that neither the U.S. Geological Survey nor the U.S. 

Government may be held liable for any damages resulting 

from the authorized or unauthorized use of the assessment. 

References Cited

Bonnin, G.M., Martin, Deborah, Lin, Binghang, Parzybok, 

Tye, Yekta, Michael, and Riley, David, 2006, Precipitationfrequency atlas of the United States: Silver Spring, 

Md., National Weather Service, National Oceanic and 

Atmospheric Administration (NOAA) atlas 14, v. 1, version 

5, accessed November 14, 2012, at http://hdsc.nws.noaa.

gov/hdsc/pfds/.

Cannon, S.H., and Gartner, J.E., 2005, Wildfire-related 

debris flow from a hazards perspective, chap. 15 of Jakob, 

Matthias, and Hungr, Oldrich, eds., Debris-flow hazards 

and related phenomena: Chichester, U.K., Springer–Praxis 

Books in Geophysical Sciences, p. 321–344.

Cannon, S.H., Gartner, J.E., and Michael, J.A., 2007, Methods 

for the emergency assessment of debris-flow 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/.)

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

Rea, A.H., and Parrett, Charles, 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. 

DeBano, L.F., 1981, Water repellant soil—A state-of-the-art: 

Berkeley, Calif., U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Forest and Range Experiment 

Station, General Technical Report PSW–46, 21 p.

DeGraff, J.V., Wagner, D.L., Gallegos, A.J., DeRose, 

M.B., Shannon, Casey, and Ellsworth, Todd, 2011, The 

remarkable occurrence of large rainfall-induced debris flows 

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Tillery and Matherne—Postwildfire Debris-Flow Hazard Assessment of the Area Burned by the 2012 Little Bear Fire, South-Central New Mexico—OFR 2013–1108