Probability and Volume of Potential Postwildfire Debris 

Flows in the 2011 Horseshoe II Burn Area, Southeastern 

Arizona

By Barbara C. Ruddy

Open-File Report 2011–1197

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

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

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Suggested citation:

Ruddy, B.C., 2011, Probability and volume of potential postwildfire debris flows in the 2011 Horseshoe II burn area, 

southeastern Arizona: U.S. Geological Survey Open-File Report 2011–1197, 10 p.

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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.
iii

Contents

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

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

Estimated Debris-Flow Probabilities and Volumes ........................................................................................................ 8

Use and Limitations of the Assessment......................................................................................................................... 9

References Cited..........................................................................................................................................................10

Figures

1. Location of drainage basins of interest and soil burn severity map of the 2011 Horseshoe II burn area, 

southeastern Arizona..................................................................................................................................................... 2

2. Estimated probability of potential postwildfire debris flows in the 2011 Horseshoe II burn area, southeastern 

Arizona in response to a 5-year-recurrence, 30-minute-duration rainfall ....................................................................... 3

3. Estimated volumes of potential postwildfire debris flows in the 2011 Horseshoe II burn area, southeastern 

Arizona in response to a 5-year-recurrence, 30-minute-duration rainfall. ...................................................................... 4

Table

1. Estimated probabilities and debris-flow volumes for the 2011 Horseshoe II burn area, southeastern Arizona...... 5
iv

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 10.76 square foot (ft2) 

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/hr) 0.03937 inch per year (in/hr) 

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)”
1

Probability and Volume of Potential Postwildfire Debris 

Flows in the 2011 Horseshoe II Burn Area, Southeastern 

Arizona

By Barbara C. Ruddy

Abstract

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

basins burned in 2011 by the Horseshoe II wildfire in southeastern Arizona. Empirical models derived 

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

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

debris-flows volumes for selected drainage basins. Input for the models include measures of burn severity, 

topographic characteristics, soil properties, and rainfall total and intensity for a (1) 2-year-recurrence, 30-

minute-duration rainfall, (2) 5-year-recurrence, 30-minute-duration rainfall, and (3) 

10-year-recurrence, 30-minute-duration rainfall. 

Estimated debris-flow probabilities in the drainage basins of interest ranged from less than 1

percent in response to the 2-year-recurrence, 30-minute-duration rainfall to a high of 100 percent in 

response to the 10-year-recurrence, 30-minute-duration rainfall. The high probabilities in all modeled 

drainage basins are likely due to the abundance of steep hillslopes and the extensive areas burned at 

moderate to high severities. The estimated debris-flow volumes ranged from a low of 20 cubic meters to a 

high of greater than 100,000 cubic meters. 

Introduction

Debris flows, fast-moving slurries of sediment and water, have been documented after many 

wildfires 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 can 

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 

additional sediment that can lead to the generation of destructive debris flows. Debris flow hazards are 

most significant 1 to 3 years following wildfires (Susan Cannon, U.S. Geological Survey, written 

commun., 2010). Debris flows have already been documented from the Horseshoe II fire, debris flows 

occurred in July 2011 along the 42 Road on the west side of Onion Saddle in response to a monsoonal 

thunderstorm (Ann Youberg, Arizona Geological Survey, written commun., 2011). 

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

basins burned in 2011 by the Horseshoe II wildfire in southeastern Arizona (figs. 1, 2, and 3, table 1). This 

assessment was done by the U. S. Geological Survey (USGS) in collaboration with the State of Arizona, 

U. S. Department of Agriculture Forest Service, and the Federal Emergency Management Agency Region 

IX. Estimates are provided of the predicted probability of debris-flow occurrence and volume of debris 

that could flow from 59 drainage-basin outlets in response to three design storms: (1) 2-year-recurrence, 

30-minute-duration rainfall (24–31 millimeters (mm), a 50 percent chance of occurrence in any given 

year), (2) 5-year-recurrence, 30-minute-duration rainfall (33–40 mm, a 20 percent chance of occurrence in 

any given year), and (3) 10-year-recurrence, 30-minute-duration rainfall (37–46 mm, a 10 percent chance 

of occurrence in any given year). The methods used for this assessment are based on the work by Cannon 

and others (2007; 2010) and Ruddy and others (2010; figs. 1, 2, and 3, table 1).
2

Figure 1. Location of drainage basins of interest and soil burn severity map of the 2011 Horseshoe II burn area, 

southeastern Arizona.
3

Figure 2. Estimated probability of potential postwildfire debris flows in the 2011 Horseshoe II burn area, 

southeastern Arizona in response to a 5-year-recurrence, 30-minute-duration rainfall.
4

Figure 3. Estimated volumes of potential postwildfire debris flows in the 2011 Horseshoe II burn area, 

southeastern Arizona in response to a 5-year-recurrence, 30-minute-duration rainfall.
5

Table 1. Probabilities and estimated debris flow volumes for the 2011 Horseshoe II burn area, southeastern Arizona.

[BAER, burned area emergency response; >, greater than; <, less than]

Drainagebasin number 

for pour point 

(figs. 1–3)

BAER Description Area Latitude Longitude

Debris flow in 

response to a 2-year, 

30-minute rainfall

Debris flow in 

response to a 5-year, 

30-minute rainfall

Debris flow in 

response to a 10-year, 

30-minute rainfall

square 

kilometers

degrees, 

minutes, 

seconds

degrees, 

minutes, 

seconds

Probability 

(percent)

Volume 

(cubic 

meters)

Probability 

(percent)

Volume 

(cubic 

meters)

Probability 

(percent)

Volume 

(cubic 

meters)

1a Sulphur HUC6 18.81 31°50'53" 109°04'42" 68 > 100,000 84 > 100,000 90 > 100,000

2b Cave Portal 109.94 31°54'43" 109°08'32" 34 > 100,000 58 > 100,000 74 > 100,000

3c East Turkey Fireline 35.93 31°58'15" 109°12'24" 35 > 100,000 59 > 100,000 75 > 100,000

4d East Whitetail Jhus Fireline 12.78 31°58'51" 109°13'24" 76 > 100,000 90 > 100,000 95 > 100,000

5 East Whitetail at Fireline 33.68 31°59'50" 109°14'33" 77 > 100,000 91 > 100,000 95 > 100,000

6 East Whitetail Oak Creek Fireline 4.61 32°01'33" 109°14'43" 17 20,800 35 23,800 52 26,000

7 Brushy at Fireline 8.03 32°04'41" 109°14'19" 18 38,600 36 44,000 52 48,000

8 Brushy Triangle Canyon 5.90 32°05'12" 109°14'51" 7 18,800 16 21,500 28 23,400

9 Wood HUC6 36.09 32°07'42" 109°17''41" 39 > 100,000 62 > 100,000 77 > 100,000

10 Littlewood at Fireline 2.80 32°08'22" 109°19'14" 80 19,400 91 22,200 96 24,300

11 Emigrant at Fireline 30.09 32°07'57" 109°22'11" 65 > 100,000 84 > 100,000 92 > 100,000

12 Lower Pinery at Fireline 3.00 32°04'08" 109°22'46" 30 12,600 57 14,500 74 15,900

13 West Whitetail at Fireline 11.13 32°02'47" 109°22'50" 56 79,500 79 91,600 89 100,000

14 Anderson at Fireline 3.64 32°05'40" 109°24'29" <1 20 <1 23 1 25

15 West Whitetail Bonita Canyon Camp 11.00 32°00'44" 109°21'18" 85 > 100,000 95 > 100,000 98 > 100,000

16e West Whitetail Bonita Creek Fireline 28.66 32°00'33" 109°23'33" 58 > 100,000 80 > 100,000 89 > 100,000

17 West Whitetail Visitor Center 9.58 32°00'20" 109°21'25" 63 70,100 84 81,000 92 88,900

18f West Whitetail Far Away Ranch 27.11 32°00'33" 109°22'23" 67 > 100,000 86 > 100,000 93 > 100,000

19g Upper Pinery Hwy 181 59.75 32°00'14" 109°24'48" 74 > 100,000 89 > 100,000 94 > 100,000

20h Pine Hwy 181 45.14 32°00'07" 109°26'20" 61 > 100,000 81 > 100,000 90 > 100,000

21i Upper Pinery at Fireline 54.25 31°59'15" 109°23'01" 4 965 11 1,110 20 1,220

22j Pine at Fireline 36.33 31°58'01" 109°24'05" 87 > 100,000 95 > 100,000 98 > 100,000

23 Five Mile Fife Canyon 23.98 31°57'32" 109°26'23" 83 > 100,000 93 > 100,000 97 > 100,000

24k Pine at Green Canyon 32.68 31°57'23" 109°21'25" 91 > 100,000 97 > 100,000 99 > 100,000

25 Pine at Methodist Camp 13.50 31°55'53" 109°19'11" 94 > 100,000 98 > 100,000 99 > 100,000

26l Upper Pinery Grave Stone 34.35 31°58'12" 109°19'46" 93 > 100,000 97 > 100,000 99 > 100,000

27 Upper Pinery Canyon Camp 2.18 31°55'59" 109°16'19" 98 19,600 100 22,800 100 25,100

28 East Whitetail Jhus Arch Site 2.73 31°57'45" 109°15'30" 94 22,000 98 25,500 99 28,000

29m Five Mile Hwy 181 53.50 31°57'04" 109°28'38" 52 > 100,000 75 > 100,000 86 > 100,000

30 Five Mile Witch Canyon 19.23 31°56'26" 109°26'48" 85 > 100,000 94 > 100,000 97 > 100,000
6

31n East Turkey Paradise 17.70 31°55'45" 109°13'11" 70 > 100,000 88 > 100,000 94 > 100,000

32o East Turkey Road 42 5.56 31°54'34" 109°15'03" 83 39,600 94 46,100 98 50,800

33 East Turkey Rustler Station 0.99 31°53'52" 109°16'25" 40 6,360 70 7,410 85 8,170

34 Rock at Fireline 38.58 31°53'55" 109°27'16" 45 > 100,000 69 > 100,000 82 > 100,000

35p Upper Turkey at Fireline 47.01 31°52'05" 109°23'56" 62 > 100,000 83 > 100,000 91 > 100,000

36q Upper Turkey Creek GS 27.07 31°51'58" 109°20'51" 79 > 100,000 92 > 100,000 96 > 100,000

37 Upper Turkey at Sycamore Camp 11.17 31°51'35" 109°20'04" 64 68,300 85 79,000 93 86,700

38r Cave Sunny Flat Camp 93.29 31°53'05" 109°10'28" 53 > 100,000 78 > 100,000 89 > 100,000

39s Cave South Fork 34.89 31°52'09" 109°11'15" 66 > 100,000 86 > 100,000 93 > 100,000

40 Cave Herb Martyr Camp 17.02 31°52'21" 109°14'02" 75 > 100,000 91 > 100,000 96 > 100,000

41 Ash at Cottonwood 11.32 31°51'03" 109°24'23" 14 44,200 32 50,800 50 55,600

42 Ash at Standford 10.27 31°48'59" 109°25'14" 78 96,000 91 > 100,000 95 > 100,000

43 Pridham at Fireline 10.11 31°48'20" 109°25'03" 72 84,900 88 97,600 94 > 100,000

44 John Long at Fireline 19.40 31°46'57" 109°25'11" 50 > 100,000 74 > 100,000 86 > 100,000

45 Rucker Forest Camp 25.50 31°47'03" 109°18'13" 70 > 100,000 88 > 100,000 94 > 100,000

46t Rucker at Fireline 40.47 31°45'55" 109°19'39" 68 > 100,000 86 > 100,000 93 > 100,000

47u Rucker Camp Rucker 90.36 31°45'27" 109°22'08" 43 > 100,000 69 > 100,000 82 > 100,000

48 Sulphur at Fireline 16.31 31°50'27" 109°07'57" 92 > 100,000 97 > 100,000 98 > 100,000

49 Horseshoe Road 21.91 31°46'26" 109°12'50" 92 > 100,000 97 > 100,000 99 > 100,000

50v Horseshoe at Fireline 70.78 31°46'37" 109°06'60" 54 > 100,000 75 > 100,000 86 > 100,000

51 Jack Wood at Fireline 17.63 31°43'51" 109°11'29" 19 80,000 39 92,000 58 > 100,000

52w Jack Wood HUC6 Adj 29.69 31°41'55" 109°08'09" 4 97,300 10 > 100,000 18 > 100,000

53 Price Brushy Canyon at Fireline 6.06 31°43'58" 109°14'03" 77 47,300 90 54,300 95 59,500

54x Price Brushy at Confluence 34.26 31°42'39" 109°13'46" 51 > 100,000 74 > 100,000 85 > 100,000

55 Price Canyon at Fireline 17.83 31°44'02" 109°14'38" 72 > 100,000 88 > 100,000 94 > 100,000

56 Blind at Fireline B 1.61 31°41'48" 109°15'32" 22 7,900 44 9,080 62 9,930

57 Blind at Fireline A 3.57 31°41'11" 109°16'47" 93 25,800 97 29,700 99 32,400

58 Tex Arch Site 17.89 31°41'05" 109°18'43" 94 > 100,000 98 > 100,000 99 > 100,000

59 Tex at Fireline 48.87 31°40'42" 109°19'05" 37 > 100,000 63 > 100,000 78 > 100,000

a includes USGS drainage-basin number 48 m includes USGS drainage-basin numbers 23 and 30 b includes USGS drainage-basin numbers 38, 39, and 40 n includes USGS drainage-basin numbers 32 and 33 c includes USGS drainage-basin numbers 31, 32, and 33 o includes USGS drainage-basin number 33 d includes USGS drainage-basin number 28 p includes USGS drainage-basin numbers 36 and 37 e includes USGS drainage-basin numbers 15, 17, and 18 q includes USGS drainage-basin number 37 f includes USGS drainage-basin numbers 15 and 17 r includes USGS drainage-basin numbers 39 and 40 g includes USGS drainage-basin numbers 21, 26, and 27 s includes USGS drainage-basin number 40 h includes USGS drainage-basin numbers 22, 24, and 25 t includes USGS drainage-basin number 45 i includes USGS drainage-basin numbers 26 and 27 u includes USGS drainage-basin numbers 45 and 46 j includes USGS drainage-basin numbers 24 and 25 v includes USGS drainage-basin number 49 k includes USGS drainage-basin number 25 w includes USGS drainage-basin number 51 l includes USGS drainage-basin number 27 x includes USGS drainage-basin numbers 53 and 55
7

A set of empirical equations (models) developed by Cannon and others (2010) and derived from 

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

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

volumes of debris flows for selected drainage basins. The regression equation (eq. 1) of debris-flow 

probability is based on empirical data described by Cannon and others (2010, model A). The model for 

debris-flow probability is as follows:

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

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

mathematical constant (approximately 2.718...); and

 

x = –0.7 + 0.03(%SG30) – 1.6(R) + 0.06(%AB) + 0.07(I) + 0.2(%C) – 0.4(LL),

 

where,

%SG30 is the percentage of the drainage-basin area with slope equal to or greater than 30 

percent; 

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 and high severity (data for this 

assessment from Jennifer Lecker, U.S. Department of Agriculture Forest Service, written commun., 

2011); 

I is average storm intensity (calculated by dividing total storm rainfall (Mike McLane, National 

Weather Service, written commun., 2011) by the storm duration, in millimeters per hour); 

%C is clay content of the soil (in percent) (U.S. Department of Agriculture, National Resources 

Conservation Service, 1991, and Schwarz and Alexander, 1995), and 

LL is the liquid limit of the soil (percentage of soil moisture by weight) (U.S. Department of 

Agriculture, National Resources Conservation Service, 1991, and Schwarz and Alexander, 1995). 

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, (2) 

 

where,

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

SG30 is the area of the 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 depth (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).

Each of the 59 evaluated drainage basins was identified by a single outlet (pour point) located at 

the drainage-basin mouth; however, some basins are included within larger basins (table 1 and figure 1). 

Conditions within the drainage-basin area upstream from the identified pour point were used to estimate 

debris-flow probability and volume for a specific design storm (Cannon and others, 2010). Locations of 
8

drainage-basin pour points were identified by the Burned Area Emergency Response (BAER) team for 

the Horseshoe II fire (indicated by drainage-basin numbers in figures 1, 2, and 3 and table 1). 

In addition to the hazard assessments for the discrete drainage basins identified by pour points, a 

continuous parameterization technique was used to map potential debris-flow hazards along the 

drainage network of each basin. With this technique, estimates of debris-flow probability and volume 

(Cannon and others, 2010) were obtained continuously along the drainage network (Verdin and 

Greenlee, 2003; Verdin and Worstell, 2008). This technique was developed as an alternative to 

traditional basin characterization approaches, which requires “a priori” definition of drainage-basin

outlets (pour points) and their corresponding basins.

Using the 1/3-arc-second National Elevation Dataset (Gesch and others, 2002) (10-meter (m)

nominal resolution) for the study area and the flow structure inherent in the digital elevation model 

(DEM), the independent variables driving the probability and volume equations were evaluated for 

every 10-m grid cell within the extent of the DEM. Rainfall totals and rainfall intensities were 

calculated from 800-m precipitation grids provided by the National Weather Service (Mike McLane, 

National Weather Service, written commun., 2011). Values for all of the independent variables driving 

the predictive equations were obtained using the continuous parameterization approach in a geographic 

information system (GIS), although “ruggedness” required a separate ArcGIS program (ESRI, 2009) to 

evaluate this variable for each grid cell in the study area. Once the surfaces of the independent variables 

were evaluated for every grid cell within the study area, the probability and volume equations were 

solved by using map algebra for each location. Identification of the probability or volume of a debris 

flow at any location within the study area is possible by querying the derived surfaces. For this 

assessment, a raster sampling technique (Verdin and Greenlee, 2003; Verdin and Worstell, 2008) was 

used to identify the values of debris-flow probability and volume at selected locations along the 

drainage network derived from a DEM.

The continuous parameterization technique allows for faster parameter characterization, and the 

ability to characterize debris-flow hazard upstream from any location, not just at predefined basin 

outlets. The continuous parameterization technique provides a synoptic view of the entire study area 

which aids in the identification of smaller basins with high probability of debris flows within a larger 

basin. This allows for rapid evaluation of potential hot spots (locations with potentially high 

probabilities of large debris flows) within the burned area. Although modeled probabilities and 

volumes at the pour point of a predefined basin might be relatively low, locations within the drainage 

basin might have substantially higher potential for debris flow. These can be easily identified with the 

continuous parameterization technique.

Estimated Debris Flow Probabilities and Volumes

The estimated debris-flow probabilities and volumes in response to the different rainfall 

scenarios are presented in table 1. Estimated probabilities of debris flows were highly variable. 

Conditions in 14 of the 59 basins resulted in debris-flow probabilities greater than 80 percent in 

response to the 2-year-recurrence, 30-minute-duration rainfall. Conditions in 35 of the 59 basins 

resulted in debris-flow probabilities greater than 80 percent in response to the 5-year-recurrence, 30-

minute-duration rainfall. Conditions in 45 of the 59 basins resulted in debris-flow probabilities greater 

than 80 percent in response to the 10-year-recurrence, 30-minute-duration rainfall. Drainage basin 14

showed the lowest probabilities, which were never greater than 1 percent for all rainfall scenarios. 

Debris-flow probabilities for drainage basins 24–28, 48, 49, 57, and 58 ranged from 91 to 100 percent 

for the three design rainfalls. These high probabilities are likely due to a combination of steep hillslopes 

burned at moderate and high burn severities, and indicate a potential for substantial debris-flow 
9

impacts to any buildings, roads, bridges, culverts, or reservoirs located both within these drainages and 

immediately downstream from the burned area. It is important to recognize that even small debris flows 

at the basin outlets could cause considerable damage to infrastructure.

Estimated debris-flow volumes ranged from 20 cubic meters (m3) for drainage basin 14 in 

response to 2-year, 30-minute rainfall to greater than 100,000 m3 at several drainage basins in response 

to all three design rainfalls. The model predicts volumes greater than 100,000 m3; however, there is high 

uncertainty at volumes greater than 100,000 m3 (Susan Cannon, U.S. Geological Survey, written 

commun., 2011.) Larger estimated debris- flow volumes usually were predicted for larger drainage 

basins except for drainage basin 19, which had the largest estimated volume and is the fifth largest basin 

(about 55 percent of the size of the largest basin). The upstream area of drainage basin 19 had a 

combination of steep hillslopes burned at moderate and high burn severities.

Hazardous areas with drainage-basin 7 identified by the continuous parameterization approach 

are shown on figure 2. The red channel reach that extends upstream from the pour point indicate debris 

flow probabilities as high as 81–100 percent. Although the probability of a debris flow resulting from 

the 5-year-recurrence, 30-minute duration rainfall occurring at the pour point of the drainage basin is 36

percent (table 1), the probabilities of a debris flow along this channel reach are much higher.

Use and Limitations of the Assessment

This assessment presents estimates of debris-flow probability and volume for selected drainage 

basins and along their channels (or drainage networks) in the area burned by the 2011 Horseshoe II

wildfire. Estimates were made in response to three design storms: (1) a 2-year-recurrence, 30-minuteduration rainfall (a 50 percent chance of occurrence in any given year), (2) a 5-year-recurrence, 30-

minute-duration rainfall (a 20 percent chance of occurrence in any given year), and (3) a 10-yearrecurrence, 30-minute-duration rainfall (a 10 percent chance of occurrence in any given year). Larger, 

less frequent storms than those considered in this study are more likely to produce even larger debris 

flows; however, the analyses indicate that even relatively common rainfall events could result in 

substantial runoff and erosion producing debris flows. Some areas within the selected basins may have 

higher debris-flow probabilities than those shown at the drainage-basin outlet, or pour point, shown on

figure 2, and debris flows may not be produced from all basins during a 2- or 5-year recurrence rainfall. 

The estimates are likely valid for up to 3 years after the wildfire (Susan Cannon, U.S. Geological 

Survey, written commun., 2010). The maps may be used to prioritize areas where emergency flood 

warnings or erosion mitigation may be needed prior to rainstorms within these basins, at their outlets, or 

in areas downstream from these basins. This assessment evaluates only postwildfire debris flows 

(Cannon and others, 2007). Substantial hazards from flash floods without debris flow may remain for 

many years after a wildfire, but are beyond the scope of this assessment.

This assessment is preliminary and is subject to revision. It is being provided due to 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. 
10

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Open-File Report 2007–1384, 10 p. (Also available at http://pubs.usgs.gov/of/2007/1384.0) 

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