Prepared in cooperation with the Association of Environmental 

& Engineering Geologists 

Mass Wasting Following the 2002 

Missionary Ridge Fire near Durango, 

Colorado, a Field Trip Guidebook 

By Jeffrey A. Coe (editor), Erica R. Bigio, Robert W. Blair, Jr., Michael Burke, 

Susan H. Cannon, Victor G. deWolfe, John Ey, Joseph E. Gartner, Mary L. 

Gillam, N.D. (Butch) Knowlton, Paul M. Santi, and William H. Schulz

Open File Report 2007-1289 

Association of Environmental & Engineering Geologists 

Special Publication No. 20 

U.S. Department of the Interior 

U.S. Geological Survey 
Frontispiece. Left: Missionary Ridge fire, early June, 2002. Photograph courtesy of La 

Plata County. Center: La Plata County warning sign along Florida River road. Right: 

Debris-flow deposits on the Stevens Creek fan. Photograph courtesy of La Plata County. 

ii
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Any use of trade, product, or firm names is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. This report has not been reviewed 

for stratigraphic nomenclature. 

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. 

Suggested citation: 

Coe, J.A. (editor), Bigio, E. R., Blair, R.W., Jr., Burke, M., Cannon, S.H., deWolfe, V.G., 

Ey, J., Gartner, J.E., Gillam, M.L., Knowlton, N.D., Santi, P.M., and Schulz, W.H., Mass 

wasting following the 2002 Missionary Ridge fire near Durango, Colorado, a field trip 

guidebook: U.S. Geological Survey Open File Report 2007-1289, 54 p. 

iii
Contents Page Number

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

The Missionary Ridge Fire.........................................................................2 

Geologic and Climatic Setting.....................................................................5 

Effects of Wildfires on Watersheds...............................................................9 

General Description of Mass Wasting Following the Missionary Ridge Fire..............10 

Road Log with Stop Descriptions................................................................13 

Pullout. Overview of Rockslide from Dakota Sandstone............................13 

Stop 1. Overview of Missionary Ridge Fire and Mass Wasting Response........13 

Stop 2. Debris Flows on the Stevens Creek Fan ......................................19 

Stop 3. Debris flows on the Halflin Creek Fan .......................................24 

Stop 4. Lions Den Picnic Shelter........................................................30 

Stop 5. Mitigation at Knight Canyon, Landslides along the Florida River........33 

 Knight Canyon at Lemon Dam.................................................33 

Log Erosion Barriers....................................................33 

Mulching..................................................................37 

Seeding......................................................................37 

Check Dams..............................................................37 

Debris Racks.............................................................39 

The Florida River Landslide....................................................39 

Stop 6. Fire and Debris Flow History North of Vallecito Reservoir...............43 

Sediment Results..................................................................45 

Tree Ring Results.................................................................46 

Discussion.........................................................................48 

Acknowledgements.................................................................................49 

References Cited....................................................................................49 

Figures 

1. Map showing the Missionary Ridge fire burn area and field trip stops........................3 

2. Map showing burn severity of the Missionary Ridge fire ...................................4 

3. Geologic map of the Missionary Ridge fire burn area........................................6 

4. Stratigraphic section of the Missionary Ridge fire burn area...............................7 

5. Fan at the mouth of Stevens Creek.............................................................8 

6. Rock fall/rock slide from Dakota Sandstone .................................................9 

7. Map showing potential peak discharges from the Missionary Ridge fire burn area ...14 

8. Comparison of debris-flow volume estimates to model predictions ......................17 

9. Map showing potential debris-flow volumes from the Missionary Ridge area .........18 

10. Debris-flow deposits on the Stevens Creek fan.............................................21 

11. Photographs showing channel migration on the Stevens Creek fan.....................22 

12. Debris-flow channel on the Stevens Creek fan .............................................23 

13. Halflin Creek drainage basin and fan.........................................................24 

14. Mountainside near the head of the Halflin Creek drainage basin........................25 

iv
15. Rills in the Halflin Creek drainage basin ...................................................26 

16. Ash/mud flow deposit in the Halflin Creek drainage basin ..............................26 

17. Debris-flow deposits in the Halflin Creek drainage basin ................................27 

18. Channel incision in the Halflin Creek drainage basin ...................................28 

19. Damage from debris flows on the Halflin Creek fan .....................................29 

20. View of lower Animas Valley to the north .................................................31 

21. Knight Canyon above the spillway intake at Lemon Dam ...............................34 

22. Staggered pattern in which log erosion barriers should be placed on a slope .........34 

23. Log erosion barriers near Lemon Dam .....................................................35 

24. Hand-spread and crimped mulch near Lemon Dam ......................................38 

25. Vegetative cover from seeding near Lemon Dam.........................................38 

26. Filled check dam in Knight Canyon near Lemon Dam ..................................39 

27. Filled debris rack in Knight Canyon near Lemon Dam...................................40 

28. Aerial view of the Florida River landslide area ...........................................40 

29. Geologic map of the Florida River landslide area.........................................42 

30. Debris flow source area in the Florida River landslide area .............................43 

31. Study basin on the northwest side of Vallecito Reservoir................................44 

32. Exposed channel reach within the study basin northwest of Vallecito Reservoir.....45 

33. Chronology of fine-grained deposition northwest of Vallecito Reservoir .............47 

34. Surface fire chronology northwest of Vallecito Reservoir ...............................47 

Tables 

1. Ages of lower Animas glacial moraines .....................................................32 

2. Summary of erosion and sediment control at Lemon Dam ...............................36 

3. Timeline of erosion and sediment control at Lemon Dam .................................36 

v
Conversion Factors 

Inch/Pound to SI 

Multiply By To obtain 

Length 

Inch (in.) 2.54 centimeter (cm) 

Inch (in.) 25.4 millimeter (mm) 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

yard (yd) 0.9144 meter (m) 

Area 

Acre 4,047 Square meter (m2) 

Acre 0.4047 hectare (ha) 

Acre 0.4047 Square hectometer (hm2

) 

Acre 0.004047 Square kilometer (km2

) 

square foot (ft2

) 929.0 Square centimeter (cm2) 

square foot (ft2

) 0.09290 Square meter (m2) 

square mile (mi2

) 259.0 hectare (ha) 

square mile (mi2

) 2.590 Square kilometer (km2) 

Volume 

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic yard (yd3) 0.7646 cubic meter (m3) 

Flow rate 

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) 

vi
vii

SI to Inch/Pound 

Multiply By To obtain 

Length 

centimeter (cm) 0.3937 Inch (in.) 

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) 0.0002471 acre 

hectare (ha) 2.471 Acre 

square hectometer (hm2

) 2.471 Acre 

square kilometer (km2

) 247.1 Acre 

square centimeter (cm2

) 0.001076 square foot (ft2) 

square meter (m2

) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2

) 

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) 

Flow rate 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s) 
Mass Wasting Following the 2002 

Missionary Ridge Fire near Durango, 

Colorado, a Field Trip Guidebook 

By Jeffrey A. Coe1 (editor), Erica R. Bigio2, Robert W. Blair, Jr.3, Michael Burke4, 

Susan H. Cannon1, Victor G. deWolfe5, John Ey6, Joseph E. Gartner1, Mary L. 

Gillam7, N.D. (Butch) Knowlton8, Paul M. Santi5, and William H. Schulz1 

1

 U.S. Geological Survey, Denver Federal Center, MS 966, Denver, CO. 

2

 Laboratory of Tree Ring Research, University of Arizona, Tucson, AZ. 

3

 Consulting Geologist, Professor Emeritus, Department of Geology, Fort 

 Lewis College, Durango, CO. 

4

 Recently retired from U.S. Forest Service, Durango, CO. 

5

 Department of Geology and Geological Engineering, Colorado School of 

 Mines, Golden, CO.

6

 Florida River Water Conservancy District, Durango, CO. 

7

 Consulting Geologist, Adjunct Professor of Geology, San Juan College, 

 Farmington, NM, and Fort Lewis College, Durango, CO. 

8

 Office of Emergency Management, La Plata County, CO. 

Introduction 

As the title indicates, this field trip guide will focus on mass wasting following 

the 2002 Missionary Ridge fire near Durango, Colorado (fig. 1). We prepared this guide 

to accompany a May 4, 2006, field trip during the second Roy J. Shlemon Specialty 

Conference, which was held in Durango, Colorado, May 3−5. The conference, entitled 

Mass Wasting in Disturbed Watersheds, was sponsored by the Association of 

Environmental & Engineering Geologists (AEG) and the AEG Foundation. The 

objective of this Shlemon Conference was to bring together practitioners and researchers 

to define the current state of practice and identify unresolved problems with regard to the 

prediction and mitigation of mass wasting in disturbed watersheds. The four technical 

session topics of the conference were (1) post-wildfire mass wasting; (2) logging and 

mass wasting; (3) mining, urbanization, volcanism, and other disturbances; and (4) 

emerging technologies and tools (for example, LiDAR, GIS applications, watershed 

modeling). Shlemon Conferences follow procedures typical of the Geological Society of 

America Penrose Conferences and the American Geophysical Union Chapman 

Conferences. Shlemon Conferences are supported, in part, by funds provided by Roy J. 

Shlemon, a Trustee of the AEG Foundation, an honorary member of the AEG, and a 

fellow of the Geological Society of America. The conferences provide for an intensive 

two- to three-day forum of technical discussions among researchers and practitioners, and 

focus on topics beneficial to society and directly related to application of the engineering 

and environmental geosciences. The conferences are intended to promote and 

1
disseminate practice and research results and to foster improvement and understanding of 

applied geology. 

The one-day field trip begins and ends in Durango. Many of the field trip stops 

are at debris-flow fans around the periphery of the burn area (fig. 1), but one stop 

examines landslide activity in the burn area that initiated during spring 2005 snowmelt 

within a dormant, deep-seated landslide, as well as an erosion/debris-flow mitigation 

effort in a drainage basin above Lemon Reservoir. Stops on fans will focus on (1) 

erosion and sediment entrainment that caused debris flows in the summer of 2002; (2) 

Holocene and 2002 debris-flow deposits; and (3) fire history, debris-flow hazards, hazard 

assessments, and mitigation schemes. A lunch stop provides a geologic overview of the 

Durango area. 

In this guidebook we provide a description of the Missionary Ridge fire, the 

geologic and climatic setting of the field-trip area, the general effects of wildfire on 

watersheds, a general description of mass wasting following the Missionary Ridge fire, 

and finally, a road log including descriptions of each stop. 

The Missionary Ridge Fire 

The Missionary Ridge fire began on June 9, 2002, and burned until about July 15, 

2002, when containment was declared. The fire occurred during the height of a severe 

drought in Colorado (Pielke and others, 2005) and was one of 30 wildfires within the 

State in the spring and summer of 2002 (Colorado Department of Local Affairs, 2002). 

The fire started in a ditch along Missionary Ridge Road (fig. 1), but the origin of the 

spark that started the fire is unclear. After inspecting the ditch, a U.S. Forest Service fire 

investigator concluded that the fire was probably started by a carbon particle spewed 

from a vehicle exhaust pipe (Greenhill, 2002). The resulting fire was responsible for the 

death of one person, burned about 73,000 acres (29,000 ha), and destroyed 83 structures 

(Burn Area Emergency Rehabilitation Team, 2002; La Plata County, 2006). 

The fire burned areas north and northeast of Durango, including parts of the 

Animas, Florida, and Los Pinos River Valleys. The fire burned Ponderosa pine, mixed 

conifer, aspen, and spruce-fir forests at elevations ranging from about 6,500 ft (1,980 m) 

along the Animas Valley to about 11,400 ft (3,475 m) near the northern boundary of the 

fire. Most of the burned area is characterized by steep mountain sides and canyons and is 

inaccessible by roads. Burn severity mapping following the fire classified 31 percent of 

the area as high burn severity, 30 percent as moderate severity, and 39 percent as low 

severity or unburned (fig. 2; Burn Area Emergency Rehabilitation Team, 2002). About 

one third (23,000 acres, 9,300 ha) of the total area burned was recommended by the Burn 

Area Emergency Rehabilitation (BAER) Team for remediation and stabilization efforts 

(aerial seeding, mulching, log erosion barriers). 

2
Figure 1. Map showing the Missionary Ridge fire burn area (outlined in red) and field 

trip stops (indicated by red stars). Base map has been modified from Greenhill (2002). 

3
4
Geologic and Climatic Setting 

Most of the burned area is underlain by Paleozoic and Mesozoic sedimentary 

rocks (figs. 3 and 4). About 50 percent of the burn area is underlain by the Permian 

Cutler Formation, which consists of a sequence of red-colored sandstones, siltstones, and 

shales deposited by streams flowing from the Ancestral Rocky Mountains (Blair, 1996; 

Carroll and others, 1997; Carroll and others, 1998; Carroll and others, 1999; Gonzales 

and others, 2002). Additional sedimentary formations that underlie the burn area include 

the Hermosa, Molas, Junction Creek, Wanaka, Entrada, Dolores, Dakota, and Morrison 

Formations (fig. 3). These formations consist of interbedded sandstones, siltstones, 

shales, limestones, and conglomerates. The Precambrian Eolus Granite also underlies the 

burn area near the northern boundary. Colluvial and glacial deposits have also been 

mapped in the burn area. 

The area has a semi-arid climate with warm summers and cold winters. Average 

annual precipitation in Durango (elevation 6,512 ft, 1,985 m) is about 19 in (480 mm). 

About one half of this amount falls between July and October during the summer-fall 

monsoon season, which is characterized by severe thunderstorms. Data from Snotel 

stations (U.S. Department of Agriculture), located at higher elevations near the burn area, 

indicate that annual precipitation is about two times greater than that in Durango. At the 

Vallecito Snotel station (fig. 1; elevation 10,880 ft, 3,316 m), average annual 

precipitation is 35 in (889 mm), with about 50 percent falling in the form of snow. At the 

Stump Lakes Snotel station (fig. 1; elevation 11,200 ft., 3,414 m) average annual 

precipitation is 40 inches (1,016 mm), with 50 percent falling as snow. 

Mass wasting is common in the area, as evidenced by extensive deep-seated 

landslide deposits (fig. 3), debris-flow fans (for example, fig. 5), and fresh rock-fall scars 

and deposits (for example, fig. 6). Deep-seated and shallow landslides are common in 

the Morrison, Burro Canyon, Dakota, Mancos Shale, and Lewis Shale Formations (for 

example, Colton and others, 1975; Blair, 1977; Carroll and others, 1997; Carroll and 

others, 1999; Krochenski and Kenny, 2003; Coe and Burke, 2003). Debris flows are 

frequently initiated from colluvium derived from the Cutler Formation and Eolus Granite, 

and less frequently from colluvium derived from the Hermosa and Morrison Formations 

(Burn Area Emergency Rehabilitation Team, 2002). 

 

5
6
Figure 4. Stratigraphic section of the Missionary Ridge fire burn area. From Blair 

(1996). 

7
Figure 5. Fan at the mouth of Stevens Creek (Stop 2, fig. 1). Photograph 

taken after the Missionary Ridge fire. Photograph courtesy of La Plata County.

A 

8
 

B

Figure 6. Rock fall/rock slide from the Dakota Sandstone along the east side of the 

Animas River valley about 4 km north of Durango. A, Photograph taken in the early 

2000’s; B, Photograph from 2005. 

Effects of Wildfires on Watersheds 

The effects of wildfires on watersheds have been described by Cannon and others 

(2003a) and the text in this section has been extracted from this previously published 

work. Wildfire can have profound effects on a watershed. Consumption of the rainfallintercepting canopy and of the soil-mantling litter and duff, intensive drying of the soil, 

combustion of soil-binding organic matter, and the enhancement or formation of waterrepellent soils can result in decreased rainfall infiltration into the soil and can 

significantly increase overland flow and runoff in channels (for example, Doerr and 

others, 2000; Martin and Moody, 2001; Moody and Martin, 2001). Removal of 

obstructions by wildfire can enhance the erosive power of overland flow, resulting in 

accelerated erosion of material from hillslopes (Meyer, 2002). Increased runoff can also 

9
erode significant volumes of material from channels, the net result being the transport and 

deposition of large volumes of sediment both within and down-channel from the burned 

area. 

Debris flows are frequently produced in response to convective thunderstorm 

activity over basins burned by wildfire (Parrett, 1987; Meyer and Wells, 1997; Cannon, 

2001; Cannon and Gartner, 2005), as well as in response to winter frontal storms 

(Morton, 1989; Cannon, 2000). Debris flows pose a hazard distinct from other sedimentladen floods because of their unique destructive power; debris flows can occur with little 

warning and exert large impulsive loads on objects in their paths. Even small debris 

flows can strip vegetation, block drainage ways, damage structures, and endanger human 

life. 

 

General Description of Mass Wasting Following the 

Missionary Ridge Fire 

In general, the Missionary Ridge fire increased the frequency and volume of 

mass-wasting events (particularly debris flows) in the burn area. The following narrative 

describes (1) the runoff response of the burn area to monsoon rainstorms in the summer 

and fall of 2002 (from Cannon and others, 2003a; see fig. 7 for basin locations and fig. 1 

for County roads mentioned below) and (2) the response of a large, deep-seated landslide 

near Lemon Reservoir following rapid snowmelt of heavy winter snowpack in the spring 

of 2005 (Schulz and others, 2006). 

July 11, 2002.⎯The first storm to affect the area burned by the Missionary Ridge 

fire was a localized thunderstorm that impacted the area south of the Vallecito Reservoir 

dam. La Plata County Road (CR) 501 and a softball field were flooded with ash and mud 

flushed from the hillslopes. This storm occurred before a rain-gage network had been 

installed; thus no precipitation record is available. 

 July 22, 2002.⎯This storm resulted in a similar flush of ash and mud from the 

burned hillslopes near Vallecito Reservoir. CR 501 was closed between the Vallecito 

Reservoir dam and the intersection of CR 240 in order to clear ash, mud, debris, and a 

400-lb (180-kg) boulder that was moved by the flood waters (Kostka, 2002). Red Creek 

also showed significant flood activity. 

 July 23, 2002.⎯This first significant storm of the season impacted the Florida and 

Los Pinos River basins. Debris flows were produced from an unnamed basin just below 

Lemon Reservoir, and significant sediment-laden floods were produced from Dry, 

Shearer, and True Creeks. At least five vehicles, four of which contained people, were 

swept off CR 240 by the advancing water (Kostka and others, 2002). The debris flow 

from unnamed basin 1 crossed CR 243, blocked a driveway, and damaged a guardrail on 

a bridge. Material consisting of ash, fine-grained sand, gravel, and clasts up to 3 ft (1 m) 

in diameter was deposited up to 4 ft (1.2 m) deep on the road. This storm also resulted in 

sediment-laden floods that crossed CR 501 along Vallecito Reservoir in several places. 

 August 3, 2002.⎯This storm resulted in the generation of debris flows from many 

basins within the Los Pinos River drainage and along CR 240. A debris flow was 

produced from Freeman Creek at about 12:15 p.m. Material up to 6 ft (2m) in diameter 

was moved in this event, and passage of the flow resulted in nearly 5 ft (1.5 m) of 

incision of the channel bed. Debris flows crossed CR 501 in several places near Freeman 

10
Creek. Root Creek also produced a debris flow that deposited material approximately 10 

ft (3 m) deep, including boulders up to 6 ft (2 m) in diameter, at the junction with CR 

501. Debris flows also crossed CR 240 in several places. 

 August 5, 2002.⎯Debris flows were again reported to have been produced from 

unnamed basin 1, where materials up to 4 ft (1.2 m) in diameter in abundant muddy 

matrix crossed CR 243. Sediment-laden streamflow events occurred along CR 501 near 

Vallecito Reservoir. 

 August 8, 2002.⎯This storm brought rain to the Animas, Florida, and Los Pinos 

River basins. Unnamed basin 2 produced a debris flow that deposited ash, mud, and 

debris at a business located across CR 501. This event moved material up to 3 ft (1 m) in 

diameter across the road. Sediment-laden streamflow was produced from many of the 

smaller drainages north of unnamed basin 2, and flooding in Freeman Creek led to 

additional incision in the channel bed. Unnamed basin 3 (fig. 7) produced a debris flow 

that deposited levees approximately 3 ft (1 m) high that lined the channel; material up to 

3 ft (1 m) in diameter was transported in a muddy matrix by this event. Adjacent Dry 

Creek showed evidence of sediment-laden streamflow. Root Creek also experienced a 

debris flow, and a sediment-laden flood was produced in Elkhorn Canyon along the 

Animas River. Material from the Elkhorn Canyon event covered CR 250 with about 1 ft 

(0.3 m) of ash and mud. 

 August 20, 2002.⎯This storm resulted in the generation of sediment-laden floods 

from Freeman and Dry Creeks, as well as from unnamed basin 3. 

 August 21, 2002.⎯An apparently localized storm resulted in the production of 

another debris flow from Root Creek and light flooding in adjacent drainages. Passage of 

the flow in Root Creek resulted in approximately 3 ft (1 m) of channel incision, and 

significant deposition along the channel banks. Material up to 3 ft (1 m) in diameter was 

transported by this event. Deposits from sediment-laden floods were also observed at the 

mouth of Freeman Creek. 

 August 29, 2002.⎯A localized rainstorm triggered debris flows from Haflin and 

Kroeger Canyons in the Animas Valley. The flows carried material no greater than 3 ft 

(1 m) in diameter in a muddy matrix. 

 September 7, 2002.⎯This was the first storm that produced debris flows in 

multiple basins that drain into the Animas River. Debris flows were produced from Coon 

Creek, Stevens Creek, Freed Canyon, and Woodard Canyon. These events blocked CR 

250 in three places with several feet of debris. The event on Stevens Creek covered 

approximately one quarter of the fan area with deposits up to 5 ft (1.5 m) deep in places 

and inundated a home with tons of rock and mud. Prior to the event, the home was at 

least 200 ft (62 m) from the active channel. This event moved material up to 8 ft (2.5 m) 

in diameter in a muddy matrix. Evidence on the fan surface suggests that channels were 

blocked by large boulders and were diverted many times during the event. Upstream 

from the fan, the channels showed up to 8 ft (2.5 m) of incision into extensive valley-fill 

deposits. The debris flow produced from Freed Canyon moved materials up to 6 ft (2 m) 

in diameter in a muddy matrix. The high mud line produced from this event was 

measured 12 ft (4 m) above the post event channel base, and up to 6 ft (2 m) above the 

original bank levels. The area downstream from the waterfall at the head of the fan was 

heavily scoured in this event. Deposition by the debris flow in Coon Creek resulted in 

the shifting of the channel bed to the north by about 10 ft (3 m). Material from this event 

consisted of up to cobble-sized material in a muddy matrix. In addition to these events, 

11
sediment-laden floods near Red, Shearer, and True Creeks blocked CR 240 in the Florida 

River basin. 

September 10−12, 2002.⎯This storm had the longest duration of any that 

occurred during the 2002 monsoon season. The storm started on the evening of 

September 10 and continued into the early morning hours of September 12. Debris 

flows were again produced from Coon and Stevens Creeks and Woodard Canyon, but 

these were smaller discharge events than those of September 7, carrying smaller (up to 

cobble-sized) material. Deposition by debris flow on Coon Creek filled in the channel 

and shifted the channel to the south. Soil slip scars that formed during this storm were 

observed in the upper reaches of Stevens Creek. These scars were relatively small⎯at 

most 100 ft2 (10 m2) in area. Material from these scars mobilized into debris flows that 

traveled a few hundred feet down the hillslopes on which they originated. Debris flows 

were also produced from Root Creek near Vallecito Reservoir and from unnamed basin 1 

near Lemon Reservoir. Sediment-laden floods also resulted in the closure of CR 501 at 

Jack and Dry Creeks near Vallecito Reservoir. 

 September 20, 2002.⎯A storm focused on the southern end of the Animas Valley 

front produced debris flows from Haflin and Kroeger Canyons and from Stevens Creek. 

These events carried materials only up to about 1.5 ft (0.5 m) in diameter, a significant 

decrease in size from previous events. These events appear to have been triggered by a 

fairly localized storm, as rain gages located nearest to the canyons did not record any 

rainfall on this date. 

 October 2, 2002.⎯The last of the monsoon storms of 2002 to impact the area 

produced sediment-laden floods on Coon Creek. These events reworked the existing 

deposits and scoured the south side of the channel. 

Summary of 2002 events⎯Evaluation of the storm conditions that resulted in the 

2002 events shows that most of the debris-flow and flood-triggering storms had 

recurrences of less than 2 yrs; only one gage (in the July 23, 2002, storm) recorded a 10-

yr recurrence event (Cannon and others, 2003a). 

April and May, 2005.⎯Landslide activity on a steep, burned hillslope southeast 

of Lemon Dam included the formation of at least nine shallow (approx. 2-m (6-ft) thick) 

colluvial landslides that mobilized into more than a dozen debris flows. Activity was 

concentrated on a very large, apparently dormant landslide. Landslides within the 

colluvium were translational debris slides (Cruden and Varnes, 1996) with volumes 

between 2,600 and 46,000 yd3 (2,000 and 35,000 m3). Debris flows were generally 

mobilized from dilated, mostly saturated, toe regions of the slides and ranged in volume 

from 200 to 2,000 yd3

 (150 to 1,500 m3). Although generally quite small, debris flows 

traveled as far as about 980 ft (300 m) downslope onto the Florida River floodplain. The 

landslides were triggered by rapid melting of deep snow with a high water content. Area 

residents had not observed landsliding on the hillslope previously (during about the past 

30 yrs), although similar snowmelt events have occurred historically. 

12
Road Log with Stop Descriptions 

From downtown Durango proceed north on Highway 550. About 2.5 miles (4 

km) north of the Durango city limit (just north of the Days Inn), pull off the road into the 

large pullout on the west side of Highway 550. 

Pullout. Overview of Rockslide from Dakota Sandstone 

GPS location: N 37° 19’ 44.5” W 107° 51’ 07.0” Elevation: 6,589 ft. (2,008 m). 

Discussion Leader: Rob Blair. 

A glance to the east reveals a landslide scar from Dakota Rim rock (fig. 6). This 

rockslide occurred at approximately 10:30 a.m. on July 5, 1998. Eye-witness reports 

ranged from a forest fire (from all the dust), to an airplane crash, to rock fall. 

Approximately 37,000 yd3 (28,500 m3 or 74,000 metric tons) of Dakota Sandstone rock 

toppled and slid downslope after being undercut by natural weathering of clays that 

underlie the rock stratum. Years of snowmelt and water seepage down through joints and 

along the sandstone-clay boundary allowed gravity creep of the overlying rock to occur. 

As an interesting side note, the property owners held a Forth of July party to view the 

Durango fireworks on the very rocks that collapsed the following day. They videotaped 

the picnic with astute comments about “someday this will fail” as they leaped across a 

gaping crack from bedrock to the block that fell. 

Recent photos and observations from residents living near the base of the 

rockslide slope suggest that parts of the deposit have subsequently mobilized as debris 

flows (compare fig. 6A to 6B). 

 

Continue north on Highway 550, then turn east on CR 252. Cross over the 

railroad tracks and turn left into a large pullout on the north side of the road. 

Stop 1. Overview of Missionary Ridge Fire and Mass Wasting Response 

Pullout on north side of CR 252 immediately east of the railroad tracks. GPS 

location: N 37° 23’ 24.8” W 107° 50’ 44.0” Elevation: 6,530 ft. (1,990 m). Stop Leaders: 

Butch Knowlton, Sue Cannon, Joe Gartner. 

The west-facing flank of the Animas Valley is visible to the east from this stop. 

The Missionary Ridge fire started about 4.5 miles (7 km) northeast of this stop, but 

burned primarily to the east and south toward Durango. The fire burned portions of about 

90 drainage basins (fig. 7). Most of these basins were uninhabited, but many of the fans 

at the mouths of the basins were populated with houses and public and private roads. A 

large part of the response of local, State, and Federal officials following the fire dealt 

with providing warnings and predictive maps for fan areas. 

A system of warning signs, signals, and maps was established by several 

cooperating agencies. First, La Plata County installed numerous warning signs along 

county roads around the periphery of the burn area (for example, see Frontispiece). 

These signs served as a reminder to motorists to be aware of weather conditions and take 

appropriate precautions. 

13
14
Second, after recognizing the extreme flood and debris-flow potential for areas 

around the periphery of the burn area, the Federal BAER Team requested that the U.S. 

Geological Survey install a series of rain gages to monitor rainfall in the burn area. These 

rain gages were positioned at 12 locations and equipped with satellite telemetry 

communication. The National Weather Service office in Grand Junction received the 

real-time rain gage information and utilized a National Oceanic and Atmospheric 

Administration all-hazards weather radio transmitter installed on Missionary Ridge to 

broadcast weather and emergency information. All citizens in La Plata County, 

particularly in areas near the burn area, were encouraged to purchase a weather radio 

receiver. La Plata County cooperated with the National Weather Service to activate the 

radios to provide warnings and weather information to areas near the fire perimeter that 

could be affected by storms and flooding. 

 Third, predictive hazard maps for peak debris-flow discharges and debris-flow 

volumes were produced for the burn area. Peak-discharge maps were generated 

immediately after the fire, and are based on a preliminary regression model developed 

using debris-flow and basin characteristic data from 53 recently burned basins throughout 

the intermountain west (Cannon and others, 2003c). The model used to estimate peak 

debris-flow discharges (Qp) for basins in the Missionary Ridge area was: 

Qp/I = -4.5x107

 – 4.2x104 (S50) +1.1x107 (logAb), (1) 

where I is average storm rainfall intensity (m/s), S50 is the percentage of slopes greater 

than or equal to 50 percent within a basin, and Ab is the area burned within a basin. Slope 

values were determined from 10-m or 30-m Digital Elevation Models (DEMs) and 

burned area values were determined by BAER Teams or from Landsat Thematic Mapper 

data. Results from this model for a 1-hr rainstorm of 1.3 in (33 mm) and a 25-yr return 

period are shown in figure 7. Assessments for 1-hr duration, 5- and 100-yr recurrence 

storms were also generated (Cannon and others, 2003a). 

Unfortunately (or fortunately, as the case may be) most of the storms that 

impacted the Missionary Ridge area had recurrence intervals of less than 2 yrs, and only 

one rain gage during the July 23, 2002, storm recorded a 10-yr recurrence storm (Cannon 

and others, 2003a). Thus, comparisons of actual responses with the predicted values in 

individual basins were difficult. However, some broad-based estimates of peak 

discharges using surveys of channel cross sections following the September 7, 2002, 

event, and an assumed velocity of 16 ft/s (5 m/s), produced values of about 5,500 ft3

/s 

(165 m3/s) at the head of the Stevens Creek fan (Cannon and others 2003a). This value is 

within the range of the predicted value for a 5-yr recurrence storm (4,001−6,000 ft3

/s, 

113−170 m3

/s, from Cannon and others, 2003b). In general, the peak-discharge maps 

gave a good indication of those basins that produced the largest events. 

Elliott and others (2004) modeled a 100-yr recurrence, 1-hr duration storm in this 

setting using the U.S. Army Corps of Engineers Hydrologic Modeling System (HECHMS). Estimated peak discharges for Stevens Creek were between 2,186 and 3,835 ft3

/s 

(62−109 m3/s). For the same storm, Cannon and others (2003a) estimated peak 

discharges of greater than 8,000ft3/s (227 m3/s). Comparisons of predicted values with 

15
actual response were again not possible because a 100-yr recurrence storm has not 

occurred in the burn area since the fire. 

 Pierson (2005) described the limited usefulness of peak-discharge measurements 

for debris flows due to orders-of-magnitude changes over short distances, and the 

difficulties in characterizing velocities using standard methods. For this reason, Gartner 

(2005) developed a model for characterizing debris-flow magnitude in terms of volume 

rather than in peak discharge. 

 Gartner (2005) derived potential debris-flow volume estimates based on a 

regression model developed using debris-flow channel and basin characteristic data from 

56 recently burned basins in Colorado, Utah, and California. In each of the 56 basins, 

debris-flow volumes were estimated by quantifying the amount of material eroded from 

the main channels using post-debris flow surveys of closely spaced channel cross 

sections. Post-debris-flow cross sections were compared to interpolated cross sections of 

the original channel surface to estimate the amount of material eroded at each location. 

The original channel surface was estimated based on the assumption that the channel was 

v-shaped prior to erosion during post fire debris flows. Measures of basin morphology 

were calculated from 10- or 30-m digital-elevation models. Maps of burn severity were 

used to quantify the basin areas burned at low, moderate, and high severities. Material 

properties for each basin were characterized by different measures of the grain-size 

distribution determined from field samples of burned soil. The dominant rock type 

underlying the debris-flow-producing basin was characterized either from field 

observations or from geologic maps. Networks of rain gages installed throughout the 

burned areas provided rainfall amounts and intensities for debris-flow-triggering storms. 

 Gartner (2005) used a series of stepwise multiple-regression analyses to develop a 

best-fit model to estimate potential debris-flow volumes in the Western U.S. The model 

is: 

ln V = 0.65(ln S) + 0.86(B)

1/2 + 0.22(R)1/2 + 6.46, (2) 

where V is debris-flow volume (m3

), S is basin area with slopes greater than or equal to 

30 percent (km2), B is basin area burned at moderate and high severity (km2), and R is 

total storm rainfall (mm). As indicated by this equation, material properties and geology 

units had low correlation coefficients and were not used in the model. 

The model for debris-flow volume was validated by comparing predicted debris 

flow volumes with reported volumes using data from 21 basins not used in the generation 

of the model (Gartner, 2005). The validation indicated that all but seven, or 67 percent, 

of the reported volumes were within one standard error of the volumes predicted by the 

model (fig. 8). 

16
Figure 8. Comparison of debris-flow volume estimates from the literature to predictions 

of the volume model of Gartner (2005). The solid black line indicates a perfect fit and 

the dotted lines represent the 68 percent error estimate of the model. 

Results from the application of this model to basins in the Missionary Ridge burn 

area using a 1-hr duration, 25-yr recurrence storm of 1.3 in (33 mm) are given in figure 9. 

Proceed east on CR 252 to CR 250. Turn north on CR 250 and drive about 2.2 

miles (3.5 km) to the Stevens Creek fan. Turn east onto a small dirt road on the north 

side of the fan and drive about 0.5 mile (0.8 km) to the east. 

17
18
Stop 2. Debris Flows on Stevens Creek Fan 

GPS location: N 37° 24’ 35.0” W 107° 48’ 32.2” Elevation: 6,791 ft (2,070 m). 

Stop Leaders: Sue Cannon, Jeff Coe, Joe Gartner. 

The Stevens Creek fan was burned by the wildfire and then impacted by debris 

flows several months later. Debris flowed onto the fan on September 7, September 

10−12, and September 20, 2002. The September 7 debris flow covered approximately 

one-quarter of the fan area with deposits as deep as 5 ft (1.5 m) and filled part of a house 

with tons of rock and mud (fig. 10). Prior to the debris flow, the house was at least 200 ft 

(61 m) from the active channel. During the event, the channel moved from a position on 

the north side of the house to a position on the south side of the house (fig. 11). The 

event moved material up to 8 ft (2.5 m) in diameter in a muddy matrix. Evidence on the 

fan surface suggested that channels were blocked by large boulders and diverted many 

times during the event. The current Stevens Creek channel on the fan has been excavated 

(by excavating equipment) and made deeper and straighter than the natural channel. This 

excavation provides an excellent longitudinal exposure of Holocene fan deposits (fig. 

12). Upstream from the fan, the channels showed up to 8 ft (2.5 m) of incision into 

extensive valley-fill deposits. 

Debris flows produced by the September 10−12 and September 20 rainstorms 

were smaller discharge events carrying smaller (up to cobble-sized) material than those of 

September 7. Soil-slip scars that formed during these storms were observed in the upper 

reaches of Stevens Creek. These scars were relatively small⎯at most 100 ft2

 (10 m2) in 

area⎯and were produced when hillslope material mobilized into debris flows that 

traveled a few hundred feet down the hillslopes on which they originated. 

Two primary processes for the initiation of debris flows were identified in the 

Missionary Ridge burned area: runoff-dominated erosion by surface overland flow, and 

infiltration-triggered failure and mobilization of discrete landslide masses. Debris flows 

generated through runoff and erosion occurred in response to short-duration, highintensity storms (and are thus the most frequent), while landslide-triggered debris flows 

were observed following a long-duration (multi-day), low-intensity storm.

Debris-flow initiation in recently burned areas is most frequently attributed to 

significantly increased rates of rainfall runoff. In the Missionary Ridge fire, Cannon and 

others (2003a) observed that runoff high on hillslopes resulted in the generation of rills. 

Convergence and concentration of flow within hollows and in low-order channels 

resulted in considerable erosion, often to bedrock, and the transport of material 

downslope. Debris-flow deposits first occurred in the lower reaches of the drainage 

networks. The debris flows appeared to initiate through progressive bulking of surface 

runoff with sediment entrained by rill erosion in steep, upper-basin slopes and from deep 

incision as the flows progressed down channels. The sheer volume of material that was 

excavated from the drainage network suggests that a large proportion of the material 

within the debris flows originated from the channel. 

 

19
A 

B 

20
C 

D 

Figure 10. Debris-flow deposits on the Stevens Creek fan caused by the September 7, 

2002, rainstorm. A-C, Deposits in and near residence on central part of fan; D, Deposits 

in channel near County Road 250. Photographs courtesy of La Plata County. 

21
A 

B 

Figure 11. Photographs showing channel migration on the Stevens Creek fan. A, Predebris flow channel location; B, Post-debris flow channel location. Photographs courtesy 

of La Plata County. 

22
A 

B 

Figure 12. Debris-flow channel on the Stevens Creek fan. A, Excavated channel and 

barrier; B, Holocene debris-flow deposits exposed on the south side of the channel. 

23
Return to CR 250 and proceed south about 3.6 miles (5.8 km) to the trail head on 

the north side of the Haflin Creek fan. Walk up the trail to the Haflin Creek channel. 

Stop 3. Debris Flows on Haflin Creek Fan 

GPS location: N 37° 22’ 02.0” W 107° 50’ 25.0” Elevation: 6,620 ft. (2,018 m). 

Stop Leaders: Joe Gartner and Butch Knowlton. 

The entire Haflin Creek drainage basin was burned by the fire (figs. 13 and 14) 

and then impacted by debris flows several months later. As described at the last stop, 

debris flows were generated by rilling and erosion of channel materials (figs. 15−18) and 

impacted structures and CR 250 on the fan at the mouth of the drainage (fig. 19). Two 

storms generated the debris flows, one on August 29 and the other on September 20, 

2002. The August 29 debris flows carried material no greater than 3 ft (1 m) in diameter 

in a muddy matrix. The September 20 debris flows carried materials only up to about 1.5 

ft (0.5 m) in diameter, a significant decrease in size from the August event. 

Figure 13. Haflin Creek drainage basin and fan. Photograph courtesy of La Plata 

County. 

24
A 

B 

Figure 14. Mountainside near the head of the Haflin Creek drainage basin. A, About 1 

month after the fire was extinguished; B, About 1 yr after the fire. Photographs courtesy 

of La Plata County. 

25
Figure 15. Rills in the Haflin Creek drainage basin. Photograph courtesy of La Plata 

County. 

Figure 16. Ash/mud flow deposit in the Haflin Creek drainage basin. Photograph 

courtesy of La Plata County. 

26
Figure 17. Debris-flow deposits in the Haflin Creek drainage basin. Photograph 

courtesy of La Plata County. 

27
Figure 18. Channel incision in the Haflin Creek drainage basin. Photograph courtesy 

of La Plata County. 

A 

28
B 

Figure 19. Damage from debris flows on the Haflin Creek fan. A, Structure impacted 

by debris flow; B, Debris-flow deposits on County Road 250. Photographs courtesy of 

La Plata County. 

The Haflin Creek channel is also an excellent location to demonstrate the method 

used by Gartner (2005) to estimate the material eroded from channels by debris flows. 

Recall that to generate the statistical model used to predict potential debris-flow volumes 

generated from burned basins, debris-flow volumes from 56 events were estimated by 

quantifying the amount of material eroded from the main channels of a basin by the 

passage of a debris flow. Channel cross sections were measured at 50- to 300-ft (15- to 

91-m) intervals along the length of the scoured channel using a 1-yd (1-m) length of 

lumber with legs and an inclinometer attached to measure the slope of the ground surface. 

The device was used to record angles across the surface of a channel to quickly measure a 

channel cross section. For each cross section, notes were taken to identify which 

measurements correspond to incised channel, bedrock, and(or) levees. These notes 

helped to interpolate the original channel surface and determine the amount of material 

removed from each cross section. Scoured areas of adjacent cross sections were averaged 

and multiplied by the distance between them to define a volume of material scoured by 

the debris flow. The sum of all segments of scoured channel represents the total 

estimated volume of material eroded from the channel and corresponds to the amount of 

material deposited at the mouth of the basin. 

29
The predictive peak discharge and volume maps for the 25-yr recurrence storm 

(figs. 7 and 9) indicate that Haflin Creek should have a smaller debris-flow response than 

Stevens Creek. The peak-discharge model estimates a value of approximately 8,000 ft3/s 

(230 m3

), and the volume model estimates a yield of approximately 12,800 yd3 (9,750 m3) 

in response to 1.3 in (33 mm) of rain in 1 hr. None of the storms to impact the area after 

the fire approached this recurrence level, so comparisons of actual and predicted values 

are not possible. 

Return to CR 250 and proceed south to CR 240. Turn west on CR 240, then south 

on North College Drive. Turn right on Rim Drive. Stop 4 (lunch) is at the Lions Den 

Picnic Shelter on the right hand side of Rim Drive.

Stop 4. Lions Den Picnic Shelter 

Lunch. Overview of Lower Animas Valley, Fort Lewis College Mesa. GPS 

location: N 37° 17’ 17.5” W 107° 51’ 41.3” Elevation: 6,970 ft. (2,124 m). Stop 

Leaders: Rob Blair and Mary Gillam. 

The Lions Den Shelter is located at the northern edge of the Fort Lewis College 

Mesa at an elevation of about 6,970 ft (2,124 m). It provides stunning views of the lower 

Animas Valley (fig. 20) to the north and the La Plata Mountains to the west. On the 

horizon to the north up the valley one can just make out peaks making up the Twilight 

Gneiss (1,780 Ma), the oldest rocks seen from this vantage. It is clearly evident from the 

dipping Paleozoic and Mesozoic strata that are exposed in the valley walls that as one 

travels north one travels back into geologic time (fig. 4). 

Animas City Mountain, located northwest of Durango, displays 5- to 7-degree dip 

slopes of the late Cretaceous Dakota Sandstone and is expressed as westward tilted rim 

rock. Below the Dakota, the Jurassic Morrison Formation makes up the lightly forested 

slopes. This is followed by the whitish cliffs of the Junction Creek Sandstone, the 

Wanaka Formation, and the lower whitish cliffs of the Entrada Sandstone, all Jurassic in 

age. Looking upvalley, the thin pinkish red rocks below the Entrada are probably the 

most eastern remnants of the Late Triassic Wingate Sandstone that overlay the Dolores 

Formation and the reddish Permian Cutler Formation (Blair, 1996). 

To the west of Durango, the grey Mancos Shale dominates the landscape. Local 

geologic lore suggests that most of the particulates (maybe up to 80 percent) of the 

Mancos Shale have passed through the bowels of some organisms. This shale underlies 

the Lions Den Shelter and makes up the slopes rising to the hogback rim immediately to 

the east of the Shelter. The Point Lookout Sandstone (eastern horizon) forms the 

Hogback monoclinal flexure and marks the approximate boundary between the Colorado 

Plateau to the west and the Southern Rocky Mountains to the east. 

The La Plata Mountains (Silver Mountains in English) to the west are the 

erosional remnants of a structural dome uplifted from the injection of magma into the 

overlying Phanerozoic strata (65 Ma ago). The exposed rocks reveal a complex labyrinth 

of anastomosing sills, dikes, laccoliths, pipes, and sedimentary layers (Blair, 1996, p. 

339). 

30
Figure 20. View of the lower Animas Valley to the north, from the Lions Den Picnic 

Shelter in Durango. 

The lower Animas Valley is a glacial valley made flat by stream and lake 

sediment. The gravel hill upon which the Lyons Shelter is built and the ridges seen 

between the shelter and the flat floor of the valley are glacial end moraines. At the time of 

the melting and retreat of Pleistocene glaciers, “Lake Durango” formed. It was, however, 

quickly filled to capacity with fine sediment and gravel, the height of which was 

controlled in part by the height of moraines and the base of the Dakota Sandstone. 

Current age estimates for the moraines are shown in table 1. The outwash terrace 

(approx. 250 ka) seen to the south of the Lions Den Shelter is now the site of a golf 

course. 

31
Moraine Age (ka) Basis 

Bakers Bridge ≥17.1 ±1.2 (nuclide) 10Be on ice-scoured granite 

(one) 

Animas City 3 and 2 > 17−approx. 21 (nuclide) 36Cl, Rio Grande area; 10Be 

on terrace (one) 

Animas City 1 Approx. 22−<60 Regional correlation 

Spring Creek 2 <61 +12/-11 (?) Optically Stimulated 

Luminescence (two, see 

note below) 

Spring Creek 1 Approx. 130−190 Regional correlation 

Durango 2 Approx. 250−275 Long-term river incision 

and Lava Creek B ash 

Durango 1 Approx. 345−360 Long-term river incision 

and Lava Creek B ash 

Table 1. Ages of Lower Animas Valley glacial moraines. Note: Dates on one outcrop of 

Spring Creek 2 may not apply to all deposits now assigned to that unit (the deposits are 

very poorly preserved, so they could have formed as parts of more than one moraine 

between Spring Creek 1 and Animas City 1). Summary by Gillam, 1998; Pierce, 2003; 

Applegate, 2005; Benson and others, 2005; Z. Guido, written communication, 2006. 

During the spring meltoff of winter snow, the Animas River will usually flood the 

lower valley. The oxbows fill first, then spill over to form a large shallow lake. As one 

travels north up the Animas Valley, note the relative abundance of alluvial fans on the 

east side compared to the west. This topographic asymmetry is due to the influence of 

Hidden Creek, which forms a small “perched” valley found parallel to and west of the 

Animas Valley. Its origin is debatable but is tied to events during the last glacial maxima. 

This valley intercepts eastward-heading drainages and prevents fans from building along 

the west side of the Animas Valley. In contrast, the drainages flowing westward from the 

eastern cliffs are associated with debris fans. These drainages flood every 5−10 yrs, much 

to the consternation of the County highway department, which has to close East Animas 

Road (CR 250) and clear the debris. The Missionary Ridge fire during the summer of 

2002 burned on the eastern slopes of the Animas Valley and temporarily increased the 

frequency and volume of debris-flow events. 

Very little debris from the Missionary Ridge fire has reached the Animas River. 

During late Holocene time, the valley floor has aggraded very slowly with sediment from 

the upper Animas drainage and local tributary streams. However, since the early 1970s, 

in-stream gravel mining has caused the channel to incise in several places by as much as 

13−30 ft (4−9 m) (M. Gillam, unpub. data obtained from several sources). 

Return to CR 240 and proceed east to the junction with CR243. Follow CR243 

north to Lemon Dam. Turn left onto the dam and drive across. Stop 5 is at the west end 

of Lemon Dam. 

32
Stop 5. Debris Flow Mitigation at Knight Canyon and Landslides 

along the Florida River. 

West end of Lemon Reservoir dam. GPS Location: N 37° 21’ 53.7” W 107° 39’ 

06.6” Elevation: 8,630 ft (2,630 m). Stop Leaders for Knight Canyon: Paul Santi and 

John Ey. Stop Leaders for Florida River landslides: Bill Schulz and John Ey. 

Knight Canyon at Lemon Dam 

 Lemon Dam is a critical part of the water supply system for the City of Durango. 

Therefore, erosion of sediment and(or) debris flows into the reservoir following the 2002 

Missionary Ridge fire could have had severe consequences. The Florida Water 

Conservancy District (FWCD) was given the resources and the mission to prevent 

significant sediment movement into critical portions of the reservoir. They concentrated 

their efforts on the drainage basin known as Knight Canyon, which empties near the 

intake structure for the water supply system (fig. 21). The mitigation efforts carried out 

by the FWCD consisted of construction of Log Erosion Barriers (LEBs), mulching, 

seeding, and construction of check dams and debris racks (deWolfe and others, 2005), as 

summarized in tables 2 and 3. These methods were effective in virtually eliminating 

sedimentation into the reservoir. This success can be attributed to a number of factors: the 

density of application of each mitigation method, the enhancement of methods working 

in concert, the quality of installation, and the rehabilitation of mitigation features to 

extend their useful life. The application of each method is summarized below. 

Log Erosion Barriers 

Log Erosion Barriers (LEBs) serve as small hillside check dams to disrupt 

overland flow, enhance infiltration, and capture eroded sediment. A typical layout 

pattern for LEBs is shown in figures 22 and 23. Some technical studies indicate that 

LEBs are inconsistent in their ability to reduce erosion and that they are ineffective 

during heavy rains (Gartner, 2003; Wohlgemuth and others, 2001; Robichaud, 2005). 

However, the LEBs at Lemon Dam appeared to have reduced hillslope erosion and 

enhanced runoff infiltration because they were applied in heavy concentrations and with 

other erosion control measures. At Lemon Dam, 231 acres (93 ha) of a severely burned 

basin above the dam’s spillway and intake structures were treated with concentrations 

between 90 and 250 LEBs/acre (223−619 LEBs/ha) and rehabilitated multiple times after 

being filled by the runoff accompanying heavy rainfall (John Ey, personal commun., 

2004). Rehabilitation consisted of removing accumulated debris from the upslope side of 

the LEB and placing the removed debris on the downslope side of the LEB. LEBs are 

usually applied to large areas in National Forests in concentrations of 40 LEBs/acre (99 

LEBs/ha, Burn Area Emergency Rehabilitation Team, 2002). 

LEB installation is difficult and expensive, which limits the size of an area on 

which LEBs can be installed. For example, about $1.38 million were spent on LEB 

installation on only 4,626 acres (1,872 ha) of the 73,000 acres (29,500 ha) burned by the 

Missionary Ridge fire (Burn Area Emergency Rehabilitation Team, 2002). 

33
Figure 21. Knight Canyon (upper center/right side of photograph) above the spillway 

intake at Lemon Dam. Photograph courtesy of La Plata County. 

Figure 22. Staggered pattern in which log erosion barriers should be placed on a slope. 

34
Figure 23. Log erosion barriers near Lemon Dam following the Missionary Ridge fire. 

35
Table 2. Summary of the application data for the erosion and sediment control 

treatments implemented at Lemon Dam (Wright Water Engineers, 2005). 

2004 2005

May June July Aug. Sept. Oct. Nov. Dec. Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.

Missionary 

Ridge Fire

* †

1 Check Dam 

Construction

2 LEB 

Construciton

3 Debris Rack 

Construction

4 Hand Mulching 

and Crimping

5 Hand Seeding

6 LEB 

Rehabilitation

* Isolated storm delivers 61.75 mm of rain in 13 hours on 9/9/2003

† Clean water flows into check dam basins, September 2004

2002 2003 Priority Event

Table 3. Timeline of the erosion and sediment control program implemented at Lemon 

Dam, listed in order of priority (information from Florida Water Conservancy District, 

unpub. data). 

36
Mulching 

At Lemon Dam over 190 tons (172 metric tons) of mulch were spread by hand 

and crimped (fig. 24) into over 250 acres (101 ha) of burned slopes (Wright Water 

Engineers, 2005). The hand-spread application was effective as an erosion-control 

treatment because it was spread thinly and evenly over the targeted slopes, a 

characteristic that provided good temporary ground cover. Crimping helps mulch to 

maintain contact with the soil and, if hydrophobic layers are detected in burned soils, 

crimping can break through the layer, allowing water to infiltrate rather than run off the 

surface. Uncrimped hand-spread mulch is vulnerable to clumping effects because wind 

can blow it around and deposit it in piles that can inhibit vegetation reestablishment. 

Mulch can also be spread by helicopter, and the resulting application is often 

characterized by thick mounds and relatively uneven spreading patterns. The mounds are 

primarily due to insufficient breakup of airborne straw bales or to wind blowing the 

mulch into piles in the lee of obstacles such as trees. During heavy rains in these areas, 

the soil between mounds is highly vulnerable to erosion. Clumps will also suffocate the 

soil beneath, limiting vegetation regrowth even 2−3 yrs following a fire. 

Seeding 

The goal of seeding is to establish a protective vegetative cover until natural 

regrowth can occur, usually after one or two growing seasons. Typical application 

concentrations for larger areas are around 40 lbs/acre (45 kg/ha), although the erosioncontrol program at Lemon Dam used 60−75 lbs/acre (66−84 kg/ha). Observations of 

those slopes show that spreading this concentration among crimped mulch helped 

reestablish a vegetative cover during the first growing season (fig. 25). When seeds are 

applied in combination with straw mulch and(or) LEBs, plant reestablishment can occur 

more effectively after the first growing season, as mulch and LEBs may reduce hillslope 

erosion and hold seeds in place until germination. 

Check Dams 

Thirteen earthen check dams were constructed in the main channel of Knight 

Canyon above Lemon Dam. Figure 26, a photograph taken on September 9, 2003 (14 mo 

after the fire), shows a check dam filled with an ash/mud deposit during a heavy rain in 

the basin. In the background, another check dam can be seen in the series. The dams 

were monitored by the FWCD and cleaned out after such erosional events. Construction 

of check dams is expensive, difficult, and labor intensive, especially in steep upper 

reaches of a channel where access is limited. If debris-flow volume is too large for a 

check dam, or if the dams are improperly constructed, failure can occur and the debrisflow will incorporate the check-dam materials into the flow, increasing both its volume 

and erosive potential. This occurred in a basin burned by the Piru fire in southern 

California (Sue Cannon, oral commun., 2005). 

37
May 2003 

Figure 24. Condition of hand-spread and crimped mulch near Lemon Dam. Notice the 

vertical straws, products of crimping (photograph from Florida Water Conservancy 

District, unpub. data, 2003). 

Figure 25. Photograph showing the amount of vegetative cover resulting from hand 

spread seeding near Lemon Dam in August 2003, approximately 13 mo after the fire. 

Some of the vegetation is natural growth (photograph from Florida Water Conservancy 

District, unpub. data, 2003). 

38
September 9th, 2003 

Figure 26. A check dam full of mud and debris in the upper reaches of Knight Canyon 

above Lemon Dam (photo from Florida Water Conservancy District, unpub. data, 2003). 

Debris Racks 

Five debris racks were constructed in the Lemon Dam sediment-control program 

between October and December 2002. Only one rack was located in a channel that 

produced a debris flow and it intercepted approx. 170 yd3 (130 m3) of a debris flow 

measured to be about 586 yd3

 (448 m3) in total volume (fig. 27). The design of this 

debris rack prevented failure and allowed the fine material to continue down channel, 

where part of it was held back at a second debris rack. Only muddy water was passed on 

to the Florida River. The effectiveness of a debris rack in reducing debris-flow volume 

depends on its strength, design to pass fine-grained material, its position in the channel, 

and the size of the channel. 

The Florida River Landslide Area 

Landslide activity was observed by residents of the Florida River area during 

April and May 2005 on a hillslope burned by the 2002 Missionary Ridge fire. This 

hillslope is located above the Florida River and CR 243 about 1 km downstream from 

Lemon Dam (fig. 28). Landslide activity on the hillslope poses hazards to area residents 

and Lemon Dam, should activity be great enough to cause damming of the river. 

Members of the U.S. Geological Survey Landslide Hazards Project investigated the 

potential for additional landslide activity on the hillslope and associated hazards (Schulz 

and others, 2006). The investigation included engineering geologic mapping, subsurface 

exploration, and monitoring of surface displacement and shallow groundwater conditions. 

39
Figure 27. Debris rack holding back a slug of coarse debris and wood above a road and 

the spillway of Lemon Dam (photo from Florida Water Conservancy District, unpub. 

data, 2003). 

Figure 28. Aerial view of the Florida River landslide area within the Missionary Ridge 

burn area near Lemon Reservoir. Photograph courtesy of La Plata County. 

40
Landslide activity during the spring of 2005 included the formation of at least 

nine individual landslides that produced more than a dozen debris flows (fig. 29). 

Landslide activity was concentrated upon a very large, apparently dormant landslide. 

Observed activity during 2005 was confined to a shallow (generally about 6 ft (2 m) 

thick) mantle of colluvium. Landslides within the colluvium were translational debris 

slides (Cruden and Varnes, 1996) with volumes between 2,600 and 46,000 yd3 (2,000 and 

35,000 m3

). Debris flows were generally mobilized from dilated, mostly saturated, toe 

regions of the slides and ranged in volume from 200 to 2,000 yd3 (150 to 1,500 m3) (fig. 

30). Although generally quite small, debris flows traveled as far as about 980 ft (300 m) 

downslope onto the Florida River floodplain. The landslides appeared to have been 

triggered by rapid melting of deep snow with high water content. Area residents had not 

observed landsliding on the hillslope previously (during about the past 30 yrs), although 

similar snowmelt events have occurred historically. It thus appears that landslide 

initiation was assisted by incineration of vegetation, which resulted in loss of root 

strength and evapotranspiration. 

The dormant landslide upon which recent activity was concentrated occurs 

primarily in bedrock of the Morrison Formation. Dip-slope bedrock landslides in the 

Morrison and other units are common in the area (e.g., Blair, 1977; Carroll and others, 

1997). However, the dormant landslide does not occur on a dip slope; rather bedrock 

orientation and landslide geometry indicate that the landslide truncates a 1,066-ft-thick 

(325 m) section of Jurassic and Cretaceous bedrock (fig. 29). The dormant landslide 

appears to be as much as about 230 ft (70 m) thick with an approximate volume of 25.7 

million yd3

 (19.7 million m3). When active, the landslide translated about 300 ft (100 m) 

downslope, likely damming the Florida River. Morphologic features suggest that the 

landslide may be quite old (thousands of years), but features were also observed that 

indicate that significant areas of the landslide have reactivated in the recent past. Samples 

obtained from a closed depression in the head of the landslide and at an apparent thrust of 

bedrock over soil have been submitted for radiometric dating. Abundant charcoal was 

identified at both locations, suggesting an influence of wildfire in initiating the landslide. 

Wildfire dating activities in the area (Frechette and others, 2003; Gonzales and others, 

2003; Gonzales and others, 2005; Bigio and others, 2005) have identified many wildfires 

that occurred during the past several thousand years. 

Future shallow landslide activity and associated debris flows will probably follow 

significant snowmelt events or periods of sustained, heavy rainfall. Characteristics of 

rainfall, snowmelt, and loss of vegetation due to wildfire that may result in future shallow 

landslide activity are being evaluated through limit-equilibrium and finite-element 

modeling. The potential for reactivation of the dormant landslide is largely unknown due 

to the lack of deep groundwater data. Renewed activity that occurs through the basal 

rupture zone of the dormant landslide would probably not result in rapid slope failure 

because that zone is most likely at relatively low residual strength, but the possibility 

remains for shallower bedrock landslides to form and mobilize rapidly. 

41
Figure 29. Geologic map of the Florida River landslide area. From Schulz and others 

(2006). 

42
Figure 30. Debris flow source area formed in April 2005 in the Florida River landslide 

area. 

Return to CR 243; drive south to the junction with CR 240. Turn east on CR 240 

and proceed to the junction with CR 501. Turn north on CR 501 and proceed about 4 mi 

to Stop 6 on the north side of Vallecito Reservoir. 

Stop 6. Fire and Debris Flow History in a Small Basin North of Vallecito 

Reservoir 

GPS location: N 37° 26’ 14.7” W 107° 33’ 35.4” Elevation: 7,847 ft (2,392 m) 

Stop Leader: Erica Bigio. 

This stop focuses on a 1 km2

 unnamed basin just north of Freeman Creek, burned 

almost entirely at moderate and high severity by the Missionary Ridge fire (fig. 31). The 

basin is underlain by sedimentary bedrock, and has hillslopes with gradients between 20 

and 50 percent. The forest type is mixed-conifer with a species composition of Douglas 

fir, white fir, aspen, and ponderosa pine. Following the fire, several episodes of debris 

flow and flooding incised the channel and alluvial fan deposits (Cannon and others, 

2003a), exposing a late Holocene record of fire-related sedimentation events (Gonzales 

and others, 2003). The exposed sediment record exhibited abundant charcoal within the 

deposits, providing an ideal opportunity to compare fire-related sedimentation events 

with tree-ring records of fire history. The goal of research in the basin has been to 

combine tree-ring and alluvial records at the scale of a single low-order basin. The 

43
annual resolution of the tree-ring records helps to interpret the timing of the younger 

alluvial deposits. In turn, the length of the alluvial records will extend the fire history for 

the study site. 

Figure 31. The Stop 6 study basin on the northwest side of Vallecito Reservoir. The 

basin was burned at approximately 80 percent moderate and high severity. Locations of 

tree-ring and alluvial sediment samples are noted. The fire-scar sample numbers 

correspond with the surface fire chronology in figure 34. 

 The alluvial deposits were sampled within a 245-ft (75-m) reach of the exposed 

channel, feeding into the alluvial fan (fig. 32). Three stratigraphic sections were chosen 

for their height of natural exposure and the presence of charcoal concentrated in “burned” 

layers. The sediment characteristics of each stratigraphic unit were used to infer 

depositional processes, which were then used to infer the relative extent and severity of 

past fires. Debris-flow deposits with abundant angular charcoal indicate that a significant 

portion of the contributing basin burned at moderate and high severity (Meyer and others, 

1995; Pierce and others, 2004). This inference comes from understanding the conditions 

that promote debris flow initiation in severely burned basins. For the Missionary Ridge 

fire, Cannon and others (2003a) found that all debris-flow-producing basins were burned 

with at least 50 percent moderate and high severities. Alternatively, fine-grained 

sheetflood and hyperconcentrated-flow deposits may represent a subset of low- and 

moderate-severity fires in the contributing basin (Pierce, 2004). The timing of past fire 

events was achieved by radiocarbon dating of charcoal pieces within each stratigraphic 

unit. 

 The goal of the tree-ring sampling was to identify both low- and high-severity 

fire behavior using a combination of tree-ring methods, including fire scars and agestructure data for aspen and conifers. Fire-scarred trees provided an annual record of 

low-severity surface fires, which were combined to identify the spatial and temporal 

patterns of surface fires. Aspen and conifer age-structure data show decadal patterns of 

44
forest regeneration, which may provide evidence of high-severity fire when associated 

with the surface-fire history. Ultimately, the component of high-severity fire, which 

occurs during widespread surface-fire years, may relate to young sediment deposits in the 

channel. 

Figure 32. Exposed channel reach within the study basin northwest of Vallecito 

Reservoir, where three stratigraphic sections (A, B, C) were sampled. The deposit types 

included a fine-grained, gravel-rich debris flow and a colluvial slump. Several buried 

logs were exposed in stratigraphic section B. 

Sediment Results 

 The majority of the alluvial deposits were fine-grained and well-sorted, and 

exhibited a range of silt- to gravel-sized sediment. Gravel-sized clasts were usually 

supported by a silt-rich matrix with little evidence of clast orientation. Within thicker (3 

ft, 1 m) fine-grained deposits, the gravel-sized clasts were concentrated in 0.3-ft-thick 

(10-cm) lenses, indicating weak stratification of the deposits. The key feature of the fine45
grained deposits was the light yellow color, which reflected a source of travertine within 

the channel in the center of the basin. The travertine source location suggests that the 

fine-grained deposits represented reworked channel sediments, with little entrainment of 

hillslope material. 

Some fine-grained deposits had a gray matrix supporting scattered gravel- and 

cobble-sized clasts. The darker color of the matrix suggested a component of hillslope 

sediment mixed with the channel material. The matrix of these deposits did not exhibit 

stratification, yet the range of clast sizes was not sufficient to interpret these deposits as 

debris-flow deposits. They were possibly deposited by hyperconcentrated flow 

conditions. 

Two other deposits more clearly indicated debris-flow deposition. The first was a 

broad deposit exposed for several meters along the length of the exposure between 

stratigraphic sections A and B, and the second was a thin, localized deposit exposed at 

stratigraphic section C (fig. 32). The darker matrix of these deposits indicated a hillslope 

sediment source. The matrix supported a poorly-sorted mix of clasts, composed of both 

local and regional lithologies, relating to reworked glacial-till deposits on the hillslopes. 

Some of the stratigraphic units were also bound by charcoal-rich “burned” layers. 

From visual inspection, these appeared to have been deposited by hydrological processes, 

rather than being in-situ burned soil horizons. They sometimes defined weak 

stratification, and were primarily composed of silt- and sand-sized charcoal fragments. 

Occasionally the burned layers were very prominent and could be traced for several 

meters along the length of the exposure. One such layer was a 0.3-ft-thick (10-cm) 

deposit near the top of stratigraphic section B (fig. 32), which had burned wood chunks 

(0.6−1.0 ft, or 20−30 cm, in length) and smaller angular charcoal pieces supported by a 

dark gray, silty matrix. 

All deposits contain abundant charcoal and can therefore be linked to fire activity 

in the contributing basin. The two debris-flow deposits have very similar radiocarbon 

ages and likely represent the same disturbance event. The event was likely a highseverity fire that occurred between 2,800 and 2,400 cal yr B.P. (2,500 14C yr B.P.). This 

was followed by continuous fine-grained deposition, which has been aggrading in the 

channel over the past 2,000 years. Locally within each stratigraphic section, the 

deposition has been episodic, with periods of rapid sediment deposition followed by 

depositional hiatuses (fig. 33). The youngest alluvial deposits occurred within the past 

500 calendar years and have broad age ranges due to the conversion between radiocarbon 

years and calendar years. The thick burned layer described above has an age range of 

A.D. 1600–1950. 

Tree-Ring results 

The evidence from fire-scarred trees shows that low-severity surface fire has been 

frequent over the past 400 yrs, with a clear cessation following the last widespread 

surface fire in 1879. The surface-fire chronology (fig. 34) shows that fires were more 

frequent and patchy during the late 17th and 18th centuries, but became more synchronous 

and widespread during the 19th century. 

46
Figure 33. Chronology of fine-grained deposition northwest of Vallecito Reservoir, 

recorded in two stratigraphic sections. The horizontal bars represent the 2-sigma 

calendar age ranges of individual charcoal pieces from each sediment layer. The depth of 

each bar is the median depth of the layer where the sediment was sampled. 

Figure 34. Surface fire chronology northwest of Vallecito Reservoir. The surface fire 

chronology represents each fire-scarred tree with a horizontal line, organized in order of 

elevation in the basin. Each tree has multiple fire scars, which are represented by the tick 

marks along the line. 

47
There is some evidence of small patches of high-severity fire occurring during 

two of the widespread surface fire years in the 19th century. The age-structure data 

collected from conifer and aspen trees suggest that there were small patches of highseverity fire burning in both the years 1851 and 1879. The aspen stand located on the 

north-facing slope has a germination age of 1879 and is the largest area (25-acre, or 10-

ha, patch) of possible high-severity fire. The evidence is not conclusive, but provides the 

best link between the tree-ring and alluvial records. 

Discussion 

The fine-grained deposition, although episodic, represents generally stable 

conditions in the basin. In this type of steep mountainous terrain, step-pools created by 

log jams and boulders are the main mechanism for trapping and storing fine-grained 

sediment (Wohl, 2000). These step-pools remain stable during low-flow events (1– to 

5−yr recurrence interval), while infrequent high-flow events (30– to 50−yr recurrence 

interval) locally scour and fill pools (Faustini and Jones, 2003). 

The fine-grained deposition indicates sediment transport in response to infrequent 

high-flow events, possibly generated by high recurrence-interval storms. Although there 

was abundant charcoal within the fine-grained deposits, the transport and deposition of 

channel sediment was probably unrelated to fire-related disturbance in the basin. The 

abundance of charcoal represents a low-severity fire regime, where frequent surface fires 

produced charcoal on the hillslopes, which was transported to the channel by surface 

runoff. Separately, high-flow events transported and deposited charcoal and sediment 

behind obstructions in the channel. In the case of stratigraphic section A (fig. 32), it 

seems as if a colluvial slump (possibly following the debris-flow event at or near 2,600 

cal yr B.P.) provided the obstruction for sediment accumulation in this portion of the 

channel. 

Fine-grained deposits with a darker matrix supporting gravel- to cobble-sized 

clasts probably represent specific fire-related disturbance events. Over the length of the 

tree-ring record, patches of high-severity fire occurred during some of the widespread 

surface-fire years. These patches could generate sufficient runoff to mobilize and 

transport channel sediment, while also incorporating hillslope sediment from the 

disturbed area of the basin. One possible example is the thick burned layer exposed at 

the top of stratigraphic section B (fig. 32). This deposit overlies a fine-grained deposit 

with a darker matrix, supporting cobbles up to 0.4 ft (15 cm) in diameter. The two 

deposits are probably related to the same disturbance event, occurring between A.D. 1600 

and 1950. These deposits probably relate to the patch of high-severity fire, indicated by 

the aspen stand, which occurred in 1879. This was also a year when a large high-severity 

fire burned in the upper elevation spruce-fir forests near Silverton, Colorado. 

In the case of the debris-flow deposit, the link to more-extensive high-severity fire 

is clear. Debris-flow initiation requires significant runoff to entrain hillslope and channel 

sediment. The dark matrix suggests that hillslope sediments were entrained in the 

deposit, while the abundant charcoal links the deposit to a fire-related disturbance. The 

timing of this event (around 2,600 cal yr B.P.) is likely related to the neoglacial period of 

cooler and wetter conditions during the late Holocene. A lake sediment record of fire 

history shows that this period had less frequent, but more intense, fires in the upper 

elevation spruce-fir forests near Silverton, Colorado (Toney and Anderson, 2006). 

48
The Missionary Ridge fire appears to be unusual in its high-severity patch size 

when compared with fire events interpreted from tree-ring records and fine-grained 

deposition over the past 2,000 yrs. This type of fire behavior has likely not occurred in 

this study basin since the last debris-flow event at or near 2,600 cal yr B.P. This previous 

debris-flow event may relate to longer-term changes in forest structure relating to 

climatic conditions at this time. The high-severity fire behavior of the recent event, 

however, seems predominantly influenced by human-induced changes in forest structure 

as a result of fire exclusion in this ecosystem. 

Return to Durango via CR501 and CR240. End of Field Trip. 

Acknowledgements 

Several of the contributors to this guidebook participated in a May 2003 

Geological Society of America (GSA) field trip entitled “Erosion and sedimentation 

following the 2002 Missionary Ridge fire”. The GSA trip was organized and led by 

Andy Gleason of the Colorado Geological Survey. This Shlemon Conference field trip 

follows a similar route and has greatly benefited from Andy’s observations, insights, and 

contacts. Additionally, we thank Bob Schuster, Bill Ellis, and Melanie Parker for 

providing constructive reviews of this guidebook. 

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