Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

In cooperation with the

MONTANA DEPARTMENT OF TRANSPORTATION and the

U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE

 Bitterroot National Forest

 Custer National Forest

 Helena National Forest

Water-Resources Investigations Report 03-4319

U.S. Department of the Interior

U.S. Geological Survey
Front cover photograph:

Debris fan from storm event of July 2001 on tributary stream to Skalkaho Creek near Hamilton, 

Montana. Debris fan overlays ancient, massive debris fan on same stream. Photograph by 

Richard M. Jones, U.S. Department of Agriculture Forest Service.
U.S. Department of the Interior

U.S. Geological Survey

Wildfire-Related Floods and Debris Flows in Montana in 

2000 and 2001

By Charles Parrett, Susan H. Cannon, and Kenneth L. Pierce

Water-Resources Investigations Report 03-4319

In cooperation with the

Montana Department of Transportation and the 

U.S. Department of Agriculture Forest Service

Bitterroot National Forest

Custer National Forest

Helena National Forest
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

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CONTENTS iii

CONTENTS

Page

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

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

Purpose and scope...................................................................................................................................................... 4

Methods of study........................................................................................................................................................ 4

Flood runoff and debris flows from a storm in 2000............................................................................................................. 5

Flood runoff and debris flows from storms in 2001.............................................................................................................. 6

Canyon Ferry area...................................................................................................................................................... 6

Ashland area............................................................................................................................................................... 13

Bitterroot area ............................................................................................................................................................ 14

Storm and flood data ...................................................................................................................................... 14

Debris-flow data and analysis ........................................................................................................................ 16

Debris-flow process in the Sleeping Child Creek basin................................................................................. 20

Implications for future floods and debris flows..................................................................................................................... 20

Summary................................................................................................................................................................................ 21

References cited..................................................................................................................................................................... 22

ILLUSTRATIONS

Figure 1. Map showing location of precipitation and streamflow-gaging stations and flood and debris-flow

discharge estimation sites in three burned areas, Montana........................................................................... 2

2-4. Photographs showing:

2. Flooding aftermath from the storm of May 27, 2001, in a storage building constructed in the

poorly defined Cave Gulch stream channel, near Helena, Montana................................................... 7

3. Flood damage on Crittenden Gulch at its mouth, near Helena, Montana ................................................ 12

4. Sediment-laden floodwaters on Brian Creek near Ashland, Montana, overtopped the road 

and deposited a thick layer of muddy sediment and large, woody debris........................................... 14

5. Graph showing maximum known peak discharges at gaged sites in the Southeast Plains Region

of Montana compared to 2001 peak discharges in the Ashland study area.................................................. 15

6. Photograph showing flood deposits from Laird Creek tributary partly blocked the Laird Creek

channel following storms of July 20 and 21, 2001, near Sula, Montana...................................................... 16

7. Hydrographs of floods for Laird Creek near Sula, Montana, July 20 and 21, 2001......................................... 17

8. Photographs showing measuring debris-flow channel section at two unnamed tributaries to Sleeping

Child Creek, Montana................................................................................................................................... 18

9. Graph showing comparison of surveyed channel cross sections at Sleeping Child Creek near

Hamilton, Montana, before and after debris flows of July 15, 2001............................................................ 19

TABLES

Table 1. U.S. Geological Survey precipitation gages in and near Montana burned areas and dates of

significant rainstorms in 2001 ..................................................................................................................... 7

2. Data for significant storms during 2001 at U.S. Geological Survey precipitation stations in Montana

burned areas................................................................................................................................................. 8

3. Peak streamflow discharges during 2001 and recurrence intervals at streamflow-gaging stations and

flood-estimation sites in and near Montana burned areas........................................................................... 10

4. Peak debris-flow discharges at selected tributary sites in the Sleeping Child Creek drainage,

July 15, 2001 ............................................................................................................................................... 11
iv Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88), unless otherwise noted. Horizontal 

coordinate information is referenced to the North American Datum of 1927 (NAD 27), unless otherwise noted.

Water year: The 12-month period October 1 through September 30. It is designated by the calendar year in which it ends.

Acronyms used in this report:

BAER Burn Area Emergency Rehabilitation

NWS U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service

USGS U.S. Geological Survey

CONVERSION FACTORS, DATUM, AND ACRONYMS

 Multiply By To obtain

acre 4,047 square meter

cubic foot per second 0.028317 cubic meter per second

cubic yard 0.7646 cubic meter

foot 0.3048 meter

foot per foot 1.0 meter per meter

foot per second squared 0.3048 meter per second squared

inch 25.4 millimeter

square foot 0.09290 square meter

square inch 6.452 square centimeter

square mile 2.59 square kilometer
WILDFIRE-RELATED FLOODS AND DEBRIS FLOWS IN 

MONTANA IN 2000 AND 2001

By Charles Parrett, Susan H. Cannon, and Kenneth L. Pierce

Abstract 1

ABSTRACT

Following extensive wildfires in summer 2000, 

Montana experienced flooding and debris flows in three 

different burned areas: (1) the Bitterroot area in southwestern Montana, (2) the Canyon Ferry area near Helena, and (3) the Ashland area in southeastern Montana. 

Flooding and debris flows in the Bitterroot study 

area began with a large, frontal storm in SeptemberOctober 2000. No precipitation data were available at 

sites in the burned area. Daily precipitation at one 

National Weather Service station near the Bitterroot 

burn area had a recurrence interval of about 10 years. 

The storm resulted in debris flows and a peak flood discharge on Little Sleeping Child Creek that had a recurrence interval of about 100 years.

Beginning in May 2001, streams in the Canyon 

Ferry area that flooded in response to thunderstorms 

were Cave Gulch, Crittenden Gulch, Magpie Creek, and 

Hellgate Gulch. Crittenden Gulch had a flood on May 

28 with a recurrence interval estimated to be about 50 

years. Small tributary streams to Magpie Creek also 

had debris flows. The maximum recurrence interval for 

recorded precipitation at U.S. Geological Survey rain 

gages was 5-10 years for the depth for the 15-minute 

duration for the July 17 storm on Crittenden Gulch. 

Recurrence intervals for the calculated peak discharges, 

based on unburned conditions, ranged from 2 years at 

several sites to 200 years for the July 17 flood on Crittenden Gulch.

Numerous small tributaries to Otter Creek and the 

Tongue River flooded in response to June and July 2001 

thunderstorms in the Ashland area. The maximum 

recurrence interval for recorded precipitation at one 

U.S. Geological Survey rain gage was 100-500 years for 

the depth for the 5-minute duration at a site near the center of the Ashland study area. Recurrence intervals for 

calculated peak stream discharges, based on unburned 

conditions, were equal to or greater than 50 years at 16 

sites and greater than 500 years at 10 sites. Three of the 

10 sites with large recurrence intervals were in 

unburned areas.

In July 2001, thunderstorms in the Bitterroot area 

caused flooding on several small streams and significant 

debris-flow activity on small, steep tributary streams to 

Sleeping Child Creek. The maximum recurrence interval for recorded precipitation at U.S. Geological Survey 

rain gages was 10-25 years for depths for the 5-, 10-,

15-, and 30-minute durations at each site on July 15, 20, 

and 21. The maximum recurrence interval for peak discharges, based on unburned conditions, was 200-500 

years for the peak discharges on two sites on Laird 

Creek on both July 20 and 21.

Calculated peak debris-flow discharge resulting 

from the storm of July 15 for six small tributaries to 

Sleeping Child Creek ranged from 1,740 to 7,860 cubic 

feet per second for drainage areas ranging from 0.07 to 

0.41 square miles. Virtually all sediment composing the 

debris flows originated in the channel. 

INTRODUCTION

The spring and summer of 2000 were unusually 

dry in most of Montana—May through August monthly 

precipitation totals ranged from 47 percent to 58 percent 

of normal at weather stations in Missoula, Helena, and 

Billings (National Climatic Data Center, 2000). Wildfires began in May, but fire suppression efforts, coupled 

with the relatively cool temperatures typical of early 

summer in Montana, generally prevented the significant 

spread of most fires until July. High July temperatures 

and lack of substantial precipitation created conditions 

conducive to fires, and by the end of July, large wildfires 

were burning in much of western Montana and in forested areas of eastern Montana near Ashland. The area 

hardest hit by wildfire was the Bitterroot study area (fig. 

1), where many smaller fires merged into a large fire 

complex that eventually burned more than 300,000 

acres. By the time most fires were extinguished or had 

died out in September, 40,000 acres in the Canyon Ferry 

study area and 60,000 acres in the Ashland study area 

had burned (Montana Department of Commerce, 2002) 

(fig. 1). 
2 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

Figure 1. Location of precipitation and streamflow-gaging stations and flood

0 5 MILES

0 5 KILOMETERS

A. Canyon Ferry study area B. Ashland study area

N

N

Canyon

Ferry

Reservoir

Ashland

Tongue

River

East

Fork

Home

Creek

King

Creek

Otter

Creek

Newell

Creek

Chromo Creek

Brian

Creek

Paget Creek

Coal

Creek

Gate Creek

Stag Creek Rock

Cow

First

Creek

Creek

Creek

O'Dell

Otter Creek

Trout

Creek

Cave

Gulch

Magpie

Creek

Missouri

River

Hellgate

Gulch

Avalanche

Creek

Spokane

Creek

Crittenden

Gulch

Horse Creek

Dry

Creek

Hole-inCreek

the-Wall

Helena

National

Forest

LEWIS

CO

CLARK

 AND

CO

BROADWATER

Forest

Custer

National

CO

ROSEBUD

CO

POWDER

RIVER

CHEYENNE

INDIAN

RESERVATION

NORTHERN

HELENA

Helena

15 miles

28

29

30

22,

26

23 31

25 32

24,

27

48

34

49

33

35

36 37

38

39

41

40

43

44

45

46

47

50

DARBY

10

9

22

51

EXPLANATION

NATIONAL WEATHER SERVICE PRECIPITATION STATION AND NAME

U.S. GEOLOGICAL SURVEY TIPPING-BUCKET RAIN GAGE AND 

NUMBER

U.S. GEOLOGICAL SURVEY STREAMFLOW-GAGING STATION

BURN AREA--Modified from generalized preliminary maps (written

 commun., 2002: Lex Riegle, Natural Resources Conservation

 Service; John Lane, Custer National Forest; and Terry Carlson,

 Bitterroot National Forest)

LAND ADMINISTERED BY BUREAU OF LAND MANAGEMENT

FLOOD-DISCHARGE ESTIMATION SITE AND NUMBER

DEBRIS-FLOW DISCHARGE ESTIMATION SITE AND NUMBER

Otter

6 miles

42

Fort

Howes

Ranger

Station

0 5 MILES

0 5 KILOMETERS

1 Continuous-recording and number

Crest-stage and number
INTRODUCTION 3

Base modified from U.S. Geological 

Survey 1:500,000, 1965. North 

American Datum of 1927 (NAD 27)

116o

49o

Missouri

River

River

River

River

Yellowstone

0 60 MILES

0 60 KILOMETERS

0

0

10 20 MILES

10 20 KILOMETERS

C. Bitterroot study area

110o 105o

48o

47o

46o

45o

N

 Anaconda

 Pintler

Wilderness

Sula

State

Forest

GRANITE

CO

RAVALLI

CO

Bitterroot

National

Forest

Bitterroot

National

Forest

Bitterroot

National

Forest

N F

Cr

Rye

Wilderness

SelwayBitterroot

Wilderness

SelwayBitterroot

Sula

Darby

Skalkaho

Creek

Bitterroot

River

Sleeping Child

Cr

Laird

Cr

Meadow

Cr

Rye

Creek

Bluejoint

Creek

E

Fork Bitterroot

River

Sleeping

Child

Cr

Little

 Little

Bluejoint

Creek

West

Fork

Lake Como

Overwhich

Nez Perce Cr

Tin Cup Creek

Deer

Creek

Hughes Creek

Sawtooth Cr

Roaring Lion Cr

Cr

Lost Horse

Burke Gulch

Creek

51-55, 57

10

Billings

Helena

21

16

18

5

2,3

4

6,7

11,12

19 20

1, 8

17

13,14

Conner

MTS

Cr

Gilbert

30

30

56

and debris-flow discharge estimation sites in three burned areas, Montana.

15,

9,

HAMILTON SKALKAHO PASS

DARBY

GIBBONS PASS

Hamilton

Missouri

C

Missoula

NORTHERN

ROCKY

Bitterroot

SOUTHEAST

REGION

PLAINS

B

A

CONTINENTAL

DIVIDE
4 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

As concern shifted from fire suppression to landscape rehabilitation, a primary focus of governmental

fire-rehabilitation teams was the potential for subsequent flooding and debris flows within burned watersheds. Floods and debris flows are a common occurrence in the western United States following fires 

because of the lack of vegetation and forest duff to intercept precipitation and the water-repelling characteristics of freshly burned soils. Accordingly, the Burn Area 

Emergency Rehabilitation (BAER) teams working in 

the large burn areas of Montana began planning landscape rehabilitation efforts in order to mitigate expected 

flood and debris-flow damage.

On September 30-October 1, 2000, a large frontal 

storm moved through the burned areas of the Bitterroot 

National Forest and caused localized flooding and 

debris-flow activity. In anticipation of additional flooding and debris flows, the U.S. Geological Survey 

(USGS), in cooperation with the Montana Department 

of Transportation and the U.S. Department of Agriculture Forest Service (Bitterroot National Forest), 

installed a continuous-recording streamflow-gaging station (site 2), several crest-stage gages, and several tipping-bucket rain gages in the Bitterroot burned areas in 

April and May 2001. In early summer 2001, thunderstorm activity began in many areas in Montana, including several of the burned areas. In late May 2001, 

flooding and debris flows occurred on several small 

burned watersheds in the Canyon Ferry area near Helena within the Helena National Forest, and tippingbucket rain gages were installed in cooperation with 

Helena National Forest. In early June 2001, the USGS 

in cooperation with the Custer National Forest, also 

installed tipping-bucket rain gages in burned areas near 

Ashland in the Custer National Forest. Only several 

weeks later, thunderstorms in late June and early July 

resulted in severe flooding on small drainages in and 

near burned areas within the Custer National Forest. 

Finally, in mid to late July, after intense thunderstorms 

caused flooding once again in the Canyon Ferry burned 

area and flooding and debris flows from burned watersheds in the Bitterroot National Forest, additional 

streamflow-gaging stations and rain gages were 

installed in cooperation with Bitterroot National Forest. 

The storm and runoff data collected from these events 

presents a unique opportunity to document the hydrologic aftermath of large-scale wildfire in the Northern 

Rocky Mountains. Accordingly, the USGS, in cooperation with the Bitterroot, Helena, and Custer National 

Forests, conducted a study to describe wildfire-related 

floods and debris flows in Montana.

Purpose and Scope 

The purpose of this report is to document the 

storm magnitudes and durations and subsequent flood 

runoff and debris flows that occurred in three different 

burned areas in Montana in 2000 and 2001. The areas 

where data were collected include: (1) the Canyon 

Ferry study area near the capital city of Helena, where 

the affected streams drain from burned areas in the Helena National Forest and forested land administered by 

the Bureau of Land Management, (2) the Ashland study 

area in southeastern Montana, where most of the 

affected streams drain from the burned, relatively small 

mountainous area within the Custer National Forest, 

and (3) the Bitterroot study area in southwestern Montana, where most of the affected streams drain burned 

areas within the Bitterroot National Forest (fig. 1).

Methods of Study

Rainfall data were compiled from National 

Weather Service (NWS) precipitation stations, unofficial observer reports, and USGS tipping-bucket rain 

gages (fig. 1). The recurrence intervals for rainfall 

amounts for various durations were calculated using 

methods developed by Miller and others (1973). Methods described by Miller and others (1973) provide estimated rain depths for specific recurrence intervals of 2, 

5, 10, 25, 50, 100, and 500 years. Measured rain depths 

in 2000 and 2001 that are between those associated with 

specific recurrence intervals are characterized by a 

range of recurrence intervals. For example, if a measured rain depth is between the value associated with a 

50-year recurrence interval and the value associated 

with a 100-year recurrence interval, it is given a recurrence interval of 50-100 years. Peak discharges for 

floods were calculated using indirect-discharge determination methods developed by the USGS (Dalrymple 

and Benson, 1967; Hulsing, 1967; and Bodhaine, 1969). 

In some instances, survey data were incomplete or difficult to interpret and the calculated discharges are less 

reliable than those where more data or better data were 

available. The less reliable calculations are characterized as estimated discharges rather than calculated discharges. Peak discharges for debris flows were 

calculated using a simple form of the critical-flow 
FLOOD RUNOFF AND DEBRIS FLOWS FROM A STORM IN 2000 5

method (Parrett, 1987) described by O’Connor and others (2001). Because of uncertainty about the applicability of the critical-flow method to debris flows, which are 

viscous, plastic-like flows, all calculations for peak discharges for debris flows are characterized as estimates.

The distinction between flood flows and debris 

flows is important because each poses different hazards 

that often require different mitigation strategies. 

Floods, for example, are often widespread throughout 

drainages that experience heavy rains. Flood damage 

often can be at least partly mitigated by flood warning 

and conventional flood-control structures, such as larger 

bridges and culverts, levees, and dams. Debris flows, on 

the other hand, tend to be highly localized, uniquely 

destructive events that may be triggered by only small 

amounts of rain, occur with little or no warning, and 

often are not amenable to conventional flood-control 

measures.

Floods are distinguished from debris flows primarily on the basis of the concentration and character of 

sediment carried in the flow. Heavily sediment-laden 

(hyper-concentrated) flood flows may be composed of 

roughly equal volumes of sediment and water, but the 

mixture, like pure water, continuously moves (deforms) 

under application of a shear stress. Debris flows, however, carry so much sediment—generally considerably 

greater than 50 percent by volume—that the sedimentwater mixture has strength to resist deformation until 

some threshold value is reached. Fluids with strength, 

such as debris flows and freshly mixed concrete, are 

termed Bingham fluids, whereas fluids without 

strength, such as hyper-concentrated flood flows and 

pure water flows, are termed Newtonian fluids (Costa 

and Jarrett, 1981).

Flood deposits tend to be different from debrisflow deposits because of the different sediment characteristics and flow mechanics associated with the different fluid types. As described by Costa and Jarrett 

(1981), flood deposits, though often poorly sorted, are 

better sorted than debris-flow deposits. Deposits from 

debris flows also tend to form levees along the edges of 

flow that consist of very poorly sorted materials ranging 

from fine sand to boulders supported in a matrix of finegrained silts, clays, and organic material. Matrix-supported deposits and levees are not characteristic of flood 

flows, which leave only fine-grained deposits along the 

flow margins and poorly sorted clasts within the channel as flows recede. These differences in deposits were 

used as the basis for distinguishing floods from debris 

flows in this report.

 The recurrence intervals for peak discharges for 

floods were based on equations developed for ungaged 

sites by Parrett and Johnson (2004). These equations 

provide estimated discharges for specific recurrence 

intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years. 

Calculated peak discharges in 2000 and 2001 that are 

between those associated with specific recurrence intervals are characterized by a range of recurrence intervals. 

These equations were developed from long-term gaged 

data and do not reflect the changed hydrologic conditions resulting from wildfire. Thus, the large recurrence 

intervals determined for many of the flood peaks in the 

study areas indicate the rarity of such large floods under 

more normal, unburned conditions and not the rarity of 

such floods under the current, burned conditions. Large 

floods and debris flows, similar to those in 2001, may be 

a relatively common occurrence in the burned areas 

until vegetation is reestablished.

FLOOD RUNOFF AND DEBRIS FLOWS FROM A 

STORM IN 2000

The first substantial storm after the fires was a 

frontal-type storm that moved generally northeast 

through the Bitterroot study area on September 30 and 

October 1, 2000. Though fairly large in areal extent, 

this storm did not affect other burned areas in Montana. 

Twenty-four hour storm totals at four NWS gages 

ranged from about 1.9 inches near Skalkaho Pass east of 

Hamilton to less than 0.5 inch at Darby (National Climatic Data Center, 2000). The recurrence interval for 

the 24-hour total at Skalkaho Pass was about 10 years, 

while the recurrence interval for the 0.5 inch total at 

Darby was less than 2 years.

The storm of September 30-October 1 produced 

some debris flows that moved relatively small volumes 

of eroded material from small, burned drainages near 

Sula and significant flood runoff from Little Sleeping 

Child Creek near Hamilton. Based on a post-flood 

channel and high-water mark survey, the calculated 

peak discharge was 190 cubic feet per second for Little 

Sleeping Child Creek at site 1 draining 9.3 square miles 

of mostly burned forest. Based on regional flood-prediction equations developed by the USGS for unburned 

conditions, this estimated peak discharge had a recurrence interval of about 100 years. No other significant 
6 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

debris flows or large water flows were noted elsewhere 

in the Bitterroot study area as a result of the storm on 

September 30-October 1.

The particularly large peak flow on Little Sleeping 

Child Creek may have been due to more intense rainfall 

in this burned drainage than elsewhere in the storm area. 

The lack of more site-specific rainfall data or evidence 

of high-intensity rainfall, such as rill erosion on hillslopes, however, makes this conclusion speculative. 

Overall, the small amount of September 30-October 1 

storm data and general lack of flood response inside and 

outside the burned areas indicate that the storm, while 

significant in terms of 24-hour rain depths, probably did 

not have the short periods (5-minutes to 30-minutes) of 

high-intensity rainfall that triggered the flooding and 

more substantial debris flows that occurred in the summer of 2001.

 In April 2001, a continuous-recording streamflow-gaging station (site 2) and a tipping-bucket rain 

gage (site 3) were installed on Laird Creek near its confluence with the Bitterroot River near Sula to help collect precipitation data in the burned areas and to ensure 

that floods or debris flows that might occur in the 

burned areas would be measured and documented (fig. 

1). Crest-stage streamflow gages and tipping-bucket 

rain gages also were installed on Laird Creek above Gilbert Creek, near Sula (sites 4 and 5), North Fork Rye 

Creek near Conner (sites 6 and 7), Little Sleeping Child 

Creek near Hamilton (sites 1 and 8), Little Bluejoint 

Creek near Connor (sites 9 and 10), Burke Gulch near 

Darby (sites 11 and 12), Sleeping Child Creek near 

Hamilton (sites 13 and 14), and Meadow Creek near 

Sula (sites 15 and 16). A crest-stage gage also was 

placed near the mouth of an unnamed tributary to Sleeping Child Creek (site 17), but because of its proximity to 

the Sleeping Child Creek gage, a tipping-bucket rain 

gage was not installed at site 17. A second tippingbucket rain gage was installed near the mouth of 

Meadow Creek (site 18) because of the relatively large 

elevation difference between the crest-stage gage location and the mouth. Following storms and runoff in July 

2001, the USGS replaced crest-stage gages on North 

Fork Rye Creek, Little Sleeping Child Creek, and 

Sleeping Child Creek with continuous-recording, seasonal streamflow gages and installed tipping-bucket 

rain gages at two additional locations in the North Fork 

Rye Creek basin (sites 19 and 20) and one location near 

Overwhich Creek (site 21).

FLOOD RUNOFF AND DEBRIS FLOWS FROM 

STORMS IN 2001

 The following sections describe the sequential 

storm and runoff activity in each of three burned areas 

during the spring and summer of 2001. While site-specific precipitation data generally are lacking for the 

early storms, data are available from USGS precipitation gages for later storms. Stations and dates of occurrence of significant storms are summarized in table 1. 

Precipitation data and recurrence intervals for the depth 

accumulations for various durations during significant 

storms are shown in table 2. Peak-discharge (streamflow) data are summarized in table 3 while the peak 

debris-flow discharge data are summarized in table 4.

Canyon Ferry Area

Beginning in late May 2001, a series of thunderstorms moved generally northeast across the Canyon 

Ferry area near Helena. On May 27, the first storm 

caused flooding on Cave Gulch (site 22), which is a 

small (4.6 square miles drainage area), ephemeral tributary to the northeast side of Canyon Ferry Reservoir. 

The Cave Gulch drainage basin, which was heavily 

burned in 2000, also has a mine and several excavated 

mine-waste settling ponds near the middle of the drainage basin. Prior to the flood of May 27, streamflow in 

Cave Gulch near its mouth was such a rare occurrence 

that structures were built in the poorly defined channel. 

Consequently, flood discharge from the May 27 storm, 

although relatively small, had nowhere to go but 

through the structures (fig. 2). 

Although no site-specific data are available for the 

rain intensity in the Cave Gulch drainage basin, 

observer reports indicated that about 0.4 inch of rain fell 

in about a 30-minute period. The NWS precipitation 

station at the Helena airport about 20 miles west of Cave 

Gulch reported only a trace of rain during the same 

period. The recurrence interval for a 30-minute precipitation depth of 0.4 inches is about 10 years. 

Following the May 27 flood, channel cross sections and high-water marks at site 22 just upstream from 

the mouth of Cave Gulch were surveyed and the peak 

discharge was calculated to be 65 cubic feet per second 

(table 3). Because of some speculation by landowners 

that this flood may have, in part, resulted from the failure of some of the upstream mine-waste settling ponds, 

channel sections and high-water marks at a site 
FLOOD RUNOFF AND DEBRIS FLOWS FROM STORMS IN 2001 7

 Table 1. U.S. Geological Survey precipitation gages in and near Montana burned areas and dates of significant

rainstorms in 2001

[Symbol: --, no significant storm data]

Site 

number

(fig. 1)

Gage 

number

Precipitation

gage

Dates of

significant

rainstorms1

3 12343300 Laird Creek at mouth, near Sula July 15, 20, 21

5 455129114042401 Laird Creek above Gilbert Creek, near Sula July 15, 20, 21

7 455947114015401 North Rye Creek near Conner July 15, 20, 22

8 460601114055001 Little Sleeping Child Creek near Hamilton --

10 454122114015401 Little Bluejoint Creek near Conner --

12 12344300 Burke Gulch near Darby July 30

14 460636114001501 Sleeping Child Creek near Hamilton July 15

16 455037113480701 Meadow Creek near Sula --

18 455426113464901 Meadow Creek at mouth, near Sula --

19 460201114035101 Spring Hill near Darby --

20 4604351114002201 White Stallion Camp near Darby --

21 454100114160601 Overwhich near Conner --

26 463942111414801 Lower Cave Gulch near Helena --

27 463543111413701 Crittenden Gulch near Helena July 17, 30

28 464334111395601 Upper Cave Gulch near Helena --

29 464359111350301 Upper Magpie Creek near Helena July 17 

30 464149111371701 Lower Magpie Creek near Helena July 17, August 4

33 452352106144601 Upper Paget Creek near Ashland June 302

34 452030106174901 Coal Bank Creek near Ashland June 302

1

A significant rainstorm is one that produced a peak discharge at a nearby streamflow estimation or gaged site or

whose depth had a recurrence interval of 2-years or greater for a 5-, 10-, 15-, 30-, or 60-minute duration storm. 2

Data missing for storms after July 1, 2001.

Figure 2. Flooding aftermath from the storm of May 27, 2001, in a storage building constructed 

in the poorly defined Cave Gulch stream channel, near Helena, Montana.
8 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

Table 2. Data for significant storms during 2001 at U.S. Geological Survey precipitation stations in Montana burned areas

[Symbol: <, less than]

Date

Upper Paget Creek

(site 33)

Coal Bank Creek

 (site 34)

Storm 

duration 

(minutes)

Maximum

rain

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain

depth 

(inches)

Recurrence

 interval

(years)

06/30/2001 5 0.56 100-500 5 0.14 <2

10 .75 25-50 10 .28 <2

15 .86 25 15 .29 <2

30 .95 10 30 .29 <2

60 .96 5 60 .29 <2

Daily Total .96 <2 Daily Total .29 <2

Date

Laird Creek at mouth

(site 3)

Laird Creek above Gilbert Creek

(site 5)

North Rye Creek

(site 7)

Sleeping Child Creek

(site 14)

Storm 

duration 

(minutes)

Maximum

rain

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain

depth 

(inches)

Recurrence

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain

depth 

(inches)

Recurrence 

interval

(years)

07/15/2001 5 .14 2-5 5 .09 <2 5 .22 10 5 .21 5

10 .21 2-5 10 .17 2 10 .35 10-25 10 .38 10-25

15 .24 2-5 15 .25 2-5 15 .44 10-25 15 .53 25

30 .31 2 30 .31 2 30 .54 10 30 .66 10-25

60 .31 <2 60 .35 <2 60 .62 5-10 60 .76 10

Daily Total .32 <2 Daily Total .36 <2 Daily Total .64 <2 Daily Total .83 <2

Date

Crittenden Gulch

(site 27)

Upper Magpie Creek

(site 29)

Lower Magpie Creek 

(site 30)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

07/17/2001 5 .17 5 5 .13 2 5 .07 <2

10 .27 5 10 .18 <2 10 .1 <2

15 .36 5-10 15 .21 <2 15 .12 <2

30 .41 2-5 30 .30 <2 30 .19 <2

60 .43 2-5 60 .35 <2 60 .23 <2

Daily Total .70 <2 Daily Total .58 <2 Daily Total .39 <2

Date

Laird Creek at mouth 

(site 3)

Laird Creek above Gilbert Creek

(site 5)

North Rye Creek

(site 7)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

07/20/2001 5 .12 2 5 .21 10-25 5 .22 10

10 .24 5 10 .35 10-25 10 .36 10-25

15 .31 5 15 .38 10 15 .39 10-25

30 .42 2-5 30 .42 2-5 30 .4 2-5

60 .43 2-5 60 .43 <2 60 .41 2-5

Daily Total .44 <2 Daily Total .43 <2 Daily Total .41 <2
FLOOD RUNOFF AND DEBRIS FLOWS FROM STORMS IN 2001 9

Table 2. Data for significant storms during 2001 at U.S. Geological Survey precipitation stations in Montana burned areas (Continued)

Date

Laird Creek at mouth

(site 3)

Laird Creek above Gilbert Creek

(site 5)

Sleeping Child Creek

(site 14)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

07/21/2001 5 .16 5 5 .15 5 5 .19 5

10 .31 10 10 .22 2-5 10 .35 10-25

15 .47 10-25 15 .3 5 15 .42 10

30 .54 10 30 .35 2-5 30 .52 5

60 .58 5-10 60 .47 2-5 60 .55 2-5

Daily Total .58 <2 Daily Total .61 <2 Daily Total .65 <2

Date

Burke Gulch

(site 12)

Crittenden Gulch

(site 27)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

Storm 

duration 

(minutes)

Maximum

rain 

depth 

(inches)

Recurrence 

interval

(years)

07/30/2001 5 .04 <2 5 .02 <2

10 .06 <2 10 .04 <2

15 .07 <2 15 .06 <2

30 .09 <2 30 .12 <2

60 .12 <2 60 .15 <2

Daily Total .78 <2 Daily Total .28 <2

Date

Lower Magpie Gulch

(site 30)

Storm 

duration 

(minutes)

Maximum

 rain 

depth 

(inches)

Recurrence 

interval

(years)

08/04/2001 5 .18 5-10

10 .28 5-10

15 .29 2-5

30 .33 <2

60 .36 <2

Daily Total .52 <2
10 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

Table 3. Peak streamflow discharges during 2001 and recurrence intervals at streamflow-gaging stations and flood-estimation sites in and 

near Montana burned areas

[Symbols: >, greater than; <, less than; -- not determined]

Site 

number

(fig. 1)

Station

number

Station or stream name

Drainage

area 

(square 

miles)

Date of 

peak

discharge

Peak

discharge

(cubic 

feet per 

second)

Estimated 

recurrence 

interval

(years)1

Bitterroot Study Area

1 460601114055001 Little Sleeping Child Creek above Spring Gulch, near Hamilton 9.3 July 30 235 2

 2 12343300 Laird Creek near Sula 9.3 July 20 3210 200-500

 2 12343300 Laird Creek near Sula 9.3 July 21 3220 200-500

 4 455129114042401 Laird Creek above Gilbert Creek, near Sula 5.1 July 20 3160 200-500

 4 455129114042401 Laird Creek above Gilbert Creek, near Sula 5.1 July 21 3160 200-500

6 455947114015401 North Rye Creek near Conner 17.5 July 15 260 100

9 454122114015401 Little Bluejoint Creek near Conner 4.3 April 19 4100 25

11 12344300 Burke Gulch near Darby 6.5 July 30 3.3 <2

13 460636114001501 Sleeping Child Creek near Hamilton 37.0 July 15 150 <2

15 455037113480701 Meadow Creek near Sula 9.3 Late April 470 <2

17 460633114001701 Unnamed tributary to Sleeping Child Creek at Hot Springs, near Hamilton 3.6 July 15 10 2

56 -- Unnamed tributary No. 7 to Sleeping Child Creek near Hamilton 1.8 July 15 5200 >500

Canyon Ferry Study Area

22 -- Cave Gulch at mouth, near Helena 4.6 May 27 65 10

23 -- Cave Gulch above mine tailings, near Helena 2.9 May 27 130 100

 24 -- Crittenden Gulch at mouth, near Helena 2.3 May 28 3370 50

 24 -- Crittenden Gulch at mouth, near Helena 2.3 July 17 31,020 200

 24 -- Crittenden Gulch at mouth, near Helena 2.3 July 31 3,560 5-10

25 -- Magpie Creek at mouth, near Helena 25.5 May 27 580 2-5

31 -- Magpie Creek above Bar Gulch, near Helena 17.4 July 17 405 50-100

32 -- Hellgate Gulch at Forest Service boundary, near Helena 9.2 July 17 5310 100-200

Ashland Study Area

 356 -- East Fork Otter Creek Tributary 1 near Ashland .2 June 30 5980 >500

 366 -- East Fork Otter Creek Tributary 2 near Ashland .1 June 30 5650 >500

 376 -- East Fork Otter Creek Tributary 3 near Ashland 1.0 June 30 5920 >500

38 -- Home Creek near Ashland 35.4 June 30 51,000 50-100

39 -- Newell Creek near Ashland 4.3 June 30 400 50-100

40 -- Chromo Creek near Ashland 5.2 June 30 1,220 >500

41 06307720 Brian Creek near Ashland 8.0 June 30 3,200 >500

42 -- Paget Creek near Fort Howes Ranger Station, near Otter 14.0 June 30 3,500 >500

43 -- Coal Creek near Fort Howes Ranger Station, near Otter 3.1 July 10 51,500 >500

44 -- Gate Creek near Fort Howes Ranger Station, near Otter 1.5 July 10 51,250 >500

45 -- Stag Rock Creek near Fort Howes Ranger Station, near Otter 2.2 July 10 280 50-100

46 -- First Creek near Fort Howes Ranger Station, near Otter 3.5 July 10 520 100-200

47 06307700 Cow Creek near Fort Howes Ranger Station, near Otter 8.9 July 10 500 50-100

48 -- Hole-in-the-Wall Creek near Ashland 1.5 June 30 310 50-100

49 -- Dry Creek near Ashland 4.5 June 30 2,460 >500

50 -- King Creek near Ashland 12.4 June 30 1,920 >500

1Based on equations developed for ungaged sites in unburned areas by Parrett and Johnson (2004). 2

Peak discharge from storm of September 30-October 1, 2000, was 190 cubic feet per second with recurrence interval of 100 years. 3

Multiple peak discharges from thunderstorms. 4Annual peak discharge was probably due to snowmelt.

5

Estimated discharge.

6

Site located in unburned area.
FLOOD RUNOFF AND DEBRIS FLOWS FROM STORMS IN 2001 11

upstream from the upper-most pond (site 23) were 

surveyed and a peak discharge of 130 cubic feet per 

second was calculated (table 3). Overall, the settling 

ponds, some of which were partially washed out, 

appear to have attenuated rather than increased the 

flood peak. Based on regional equations developed for 

unburned watershed conditions, the estimated 

recurrence interval for the calculated flood peaks was 

about 100 years at the upper site and about 10 years at 

the lower site.

On May 28, another thunderstorm passed through 

Helena and the Canyon Ferry study area and caused 

flooding from Crittenden Gulch (site 24), a small 

(drainage area of 2.3 square miles) west-side tributary to 

Canyon Ferry Reservoir. This flood washed out an 

access road to several summer homes, exposed a 

recently constructed septic tank (fig. 3), and deposited a 

considerable amount of sediment in a local harbor at the 

mouth of the stream. The Crittenden Gulch drainage 

basin also was heavily burned in 2000. Although no 

site-specific rainfall data were available for the May 28 

storm at the Crittenden Gulch site, this second storm 

was much more intense at the NWS station at Helena 

than the May 27 storm. The maximum 15-minute storm 

depth at the NWS site was 0.3 inch and the maximum

1-hour depth was 0.5 inch. The recurrence intervals for 

these depths and durations were about 25 and 50 years, 

respectively. 

Channel cross sections and high-water marks were 

surveyed and the peak discharge from Crittenden Gulch 

at its mouth (site 24) was calculated to be 370 cubic feet 

per second (table 3). Based on regional flood equations 

developed for unburned watershed conditions, the estimated recurrence interval for the peak discharge was 

about 50 years.

Both the May 27 and May 28 storms caused some 

debris flows from small gulches draining into Magpie 

Creek, (drainage area of 25.5 square miles at site 25) 

tributary to the east side of Canyon Ferry Reservoir. 

Although these debris flows caused no structural damage, they did block the Magpie Creek road for several 

days. The material deposited on the road, though probably soupy at the time of deposit, was a hard, concretelike matrix-supported deposit when observed several 

days after the event. The deposits were eroded from a 

burned, steep, east-facing valley wall about 400 feet 

above the road. The bedrock is a siltite of the Belt 

Supergroup of Middle Proterozoic age that erodes to 

small plates in a finer grained soil. The debris flows 

generally did not reach the Magpie Creek channel and 

significant flooding did not occur on Magpie Creek.

The peak flood discharge for the May 27-28 

storms for Magpie Creek near the mouth (site 25) was 

estimated to be 80 cubic feet per second. The estimated 

recurrence interval for this discharge, based on 

unburned watershed conditions, is 2-5 years.

Following the storms in late May, five tippingbucket rain gages were installed in burned areas (fig. 

1A) to better determine future storm intensities. The 

rain gages were located near the mouths of Cave Gulch 

(site 26) and Crittenden Gulch (site 27), at a site near the 

headwaters of Cave Gulch (site 28), and at two sites in 

the Magpie Creek basin (site 29 in the upper part of the 

basin and site 30 near the middle of the basin).

On July 17, a thunderstorm caused significant 

flooding again on Crittenden Gulch and Magpie Creek

Table 4. Peak debris-flow discharges at selected tributary sites in the

 Sleeping Child Creek drainage, July 15, 2001

Site

number

(fig. 1)

Unnamed 

tributary

to

Sleeping 

Child 

Creek

Drainage

area

(square

miles)

Average

channel

slope

(foot per 

foot)

Estimated 

peak 

discharge

(cubic feet 

per second)

51 No. 2 0.07 0.43 1,740

52 No. 3 .09 .47 1,860

53 No. 4 .10 .46 1,930

54 No. 5 .28 .31 7,860

55 No. 6 .08 .43 3,500

57 No. 8 .41 .16 2,730
12 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

on Hellgate Gulch (site 32), a tributary to the east side 

of Canyon Ferry Reservoir. Rain depths at the raingage site in the Crittenden Gulch (site 27) drainage 

ranged from 0.17 inch in 5 minutes to 0.70 inch for the 

daily total (table 2). Rain depths at the Upper Magpie 

Creek gage (site 29) ranged from 0.13 inch in 5 

minutes to 0.58 inch for the daily total. At the Lower 

Magpie Creek gage (site 30) rain depths ranged from 

0.07 inch in 5 minutes to 0.39 inch for the daily total 

(table 2). Recurrence intervals for the rain depths 

ranged from <2 years for the daily total to 5-10 years 

for the 15-minute storm duration at the Crittenden 

Gulch site. Recurrence intervals for the rain depths 

ranged from <2 years to 2 years for all storm durations 

at the two gage sites in the Magpie Creek drainage.

The estimated peak discharge on Crittenden Gulch 

(site 24) following the July 17 storm was calculated to 

be 1,020 cubic feet per second (table 3). This flood 

caused further damage to the previously flooded access 

road on the west side of Canyon Ferry reservoir and 

completely blocked a culvert on the main road with 

Figure 3. Flood damage on Crittenden Gulch at its mouth, near Helena, Montana. Vertical,

corrugated pipe on the downstream side of the road provides access to a septic tank.
FLOOD RUNOFF AND DEBRIS FLOWS FROM STORMS IN 2001 13

gravel and debris. Estimated peak discharges for Magpie Creek above Bar Gulch (site 31, drainage area of 

17.4 square miles) and Hellgate Gulch (site 32, drainage 

area of 9.2 square miles) were calculated to be 405 and 

310 cubic feet per second, respectively (table 3). As 

indicated in table 3, estimated recurrence intervals for 

these flood magnitudes were 200 years for Crittenden 

Gulch, 50-100 years for Magpie Creek above Bar 

Gulch, and 100-200 years for Hellgate Gulch.

Ashland Area

On June 30, severe thunderstorms were reported in 

the Ashland area, and golf-ball-sized hail damaged windows in the Ashland school. Several Otter Creek and 

Tongue River tributaries draining burned areas within 

the Custer National Forest severely flooded. Severe 

thunderstorms and subsequent flooding occurred again 

in the Ashland area on July 10. Most, but not all, 

streams that flooded as a result of the June and July 

storms drained basins that were burned in 2000. Sites 

35, 36, and 37 drain unburned areas (table 3).

Two tipping-bucket rain gages installed in the 

Ashland burned area in May collected data during the 

storm of June 30. One, located near the headwaters of 

Paget Creek (site 33, fig. 1B), recorded 0.56 inch in only 

5 minutes and a storm total of 0.95 inch in 30 minutes 

(table 2). The second gage, located near the southwestern edge of the burned area (site 34, fig. 1B), recorded 

0.14 inch in 5 minutes and a storm total of about 0.29 

inch in 15 minutes. Neither gage was operable during 

the July 10 storm. The calculated recurrence intervals 

for the 5-, 10-, 15-, and 30-minute rain depths at the 

Paget Creek rain gage (site 33) were 100-500, 25-50, 

25, and 10 years, respectively, whereas the calculated 

recurrence intervals for all depths at Coal Bank Creek 

(site 34) were less than 2 years. Based on observer 

reports and inspection of Doppler radar images for the 

storms, the National Weather Service (Steve Kuhl, 

National Weather Service, written communication, 

2001) estimated that the maximum intensity for the June 

30 storm was about 2.0-2.5 inches in 30 minutes and 

that the maximum intensity for the July 10 storm was 

about 5 inches in 2 hours in the burned area. The calculated recurrence intervals for these rainfall depths and 

durations exceed 500 years.

Following the storm of July 10, 16 sites (sites 35-

50) were surveyed in order to calculate peak discharges 

(table 3). As indicated in table 3, the estimated recurrence intervals were equal to or greater than 50 years for 

the calculated peak discharges at the 16 sites in the Ashland area and greater than 500 years for calculated peak 

discharges at 10 sites. Long-term peak discharge record 

was available for two sites, Cow Creek near Otter (site 

47, USGS station 06377700) and Brian Creek near Ashland (site 41, USGS station 062307720). The calculated 

peak discharge for the July 10 storm for Cow Creek was 

500 cubic feet per second and had a recurrence interval 

of 50-100 years. The calculated peak discharge for the 

June 30 storm for Brian Creek was 3,200 cubic feet per 

second. This discharge was more than three times the 

previous peak (23 years of record) and more than twice 

the 100-year flood. The recurrence interval for the June 

30 peak discharge for Brian Creek was greater than 500 

years. Muddy sediment and large, woody debris deposited near the gage by the flood on Brian Creek are 

shown in figure 4. The thick sediment grades upward 

from sand at the base to finer, fire-ash and charcoal-rich 

silt at the top. At a site about 3.5 miles upstream from 

the gage, floodwaters were about 10 feet deep. Near 

this upper site, intense rain on hillslopes produced 

sheetwash that removed burned duff and fire ash, leaving a firm soil surface laced with a mesh of fine tree 

roots.

Figure 5 shows the relative rarity of the large 

floods that occurred in the Ashland study area in June 

and July 2001. This graph shows the maximum known 

peak discharges at gaged sites in the Southeast Plains 

Region of Montana (Parrett and Johnson, 2004) and 

drainage areas, together with calculated peak discharges 

and drainage areas for the June and July floods in the 

Ashland study area. Several of the calculated peak discharges in 2001 plot as high or higher than the maximum known discharges at similar-sized gaged sites 

(sites 35 and 36). Also shown for comparison on figure 

5 is a regression line relating 100-year flood peaks 

(Q100) to drainage area for the gaged sites. Several peak 

discharges for 2001 in the Ashland study area plot well 

above the 100-year flood regression line (sites 35, 36, 

37, 40, 41, 42, 43, 44, 49, and 50). The regression line 

in figure 5 is based on only one explanatory variable 

(drainage area), whereas the regression equations developed by Parrett and Johnson (2004) are based 
14 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

on more than one explanatory variable. Consequently, 

one calculated peak discharge for 2001 (First Creek, 

site 46) plotted below the regression line, even though 

data in table 3 indicates that the recurrence interval is 

100-200 years. The recurrence intervals shown in table 

3 are considered to be more reliable than the regression 

line in figure 5. Finally, figure 5 distinguishes the 2001 

peak-discharge sites draining burned areas from those 

draining unburned areas. Surprisingly, peak discharges 

for the three sites (sites 35, 36, and 37) draining 

unburned areas generally plot as high above the 

regression line as the highest-plotting discharges for 

sites draining burned areas. This result may indicate 

that the fire of 2000 had less effect on the 2001 flood 

peaks in some watersheds than did the pattern and 

intensity of the storms producing the runoff.

Bitterroot Area

Severe thunderstorms moved generally northeast 

through the Bitterroot study area on July 15, 20, and 21. 

The storm on July 15 produced flooding on North Rye 

Creek, debris flows on several tributaries to North Rye 

Creek, and debris flows that eroded large volumes of 

material from several tributaries to Sleeping Child 

Creek (fig. 1C). The storms on July 20 and 21 caused 

flooding on Laird Creek and several Laird Creek tributaries. Because of their large size, the debris flows in 

the Sleeping Child Creek basin will be described separately from the storms and floods.

Storm and Flood Data 

Rainfall data for the July 15-21 storm periods at 

USGS precipitation gages on Laird Creek (sites 3 and 

5), North Rye Creek (site 7), and Sleeping Child Creek 

(site 14) are summarized in table 2. On July 15, rain 

depths were relatively small at the two Laird Creek 

sites, ranging from 0.09-0.14 inch in 5 minutes to 0.31-

0.35 inch in an hour, which was essentially the storm 

duration. Rain depths were greater at the North Rye 

Creek site, where 0.22 inch fell in 5 minutes, 0.44 inch 

fell in 15 minutes, and 0.62 inch fell in an hour. Rain 

depths at the Sleeping Child site were even greater, as 

Figure 4. Sediment-laden floodwaters on Brian Creek near Ashland, Montana, overtopped the 

road and deposited a thick layer of muddy sediment (foreground) and large, woody debris (road 

embankment in background). Note the person standing on the culvert for scale. View is looking 

downstream.
FLOOD RUNOFF AND DEBRIS FLOWS FROM STORMS IN 2001 15

the gage recorded 0.53 inch in 15 minutes and 0.76 

inch in an hour. On July 20, rain depths were similar at 

the two Laird Creek sites and at the North Rye Creek 

site. The 15-minute rain depths at the three sites 

ranged from 0.31 to 0.39 inch, and the hourly rain 

depths ranged from 0.41 to 0.43 inch. Rain depths on 

July 21 were similar to those on July 20 at the two 

Laird Creek sites and similar to those on July 21 at the 

Sleeping Child Creek site.

As shown in table 2, the recurrence intervals for 

the rain depths ranged from less than 2 years for the 

daily total at all sites for all storms to 25 years for the 

15-minute duration for the July 15 storm at Sleeping 

Child Creek (site 14). Each site had at least one shortduration (5-, 10-, 15-, or 30-minute) depth for at least 

one storm with a recurrence interval of 10-25 years. 

In the Bitterroot study area, the July storms first 

caused flooding in the heavily burned North Rye Creek 

basin (site 6, drainage area of 17.5 square miles). 

Although the flooding deposited debris on hay fields 

near the mouth and caused substantial channel erosion, 

it did little or no structural damage. The peak flood discharge for the July 15 storm was calculated to be 260 

cubic feet per second at site 6 (table 3, fig. 1). The estimated recurrence interval for this discharge was 100 

years. On July 20, flooding occurred in the heavily 

burned Laird Creek basin (site 2, drainage area of 9.3 

square miles). Flood deposits from several Laird Creek 

tributaries spilled onto the narrow valley floor and 

partly diverted the main Laird Creek channel (fig. 6). 

Debris and floodwaters from Laird Creek damaged several homes that had previously suffered fire damage in 

Figure 5. Maximum known peak discharges at gaged sites in the Southeast Plains Region of Montana (Parrett and Johnson, 2004)

compared to 2001 peak discharges in the Ashland study area.

DRAINAGE AREA, IN SQUARE MILES

0.01 100 1,000 10,000

PEAK DISCHARGE, IN CUBIC FEET PER SECOND

1

10

100

1,000

10,000

100,000

Regression line relating 100-year flood peak (Q ) 100

to drainage area at gaged sites

0.1 1 10

36

35 37

41 42

49

43

40

50

46

44

EXPLANATION

MAXIMUM KNOWN PEAK DISCHARGE 

AT GAGED SITES

Burned site and site number where applicable

Unburned site and site number

2001 PEAK DISCHARGE

35

46
16 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

2000. Floodwaters overtopped Highway 93 and backed 

up along the highway to flood commercial buildings 

several hundred feet south of the main Laird Creek 

channel. On the following day, a similar storm caused 

more damaging floods. Homes and structures in the 

narrow valley were again damaged by mud, debris, and 

floodwaters; Highway 93 was again overtopped. The 

peak discharges at site 2 on July 20 and July 21 were 

similar—210 and 220 cubic feet per second (table 3). 

The recurrence intervals for these peak discharges were 

200-500 years. Hydrographs for the July 20 and 21 

floods for the gage at site 2 are shown in figure 7 along 

with a line showing the 200-year flood discharge of 200 

cubic feet per second. The calculated peak discharge at 

site 4 on Laird Creek above Gilbert Creek (table 3), 

where the drainage basin size is 5.1 square miles, was 

160 cubic feet per second. Although only one set of 

high-water marks was found at this upper site, the peak 

discharges for July 20 and 21 were presumed to be 

about the same magnitude, based on the data for the 

lower site. The recurrence interval for a peak discharge 

of 160 cubic feet per second at the upper site on Laird 

Creek was 200-500 years.

Debris-Flow Data and Analysis 

Sleeping Child Creek had only minor flooding 

from the storm of July 15 but several tributaries to 

Sleeping Child Creek, mostly draining from the north 

side, produced debris flows that deposited thousands of 

cubic yards of mostly rock debris on the narrow Sleeping Child Creek valley floor. In several places, the 

debris-flow deposits completely blocked the main 

Sleeping Child Creek channel and forced the stream to 

cut a new channel along the south side of the V-shaped 

valley. The large volumes of debris deposited at the 

mouths of the tributary streams originated in the steep 

tributary channels upstream from the areas of deposition. These tributary channels all exhibited deep, vertical erosion of as much as 15 feet in some reaches. 

Peak debris-flow discharge was estimated at several of the tributary channels (sites 51-55, 57; table 4; 

Figure 6. Flood deposits from Laird Creek tributary partly blocked the Laird Creek channel

(upstream from site 2) following storms of July 20 and 21, 2001, near Sula, Montana.
FLOOD RUNOFF AND DEBRIS FLOWS FROM STORMS IN 2001 17

fig. 1) where the erosion and subsequent deposition was 

especially large (fig. 8). Calculated peak debris-flow 

discharges ranged from 1,740 to 7,860 cubic feet per 

second for drainage areas ranging from 0.07 to 0.41 

square mile. At one of the largest tributary sites (site 

56), flow deposits did not form levees, were generally 

sorted by size, and did not have any evident fine-grained 

matrix support. Consequently, the peak discharge at 

this site was considered to be a hyper-concentrated flow 

rather than a debris flow (Costa and Jarrett, 1981) and 

was estimated as a water discharge (table 3).

Debris-flow discharge was calculated at channel 

sites where the downstream gradient suddenly steepened and where a transition from subcritical flow to critical flow was considered possible. At such flow 

transitions, the discharge is more dependent upon gravitational acceleration than frictional resistance and can 

be calculated from the following equation (Henderson, 

1966):

(1)

where

Q is the peak debris-flow discharge, in 

cubic feet per second,

g is the gravitational acceleration constant, 

32.2 feet per second squared,

A is the cross-sectional flow area, in square 

feet, and

B is the top width of the flow cross section, 

in feet.

A single channel cross section encompassing the 

tops of the debris-flow deposits or scour marks on each 

side of the channel was surveyed at a location assumed 

to best represent the break in channel gradient, and 

equation 1 was used to calculate peak debris-flow discharge. In very steep mountain channels, debris flows 

may be predominantly supercritical rather than subcritical, and application of equation 1 may yield calculated

Q gA3

B

---------

⎝ ⎠ ⎜ ⎟ ⎛ ⎞ 0.5

=

DISCHARGE, IN CUBIC FEET PER SECOND

0

50

100

150

200

250

200-year flood

1200 1800 2400 0600 1200 1800 2400

July 20, 2001 July 21, 2001

Figure 7. Hydrographs of floods for Laird Creek near Sula, Montana (site 2), July 20 and 21, 2001.
18 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

Figure 8. Measuring debris-flow channel section at two unnamed tributaries to Sleeping

Child Creek (sites 53 and 54), Montana.

Site 54

Site 53
FLOOD RUNOFF AND DEBRIS FLOWS FROM STORMS IN 2001 19

peak debris-flow discharges that are smaller than actual 

discharges.

Because of some uncertainty about the applicability of the critical-flow equation to Bingham fluids like 

debris flows and uncertainty about the transition from 

an assumed subcritical flow condition to a critical flow 

condition in steep channels, the peak debris-flow discharges calculated for this report are shown as estimates. Debris-flow discharge also is not directly 

comparable to flood discharge because debris flows are 

comprised of a high percentage of solids by volume, 

whereas floods are composed mostly of water. Thus, 

debris-flow discharge from a given size drainage may 

be much larger than even a very rare flood discharge. 

Table 4 presents data for the peak debris-flow discharges estimated for sites in the Sleeping Child drainage, along with drainage areas and average mainchannel slopes estimated from USGS 7.5-minute quadrangle maps. Main-channel slope was determined by 

subtracting the elevation at the channel mouth from that 

at the drainage divide and dividing the result by the 

length of channel from the mouth to the divide. The calculated main-channel slopes represent the overall channel steepness in the small basins where debris flows 

occurred, and not necessarily the steepness of the channel at the sites where peak debris-flow discharges were 

estimated. As shown on table 4, the main-channel 

slopes for all the debris-discharge sites were greater 

than 0.15 foot per foot.

Although the peak debris-flow discharges were 

spectacularly large in comparison to flood discharges 

anywhere in the Bitterroot study area, the debris-flow 

discharges had only a small effect on streamflow in 

Sleeping Child Creek. The peak flood discharge at site 

13 on Sleeping Child Creek resulting from the July 15 

storm was only about 150 cubic feet per second. The 

recurrence interval for a peak discharge of this magnitude is less than 2 years. The crest-stage gage indicated 

several smaller peak flood discharges also occurred 

after the peak of 150 cubic feet per second. These 

smaller peaks may have been the result of short-lived 

channel blockages from tributary debris flows that subELEVATION, IN FEET (ARBITRARY GAGE DATUM)

2

3

4

5

6

7

8

April 27, 2001

August 21, 2001

October 25, 2001

0 10 20 30 40

DISTANCE FROM LEFT EDGE OF CHANNEL, IN FEET

Figure 9. Comparison of surveyed channel cross sections at Sleeping Child Creek near Hamilton, Montana (site 13), 

before and after debris flows of July 15, 2001.
20 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

sequently eroded from the channel. The upstream channel blockages contributed additional sand-sized 

sediment to Sleeping Child Creek. Figure 9 shows the 

channel section at site 13 in April 2001 (before the July 

debris flows), compared to the channel section surveyed 

in August and again in October 2001. The comparison 

indicates that the channel aggraded about 0.3 foot from 

April to August and another 0.2 foot from August to 

October. Further aggradation may take place as sediment from upstream debris-flow deposits moves downstream, but cyclic variations in runoff also may result in 

cyclic changes from aggradation to degradation. Overall, given the size of the upstream debris-flow discharges and the several main-channel blockages, the 

debris flows had a notably small effect on streamflow in 

Sleeping Child Creek downstream at site 13. Following 

the large debris flows in July, a continuous-recording 

streamflow gage was placed at the Sleeping Child Creek 

gage site (site 13).

Debris-flow process in the Sleeping Child Creek basin

The debris flows on the small steep tributaries of 

Sleeping Child Creek appeared to have originated about 

one-third of the distance down from the drainage 

divides to the tributary mouths. Soil-slips, a type of 

landslide that commonly mobilizes into debris flows, 

were not evident. Rather, the debris flows started as a 

result of bulking of overland runoff with material 

eroded from the hillslopes and channels. Field observations indicated that overland runoff high on the hillslopes coalesced into numerous small rills. The rills 

then converged in the shallow swales that serve as channels in the upper parts of the small basins. The runoff, 

once concentrated, began eroding the channel bed as a 

series of stepped plunge pools. The uppermost plunge 

pools were just a few square inches in area and a few 

inches deep, but with progression down the swales, the 

pools increased in area and depth. The plunge pools 

often eroded into unconsolidated material laced by a 

mesh of fine roots. In many cases, the pools were 

observed to form immediately below large roots or 

rocks in the flow path. Less frequently, plunge pools 

formed immediately below locations where the flowing 

material dammed, and then breeched. Localized deposits of material were observed immediately downslope 

from some of the plunge pools or along the flow path 

behind obstructions such as trees or rocks. In the upper 

parts of the basins, the deposits primarily consisted of 

well-sorted sands and silts. With progression downstream, the sorting of the deposits decreased, and the 

size of the materials and amount of matrix increased. At 

a distance about one-third of the total channel length, 

the deposits, although discontinuous, were characterized by poor sorting, random orientation of the clasts, 

and fine-grained matrix support of the larger clasts. 

These features, which are commonly associated with 

fire-related debris-flow activity (Meyer and Wells, 

1997; Cannon and others, 2001), indicate that sufficient 

material had been entrained, relative to the amount of 

runoff, to impart debris-flow characteristics to the flow. 

At a distance about midway between the drainage 

divides and channel mouths, the deposits formed continuous levees lining the channel. In addition, depths of 

channel erosion increased, and the series of discrete 

plunge pools were replaced by continuous channel incision, leaving steep, nearly vertical banks. At the narrow 

valley floor of Sleeping Child Creek, the channels were 

incised as much as 15 feet. The deep channel incisions 

revealed that the channels were filled with thick accumulations of colluvium prior to the debris-flow events 

of July 2001 and that virtually all sediment composing 

the debris flows originated in the channels. A similar 

process of progressive channel erosion in burned areas 

resulting in debris flows was found in New Mexico 

(Cannon and others, 2003) and in Colorado (Cannon 

and others, 2001), although in the Colorado case the 

major source of eroded material in that area was the hillslope and not the channel.

IMPLICATIONS FOR FUTURE FLOODS AND DEBRIS 

FLOWS 

Storm intensity and the resultant degree of flooding and debris flows that occurred in 2001 varied widely 

among the three study areas. Although data are few, the 

largest and rarest storms were those only 10 days apart 

in the Ashland study area. These storms also resulted in 

the largest floods, with many calculated peak discharges 

greatly exceeding those with 500-year recurrence intervals for unburned conditions. Ironically, however, any 

correlation between wildfire and flooding is hard to discern in the Ashland study area, because the storms were 

so large and rare. Indeed, based on a plot of peak discharge and drainage area, calculated flood peaks for 

three unburned drainages were just as large as or larger 

than any of the calculated peak discharges for streams 

draining burned areas. In addition, debris flows, which 

are relatively common in many burned areas shortly 

after fires and which were evident in both the Canyon 
SUMMARY 21

Ferry and Bitterroot study areas, were notably absent in 

the Ashland study area. On this basis, wildfire may not 

have been a major factor in the degree of flooding that 

occurred in the Ashland study area. 

In the Canyon Ferry and Bitterroot study areas, 

several factors were common in 2001. First, flooding or 

debris flows generally occurred only when storm depths 

for the shorter (5-, 10-, and 15-minute) durations were 

greater than about 0.2-0.3 inch. Second, floods 

occurred more than once on several streams and the 

degree of flooding from the second storm generally was 

as great as or greater than that from the first storm. This 

second finding indicates that thunderstorm-caused 

flood peaks as large as those in 2001 might be a common annual occurrence on some streams until permanent vegetation is re-established. Third, significant 

debris-flow activity occurred only in burned drainages 

smaller than 1 square mile with channel gradients 

steeper than about 20 percent. Thus, numerous small, 

steep drainages in burned areas currently (2003) without significant debris flows are probably still highly susceptible to this process until permanent vegetation is reestablished.

SUMMARY

In summer 2000 more than 900,000 acres were 

burned by wildfire in Montana. On September 30-

October 1, 2000, a large frontal storm moved generally 

northeast through the largest burned area in the Bitterroot River drainage basin and caused minor debris flows 

at several sites and a flood having a recurrence interval 

of about 100 years, based on unburned conditions, on 

Little Sleeping Child Creek. The sparse rainfall data 

indicated that, while 24-hour precipitation depths were 

significant in 2000, short-duration rainfall amounts evidently were small. The U.S. Geological Survey, in 

cooperation with the Montana Department of Transportation and the Bitterroot National Forest, installed a 

streamflow-gaging station, several crest-stage gages, 

and several tipping-bucket rain gages in anticipation of 

additional flooding and debris flows in 2001. 

Beginning in late May 2001, a series of convective 

thunderstorms across Montana caused significant runoff 

and debris flows in three burned areas. The first area 

affected was the Canyon Ferry area near Helena, where 

debris flows blocked a road, and flooding from Cave 

Gulch and Crittenden Gulch caused structural damage. 

Rainfall data from early storms were sparse, but shortduration rainfall amounts evidently were significant. 

Several tipping-bucket rain gages were installed in the 

Canyon Ferry area following the May storms. Storms in 

the Canyon Ferry area in mid-July again caused flooding from Crittenden Gulch, Magpie Creek, and Hellgate 

Gulch. Short-duration rainfall amounts had recurrence 

intervals of as much as 5-10 years, while flood peaks 

had recurrence intervals up to 200 years, based on 

unburned conditions.

Large thunderstorms in late June and early July 

2001 moved through the Ashland study area and caused 

substantial flooding from tributaries to Otter Creek and 

the Tongue River. Based on data at one USGS gage, 

rainfall depth for the 5-minute duration had a recurrence 

interval of 100-500 years. Based on unofficial reports, 

rainfall depths for the 30-minute duration for the June 

30 storm and for the 2-hour duration for the July 10 

storm both had recurrence intervals greater than 500 

years. Calculated flood peaks had recurrence intervals 

equal to or greater than 50 years at 16 sites and greater 

than 500 years at 10 sites. At most of the flood sites in 

the Ashland area, the streams drained burned areas. 

However, at three sites (35, 36, and 37) where the calculated flood peaks had greater than 500-year recurrence 

intervals, the streams drained unburned areas. The fires 

in 2000 may have had less effect on the large floods in 

the Ashland area in 2001 than did the severity of the 

storms.

A series of thunderstorms on July 15, 20, and 21 

caused flooding and large debris flows in the Bitterroot 

study area. Rainfall depths for the storm of July 15 had 

recurrence intervals of 10-25 years for 10- and 15-

minute durations at one USGS gage and recurrence 

intervals of 10-25 years for 10- and 30-minute durations 

at another. The calculated peak discharge on North 

Fork Rye Creek from the July 15 storm had a recurrence 

interval of 100 years, based on unburned conditions. 

The storms on July 20 and 21 resulted in flooding on 

Laird Creek. The rain depths at two USGS gages in the 

Laird Creek drainage had maximum recurrence intervals of 10-25 years for durations of 5-, 10-, and 15-minutes. The two successive storms resulted in almost 

identical flood peaks on Laird Creek. The recurrence 

interval for the peaks was 200-500 years, based on 

unburned conditions.

Significant debris-flow activity on small, steep 

tributary streams to Sleeping Child Creek resulted from 
22 Wildfire-Related Floods and Debris Flows in Montana in 2000 and 2001

the storm of July 15. The main channel of Sleeping 

Child Creek was diverted and temporarily blocked by 

several of the tributary debris flows. Peak debris-flow 

discharges were calculated at several tributary sites. 

The maximum calculated debris-flow discharge was 

7,860 cubic feet per second from a tributary draining 

only 0.28 square mile of severely burned forest. The 

debris flows were associated with steep channel slopes 

on severely burned areas. The debris flows, which originated about one-third of the distance down from the 

drainage divides to the tributary mouths, were caused by 

overland runoff that coalesced into rills and swales and 

progressively eroded the channels from a condition of 

widely spaced stepped plunge pools to a condition of 

continuous channel incision.

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Mario, 2003, Wildfire-related debris flow generation 

through episodic progressive sediment bulking 

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