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Wildfire-related floods and debris flows in Montana in 2000 and 2001

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

U.S. Geological Survey
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


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 September-October 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.

Contents

Abstract
Introduction
   Purpose and scope
   Methods of study
Flood runoff and debris flows from a storm in 2000
Flood runoff and debris flows from storms in 2001
   Canyon Ferry area
   Ashland area
   Bitterroot area
       Storm and flood data
       Debris-flow data and analysis
       Debris-flow process in the Sleeping Child Creek basin
Implications for future floods and debris flows
Summary
References cited

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
         2-4.    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
                     3.   Flood damage on Crittenden Gulch at its mouth, near Helena, Montana
                     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
              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
              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
              7.    Hydrographs of floods for Laird Creek near Sula, Montana, July 20 and 21, 2001
              8.    Photographs showing measuring debris-flow channel section at two unnamed tributaries to Sleeping
                        Child Creek, Montana
              9.    Graphs showing comparison of surveyed channel cross sections at Sleeping Child Creek near
                        Hamilton, Montana, before and after debris flows of July 15, 2001

TABLES

Table 1.    U.S. Geological Survey precipitation gages in and near Montana burned areas and dates of
                     significant rainstorms in 2001
          2.    Data for significant storms during 2001 at U.S. Geological Survey precipitation stations in Montana
                     burned areas
          3.    Peak streamflow discharges during 2001 and recurrence intervals at streamflow-gaging stations and
                     flood-estimation sites in and near Montana burned areas
          4.    Peak debris-flow discharges at selected tributary sites in Sleeping Child Creek drainage,
                     July 15, 2001


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