Title

Landslides and Wildfire: An Example from the Boise National Forest

Publication Date

5-2001

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

Walter S. Snyder

Abstract

The 1997 New Year Storm triggered numerous landslides in an area previously burned by wildfire in the Boise National Forest near Lowman, Idaho. To assess the impact of wildfire on landslides, an inventory of landslides in both the burned area and an adjacent unburned area was completed. The study area covered 195.5 square kilometers, of which approximately 48% were burned by wildfire in 1989. Twenty-one landslides were observed in a field study of the entire area; nearly all of these were debris flows. A supplemental inventory taken from digital orthophotoquads revealed an additional 225 landslides. Of the 246 landslides inventoried, 181 (74%) of those landslides occurred in the burned area and 65 occurred in the unburned area. This results in a frequency of 2 landslides per square kilometer in the burned area versus 0.6 landslides per square kilometer in the unburned area.

Slope, azimuth, and elevation were derived from Digital Elevation Models for each landslide and evaluated statistically. There were no significant differences in slope values between landslides that occurred in the burned area versus the unburned area; this was not an expected result. However, the landslides in the burned area occurred over a wider range of slopes. There was a significant difference between azimuths of landslides in the burned versus the unburned area. The landslides in the burned area occurred over a much larger range of azimuths than landslides in the unburned area. Likewise, there was a significant difference in elevation between landslides in the burned area versus the unburned area. Although the reasons for this are not clear, the landslides in the burned area occurred on much higher elevations. These results suggest that wildfire effects the range of conditions over which landslides occur in this area of the BNF. This may be important when land managers are assessing an area for potential impact of landslides under "normal" conditions. If an area has been burned land managers should also expect to take into consideration the effects of wildfire on landslides, such as landslides occurring on shallower slopes, slopes facing other directions (than south), and at higher elevations.

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