On January 4th, 2012, AZGS geologists Ann Youberg and Joe Cook traveled to Emigrant Canyon at the north end of the Chiricahua Mountains to assess post-fire erosion following the 2011 Horseshoe 2 Fire (Figure 1).
Figure 1. Location map of Emigrant Canyon. Arrows, boxes and circles show locations of photographs in the accompanying photo album. The aerial photograph was taken in August 2011 by the US Forest Service. The summit of Rough Mountain is just to the right of the light blue boxes.
The visit was prompted by observations of post-Horseshoe 2 Fire floods and erosion in Emigrant Canyon and its tributaries by Cochise County ranchers Jacob Brown and Larry Prentice, and Coronado National Forest Rangeland Management Specialists Joe Harris and Doug Ruppel (Figure 2). We rode US Forest Service horses into Emigrant Canyon under the guidance of Joe Harris and Doug Ruppel. Accompanying us was Marc Stamer, co-team leader for the Horseshoe 2 Burned Area Emergency Response (BEAR) Team, and Jacob Brown.
Figure 2. Post-fire erosion and sedimentation in an unnamed channel on the north side of Rough Mountain, Emigrant Canyon. Photo: J. Cook.
The human-caused Horseshoe 2 Fire burned 222,954 acres of the Chiricahua Mountains between May 8th and June 25th, 2011 (http://inciweb.org/incident/2225/). Heavy late summer rains produced floods and debris flows in many of the burned areas. Prior to the fire, Emigrant Canyon was densely vegetated with manzanita, oak and juniper. Jacob Brown and his grandfather, Larry Prentice, reported that the shrub cover was so thick it was not possible to walk through. Although the soil burn severity was mostly classified as moderate throughout the section of canyon we visited (Figure 3), the vegetation cover was almost completely removed.
Figure 3. Soil burn severity map of Emigrant Canyon. Low burn severity = green; moderate = orange; high = red. Same spatial extent as Figure 1.
We saw ample evidence of post-fire erosion and sedimentation in and along the main channel, tributaries and hillslopes of Emigrant Canyon. Intense erosion on the steep upper hillslopes of Rough, Wood and Dug Road Mountains scoured and transported surface materials leaving a dense concentration of rill (wide and shallow erosional features) and gully (narrow and more deeply incised features) channels. It is not surprising to see rills and gullies start high in the hillslope as they often form at the base of cliffs where storm runoff initiates erosion. In this case, however, many of the rills and some of the gullies started at the top of the ridges where there were no rock outcrops. The density of the channels on the hillslope and the location of the channel heads at the tops of the ridges are a testament to the changed post-fire hydrologic conditions of this area. On some lower hillslopes, thin, widespread surface soil erosion was likely caused by sheetflow. In larger channels we observed deposits that provided evidence of both floods and debris flows. The accompanying photo album shows the different kinds of erosion and deposits we encountered during our field visit.
Jacob Brown and Larry Prentice report that the erosion occurred during an early evening storm soon after the fire and that little rain fell after the first big event. While there is evidence of some recent re-working and scouring of the channel sediments, the largest summer 2011 deposits appear to be intact. We are evaluating National Weather Service archived radar data in hopes of determining the most likely date of major rainfall and erosion. Results of the Emigrant Canyon rapid erosion event will be used to assess the performance of post-fire debris-flow prediction models in Arizona.
