Continued...

Figure 8. Same drainage as Figure 7. This photo was taken on August 9th after the July 20th debris flows and several other rain storms. Photo: D. Fleishman, USFS.

Figure 7. Drainage swale along FR146 (waterline) taken on June 29 after the fire but before rain. Note the swale and smooth hillslopes. Photo: City of Flagstaff Water Department.

Post-Fire Erosion

Post-fire flooding and debris flows accomplished a significant amount of geologic work in a very short period of time. The first major flood event of July 20th exposed bedrock in channels that had previously contained substantial amounts of sediment. The rip-rap barriers installed to protect FR146 and the waterline beneath at drainage crossings were completely destroyed and the waterline was broken at two drainage crossings. The following photos from two drainages (Figures 7, 8 and 9) and along FR146 (waterline, Figure 10) illustrate the impact of this fire and subsequent rains on Coconino National Forest lands. There are several key points to note in these photographs. Photos taken after the fire but before monsoon rains show drainage swales with no defined channel bottom or banks (Figure 7), while post-storm channels are either filled with fresh debris (Figure 8) or scoured to bedrock with steep banks up to 3 m high (Figure 9). Notice the size of material, both rock and woody debris, and depths of scour in these channels. Over time, the channel banks will collapse and more sediment will feed into the channel from the adjacent hillslopes, refilling the channel. Before the rains, hillslopes were smooth; now they have numerous rills and gullies (Figure 10). Also note that vegetation was already growing on steep hillslopes by mid-August (Figure 11). Sediment scoured from hillslopes and channels on the steep slopes of the burned area was deposited on fans and in channels at the base of those slopes.

Figure 9. Photo taken on July 25th of FR146 at the drainage where the waterline was severed and removed by debris flows on July 20th. Half of the roadbed was removed by the debris flows; the remaining roadbed was removed during the August 16th debris flows. Photo: D. Fleishman, USFS.	Figure 10a. Example of hillslope erosion along FR146. The photo was taken on August 17th and shows gullies. Photo: A. Youberg, AZGS.	Figure 10b. Examples of hillslope erosion along FR146. The photo was taken on August 6th and shows rilling through ash. Photo: A. Youberg, AZGS.

To view additional images from the flooding, go to this month's photo gallery.

 

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