Wednesday, 9 November 2005
5

Spatial Analysis of an Ash/Sediment Flow Following a Sierran Wildfire.

Erin M. Carroll, W. W. Miller, D. W. Johnson, L. Saito, P. Szameitat, and R. F. Walker. University of Nevada, Reno Department of Natural Resources and Environmental Sciences, 1000 Valley Road, Mailstop 186, Reno, NV 89512

Heavy fuels buildup due to long-term fire suppression is now common to Sierran watersheds. As a result of high intensity wildfire, bared watersheds become highly susceptible to wind and water erosion. The “Gondola” wildfire of July 3, 2002 located approximately 2 km southeast of South Shore Lake Tahoe near Stateline, Nevada, was followed on July 18, 2002 by a severe hail and rainfall event. The wildfire site and surrounding area received 0.76 to 1.52 cm precipitation over a 3-6 hr time frame. The result was a substantive ash and sediment flow with subsequent down-gradient riparian zone deposition. Bulk density and depth of deposition, and GPS delineations of source and sink areas were collected as point data and used in conjunction with ESRI ArcView v.3.3 to spatially assess source area contribution and the extent of ash and sediment flow deposition. Total deposition mass, estimates of minimum and maximum source area (3.03 to 3.35 ha, respectively), and pre-wildfire surface bulk density measurements were used to estimate an amount of surface material removed from the source area via the ash and sediment flow. Total deposition in the riparian zone was 378.6 Mg of ash and sediment over an area of 0.82 hectares. This equated to an average loss of 6-14 mm of surface material as a result of water erosion and off-site transport. Non-glacial baseline erosion rates in mountainous granitic terrain of the Sierra Nevada have been reported to range from 20-60 mm per1000 yr. These long-term rates were only weakly affected by precipitation, which ranged from 20-180 cm yr-1 over the study sites. The single event documented in this investigation along with wildfire enhanced soil water repellency contributed approximately 25-33% of the erosion expected over 1000 yr of normal precipitation.


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