HR: 0800h
AN: H41C-0420    [Abstracts]
TI: A five-year record of sedimentation in the Los Alamos Reservoir, New Mexico, following the Cerro Grande Fire
AU: * Lavine, A
EM: alavine@lanl.gov
AF: Earth and Environmental Sciences Division Los Alamos National Laboratory, EES-9, MS D462 Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AU: Kuyumjian, G A
H41C-0420 AF: U.S.D.A., Forest Service, U.S.D.A., Forest Service 475 20th Street, Suite B, Los Alamos, NM 87544 United States
AU: Reneau, S L
EM: sreneau@lanl.gov
AF: Earth and Environmental Sciences Division Los Alamos National Laboratory, EES-9, MS D462 Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AU: Katzman, D
EM: katzman@lanl.gov
AF: Earth and Environmental Sciences Division Los Alamos National Laboratory, EES-9, MS D462 Los Alamos National Laboratory, Los Alamos, NM 87545 United States
AB: The Cerro Grande fire of May 2000 burned approximately 17,400 ha in the eastern Jemez Mountains in the vicinity of Los Alamos, New Mexico. Changes in surface characteristics caused by the fire resulted in significant increases in runoff and erosion relative to pre-fire conditions. This study documents five years of sediment delivery into Los Alamos reservoir, providing a unique datum for evaluating changes in post-fire erosion rates in a montane watershed. The reservoir was built in 1943 for water storage, and had a maximum capacity of 42,000 m3. The upstream drainage basin has an area of 16.5 km2, elevations of 2320 to 3180 m, and largely supported a mixed conifer forest prior to the fire. Thirty percent of the basin experienced moderate to high burn severity, including some of the steepest parts of the basin; 33 percent experienced low burn severity and 37 percent was unburned. Draining of the reservoir in June 2000 allowed for detailed surveying of the top of pre- and post-fire sediments. A total station survey after a single post-fire flood showed a capacity of 34,500 m3. The post-fire deposits in the reservoir were less than 0.3 m thick, comprising 1200 m3 of sediment deposited during 1 event. This yields an estimate of 8600 m3 of sediment accumulation in 57 years, or an average of 150 m3/yr prior to the fire, equivalent to an average basin-wide denudation rate of 0.009 mm/yr. This low rate is consistent with the well-vegetated nature of the basin prior to the fire, and absence of evidence of extensive surface runoff and erosion. When the total station survey was repeated in June 2001, a large delta consisting of sand and gravel had formed a subaerial platform at the head of the reservoir. The distal subaqueous portion of the delta was composed of finer sand, silt, and ash. The remainder of the reservoir basin contained predominantly silt and ash. These combined facies constituted approximately 20,000 m3 of sediment accumulated in one year, equivalent to an average denudation rate of 1.2 mm/yr or 135 times the average pre-fire rate. Assuming that all of the sediment was derived from the high- and moderate-severity burn areas, an average denudation rate of 4 mm/yr is estimated for those areas, about 450 times the pre-fire rate. Within a year after the fire, hillslope erosion changed from primarily sheet wash to rilling and gullying. Surveys performed in the second year after the fire record over 8,000 m3 of sedimentation between June 2001 and January 2002, when the reservoir was dredged. Subsequent surveys document reduced sedimentation as compared with the first two years: between April 2002 and July 2003 4000 m3 of sediment was deposited within the reservoir, with some coarse-grained material forming a small delta; between July 2003 and July 2004 4,800 m3 of sediment was deposited, with 4000 m3 comprising a coarse-grained delta; and between July 2004 and July 2005 4500 m3 of sediment was deposited, with 3500 m3 comprising a coarse-grained delta. The increase in sedimentation in the fourth and fifth year is likely due to excavation and delivery of sediments previously deposited in channels and mobilized by sustained snowmelt.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1815 Erosion
DE: 1857 Reservoirs (surface)
DE: 4863 Sedimentation (1861)
SC: Hydrology [H]
MN: Fall Meeting 2005