EBTAG Annual Workshop and Field Trip
May 16-17, 2012

¹Glorieta Geoscience, Inc., PO Box 5727, Santa Fe, NM, 87502, drakos@glorietageo.com

²Los Alamos National Laboratory, EES-16, MS D452, Los Alamos, NM, 87545

The Water Canyon/Cañon de Valle watershed is located north of Frijoles Canyon and Bandelier National Monument and originates on the flanks of the Sierra de los Valles. Water Canyon and its largest tributary, Cañon de Valle, are generally west-to east trending drainages that flow across the southern part of Los Alamos National Laboratory (LANL) on their way to a confluence with the Rio Grande in White Rock Canyon, and comprise a drainage basin with an area of 49.6 km².  Portions of the Water Canyon/Cañon de Valle watershed were burned in the 1977 La Mesa fire (6.7 km² within the burn perimeter), the 2000 Cerro Grande fire (26.2 km² within the burn perimeter), and the 2011 Las Conchas fire (16.9 km² within the burn perimeter). Post-fire sediment deposits associated with some or all of these fires have been identified during investigation of a series of reaches in this watershed, conducted in 2010 and 2011. These post-fire deposits comprise a significant portion of post-1942 sediment deposits within the watershed.

The Water Canyon/Cañon de Valle sediment investigation included detailed geomorphic mapping (1:200 scale) in a series of investigation reaches (25 reaches mapped in 2010-2011), descriptions of post-1942 sediment deposits in each reach, measurement of a series of geomorphic cross sections, and sediment sampling and analysis for an extensive suite of potential contaminants. A series of cross sections measured in 2011 will be relocated and resurveyed in May 2012 to systematically evaluate post-fire changes (erosion/deposition) to post-1942 sediment deposits. 

A series of cross sections were surveyed after the first post-Las Conchas fire flood, which occurred on August 3, 2011, to investigate post-Las Conchas fire sediment deposition and erosion. Sediment from the August 3 flood, measured at cross section locations, reached a maximum thickness of 22 cm and consisted largely of reworked ash and silt (“muck”). Some scouring also occurred during the August 3 flood. Several sections were surveyed after the second, and larger, post fire flood on August 21, 2011. Two sections surveyed after the first flood were resurveyed after the second flood. In contrast to the deposition of muck in overbank areas during the August 3 event, coarser-grained sediment deposits were observed after the August 21 event in both overbank and channel settings, associated with the deeper flow and greater stream power of the second flood event. Sediment thickness from the August 21 flood at cross section locations reached 50 cm. Considerable local scour and bank widening also occurred during the August 21 flood event, and a large fan was built at the mouth of Water Canyon.