HR: 10:50h
AN: H52A-03    [Abstracts]
TI: Sediment Studies Refute EIS Hypothesis, While Most Fundamental Process Questions Remain Unanswered: An Update on Experiments in Grand Canyon
AU: * Melis, T S
EM: tmelis@usgs.gov
AF: U.S. Geological Survey, Grand Canyon Monitoring & Research Center, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Topping, D J
AF: U.S. Geological Survey, Grand Canyon Monitoring & Research Center, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Wright, S A
AF: U.S. Geological Survey, Grand Canyon Monitoring & Research Center, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AU: Rubin, D M
AF: U.S. Geological Survey, Coastal & Marine Team, 400 Natural Bridges Dr., Santa Cruz, CA 95060 United States
AU: Schmidt, J C
AF: Utah State University,, Box 5240, Logan, UT 84322 United States
AU: Hazel, J E
AF: Northern Arizona University,, Box 4099, Flagstaff, AZ 86011 United States
AU: Kaplinski, M A
AF: Northern Arizona University,, Box 4099, Flagstaff, AZ 86011 United States
AU: Parnell, R A
AF: Northern Arizona University,, Box 4099, Flagstaff, AZ 86011 United States
AB: For three decades, sediment researchers have pondered the question of whether or not operations at Glen Canyon Dam could be adjusted to maintain downstream sand resources in Grand Canyon. Prior to the era when managed floods were proposed as a strategy to conserve sand inputs below the dam, Laursen et al. (1976) concluded that erosion of sandbars below the dam would be an inevitable, yet protracted post-dam process. Despite this earliest conclusion, the operational strategy for sandbar maintenance since 1996, has been based on two hypotheses: first, much of the sand introduced by tributaries downstream from the dam can accumulate in the channel over multiple years under operations associated with average-to-below average hydrology; and second, controlled floods can move that accumulated sand from the channel bed to shorelines, thereby rebuilding bars in a sustainable manner. Recent work has shown that the first hypothesis is false (Rubin et al., 2002). High resolution data for the ecosystem sand mass balance between 1999 and 2004, indicate no accumulation of tributary sand inputs in the main channel, despite a drought resulting in minimum annual release volumes from 2000 through 2004. Sandbar data also indicate that erosion has not been mitigated by re-operation strategies since 1991. On the basis of these data, researchers have again identified uncertainty regarding a flow strategy that will result in sustainable sandbars. If a successful flow strategy can not be devised, then managers may have to choose between abandoning sandbar restoration objectives, or pursuing sediment augmentation. Experimental fluctuating-flow treatments are also being evaluated for their potential to limit populations of introduced rainbow trout, yet these options are already known to increase sand export. While many institutional barriers to large-scale sediment experiments in Grand Canyon have recently been bridged through a science-based, adaptive management approach, protracted drought throughout the Upper Colorado River Basin now poses a natural barrier to testing the key sediment hypothesis. Downstream sand production from the Paria River remains at its lowest level in 80 years, while water storage in Lake Powell approaches 40 percent of capacity. As scientists wait out delays in sediment experimentation forced by the current drought, managers have already approved limited sediment augmentation feasibility studies aimed at identifying options for managing physical habitats. With regard to conventional thinking about regulation and management of natural hydrologic systems, some important lessons may be learned from the current situation. Rubin, D.M., Topping, D.J., Schmidt, J.C., Hazel, J., Kaplinski, M. and Melis, T.S., 2002, Recent Sediment Studies Refute Glen Canyon Dam EIS Hypothesis: {\it Eos}, vol. 83, no. 25, p. 273-278. Laursen, E.M., Ince, S. and Pollack, J., 1976, On Sediment Transport Through the Grand Canyon, Proceedings of the 3rd Federal Interagency Sedimentation Conference, Denver, CO, vol. 1, p. 4-76 - 4-87.
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting