Glen Canyon Dam adaptive management program. - Living Rivers

Downstream- Adaptive Management of Glen Canyon Dam and the Colorado River Ecosystem
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88 Downstream: Adaptive Management
budget reflect important advances toward understanding and managing
sand resources in the Grand Canyon. A revision currently under investigation
is the channel's ability to store tributary-derived sediment, which
has important implications regarding the timing of controlled floods
needed to preserve available sand. While the Glen Canyon Dam
Environmental Impact Statement was being written, it was believed that
tributary sand was stored in the channel in years without large dam
releases, leaving it available for redistribution to bars and channel margins
by occasional controlled floods. This model was based on sand budgets
developed from US. Geological Survey gauging records and was based on
the assumption that relations between sand transport and discharge were
stable over time (Handle et al., 1993; Smillie et al., 1993, cited in U.S.
Bureau of Reclamation, 1995). U.S. Geological Survey cross-sections of
the Colorado River were used in determining sand storage in the channel,
information important to planning the controlled flood of 1996.
Reanalysis of sediment gauging records (Topping et a]., 1999) and
observations during the 1996 controlled flood (Rubin et al., 1998; Smith,
1999; Topping et al., 1999) indicated the concentration and size of
sediment transported at a given discharge can vary depending on the
duration of mainstem flows and their timing relative to tributary sediment
inputs. The existence of previously assumed multi-year in-channel storage
is now in question, raising important new questions concerning the
effective timing and duration of future controlled floods.
A previous National Research Council committee recommended
several areas of research and monitoring to support management of the
sand resource, including developing triggering criteria and flow specifications
for beachhabitat-building flows, monitoring rates of beach
deposition during beachhabitat-building flows, and creating a procedure
for determining sand budgets for different parts of the Grand Canyon
(NRC, 1996a). Research and monitoring supporting all of these recommendations
is ongoing, and much of it is incorporated in the 1998
Strategic Plan. Results of ongoing work in each of these areas are also
being used to evaluate and revise management decisions. Studies of beach
deposition during the 1996 beachlhabitat-building flows (Andrews et al.,
1999; Center, 1997a; Hazel et al., 1999; Kearsley et al., 1999; Schmidt et
al., 1999b) and research on channel-eddy sand exchange (Rubin et al.,
1998; Smith, 1999; Topping et al., 1999; Wiele et al., 1999) contributed
directly to ongoing discussions of the most effective magnitude and
duration of such management events.