Geoscience Reference
In-Depth Information
This information was needed to develop and
calibrate physical models as well as helping
managers decide how dam operations could be
altered to reduce sandbar erosion in Grand Canyon
National Park.
compared with previous conditions (Topping
et al
.,
2003). Topographic sandbar data from the 1996
HFE showed that without a sufficient supply of new
sand, sandbars built during the HFE were derived
from lower elevation sandbars within eddies rather
than from sand accumulated over previous years in
the river channel (Rubin
et al
., 2002). These results
indicate that HFEs conducted under sand-depleted
conditions, such as that in 1996, will not sustain
sandbar area and volume successfully (Rubin
et al
.,
2002; Topping
et al
., 2006).
Using these findings, scientists and managers
focused on the need to time HFEs shortly after
episodic tributary floods supply new sand to the
Colorado River downstream from the dam. This
timing was tested during an HFE conducted in
November 2004, under conditions of lower than
average annual tributary sand input that occurred
during September-November 2004; the result was
a temporary increase in sandbar area and volume,
mainly in the upper half of Marble Canyon
(Topping
et al
., 2006). A third HFE, in March 2008,
was conducted following above-average levels of
sand enrichment resulting from tributary floods
in the autumn of 2006 and again in 2007; this
HFE resulted in more widespread sandbar building
(Hazel
et al
., 2010). However, almost all sandbar
gains resulting from the three HFEs were eroded
away within six months following a return to
normal modified low fluctuating flow operations.
Monitoring data on flow, sediment transport and
sandbar topography suggested that erosion of new
sandbars was correlated positively with mean daily
flows and inversely correlated with tributary sand
supply to the river following HFEs (Grams
et al
.,
2010).
Monitoring also confirmed that substantial
increases in total eddy-sandbar area and
volume were only possible by implementing
HFEs following large tributary sand inputs.
Consequently, while managers lack the ability to
increase the sand supply to the river or limit annual
releases, they may still be able to promote greater
sandbar stability by changing daily to seasonal
patterns of flow following HFEs. An experiment
that combines repeated HFEs under sand-enriched
conditions with a more stable daily-to-seasonal
High flow experiments
Various experimental flows were conducted
throughout the 1990s and beyond (Table 26.2).
The Program, established in 1997, is best known
for a series of three high-flow experiments (HFE),
or controlled water releases from Glen Canyon
Dam that were designed to mimic natural pre-
dam seasonal flooding. These HFEs took place
in March 1996, November 2004, and March
2008. The primary objective of these experiments
was to restore geomorphic processes and rebuild
sandbars that provide habitat for native plants
and animals, supply camping beaches for river
runners, and provide fine sediment that helps
to protect archaeological sites from weathering
and erosion. The first HFE was conducted in late
March 1996 to test the 'beach/habitat-building
flow' concept described in the final Environmental
Impact Statement (US Department of the Interior,
1995; Schmidt
et al
., 1999; Webb
et al
., 1999). The
preferred option for dam operation identified by
the Environmental Impact Statement was one of
modified low fluctuating flow. This was assumed
to allow 'new' sand from tributaries to accumulate
more rapidly in the river channel over several
years. The idea was that accumulated channel
sand would then periodically be mobilized during
artificial floods and re-deposited along shorelines
as new, higher elevation sandbars.
Several lessons have been learned from the three
HFEs. First, contrary to predictions, sand that enters
the Colorado River periodically from the Paria and
Little Colorado Rivers below the dam (Figure 26.1)
did not accumulate in the river channel under
normal modified low fluctuating flow operations
(Rubin
et al
., 2002). This conclusion is true even
when the quantity of sand supplied by tributaries
is above average, as was the case in the late
1990s. The rapid export of new sand is explained
by the fine grain size of the sediment, combined
with an increased daily mean flow below the dam
