PASSAGE 1
At the time Glen Canyon Dam was constructed
(1956–63), little consideration was given to how
dam operations might affect downstream resources
in Grand Canyon National Park. In fact, the dam
Line 5 was completed before enactment of the National
Environmental Policy Act of 1969 and the Endangered
Species Act of 1973. By the late 1950s, public values
began to shift, and throughout the 1960s and 1970s
recognition of the environmental consequences of
10 Glen Canyon Dam and its operation grew. National
Park Service and U.S. Geological Survey scientists
and river recreationists observed the physical
transformation of the river in Grand Canyon,
including the loss of large beaches used for camping,
15 narrowing of rapids so as to reduce navigability,
and changes in the distribution and composition
of riparian vegetation. The humpback chub and Colorado
pikeminnow, species found only in the Colorado
River Basin, were listed as endangered in 1967 by
20 the U.S. Fish and Wildlife Service, which concluded
in 1978 that the dam and its operation jeopardized
the continued existence of humpback chub in Grand
Canyon.
Annual spring snowmelt floods were the defining
25 attribute of the pre-dam flow regime. Before the
Colorado River was regulated by dams, streamflow
gradually increased from mid-December to March,
precipitously increased in April and May, and reached
its peak in early June.
30 Pre-dam floods disturbed the aquatic ecosystem,
and native fish species developed strategies to survive
periods when the velocity in the main part of the
channel was high and large amounts of suspended
sediment Were being transported. For example, several
35 of the native fish species share unusual body shapes,
including a large adult body size, small depressed
skulls, large humps on their backs, and small eyes,
which presumably developed as adaptations to life in a
turbid and seasonably variable riverine environment.
40 Sandbars, riverbanks, and their accompanying aquatic
habitats were reshaped during floods. Additionally,
the increased elevation of the river surface during
floods provided Water to native riparian vegetation
otherwise principally dependent on precipitation.
PASSAGE 2
45 Decline in the size and abundance of Sandbars
since the pre-Glen Canyon Dam era has been
documented by analysis of old aerial and ground-level
photographs and by topographic surveys that began in
the mid-1970s. Scientists have estimated that sandbar
50 area in the upstream 100 miles of Glen, Marble, and
Grand Canyons was 25 percent less in 2000 than in
average pre-dam years. This decline occurred because
releases of water from Lake Powell are virtually free of
sediment. The tributaries that enter the Colorado River
55 downstream from the dam supply only a fraction of
the pre-dam sand supply, and the capacity of the post-
dam river to transport that sand greatly exceeds this
limited supply. Normal dam operations, therefore, tend
to erode, rather than build, sandbars.
60 By experimentation, scientists have learned that
controlled floods, if released from the reservoir
immediately following large inputs of sand from
tributaries, can build sandbars. These sandbars are
built during controlled floods when sand is carried
65 from the riverbed and temporarily suspended at high
concentration in the flow. The suspended sand is
transported into eddies where it is then deposited in
areas of low stream-flow velocity. Sandbars enlarged
by this process provide larger camping beaches for
70 river-rafting trips and create backwater habitats used
by native fish. Newly deposited sandbars also provide
areas for riparian Vegetation to grow and are a source
of windblown sand. Windblown sand carried upslope from
sandbars helps to cover and potentially preserve
75 some of the culturally significant archeological sites in
Grand Canyon.
Scientists have also learned that controlled floods
may erode sandbars if the concentration of suspended
sand during a controlled flood is too low.
80 The concentration of sand during a
flood is directly proportional to the amount of the
riverbed covered by sand and the size of that sand.
Higher concentrations of suspended sand occur when
the sand is relatively fine and large amounts of the riverbed are covered by
85 sand. These findings are incorporated in the current
reservoir-release management strategy for Glen Canyon Dam,
which involves releasing controlled floods-administratively
referred to as High Flow Experiments (HFEs)–whenever the Paria River
90 has recently delivered large
amounts of sand to the Colorado River. The magnitude
and duration of the controlled floods is adjusted
to transport just the amount of sand that has
recently been delivered from the Paria River.