Geomorphology:
The Colorado River is a large, aridlands river, and in its natural condition was a flood-prone, silt-laden river with seasonally varying water temperature. Geologically constrained rivers in areas with great topographic relief, such as Grand Canyon, function very differently from low gradient alluvial rivers, which are not so confined. Whereas the Mississippi and Missouri rivers have broad floodplains, meandering channels, and migrating sand bars, the Colorado River in Glen and Grand canyons is narrowly confined by talus slopes and bedrock to a relatively narrow channel. River width and depth vary rather predictably in relation to the hardness of the 16 Paleozoic and 5 Precambrian geologic formations through which the river cuts. The channel of the Colorado River in Grand Canyon is punctuated by debris fan complexes that dictate specific locations where sand and silt bars develop, and this geomorphic prescription means that our sand bars do not migrate. Silt and sand loads from the Paria and Little Colorado rivers create three turbidity segments among 13 geomorphically distinctive reaches. This suite of reaches, affected domino-style by the river, serves as the template for life in the river corridor. Thus, the pre-dam Colorado River ecosystem was a classical example of one primarily organized by physical processes, including climate, bedrock geology, geomorphology, and tributary flooding.
Dam Impacts:
Since
1963, Glen Canyon Dam has greatly reduced and/or stabilized
three essential characteristics of the Colorado River: flood
flows, sediment transport, and water temperature (Carothers
and Brown 1991, Schmidt et al. 1998). Although the average
mainstream flow has not changed much, annual spring peak
flows of nearly 90,000 cfs have been eliminated, and maximum
flows have generally been restricted to powerplant capacity
(effectively 31,000 cfs). Low wintertime flows have also
been eliminated. Sediment transport has been reduced at
the U.S. Geological Survey’s Grand Canyon streamflow
gauge from about 68 million tons/yr to about 12 million
tons/yr, nearly all of which is now derived from the Little
Colorado River and, secondarily, the Paria River. Pre-dam
water temperature varied from freezing in winter to 85 degrees
Fahrenheit at Lees Ferry in July. Since 1970 the water has
become uniformly cold because it is withdrawn from deep
within Lake Powell; it now varies from about 47 degreed
Fahrenheit at the dam, and rising to as much as 67 degreed
Fahrenheit at Diamond Creek (Mile 226).
Tributary Flooding:
Tributaries, which flood erratically as a result of storms, contribute about 12 million tons of sand and silt to the mainstream on average. While these tributary floods provide nearly all of the sediment now transported by the river, a rarer kind of flood has very different effects. Debris flows are floods that carry large boulders. These have reached the river at the mouths of more than 500 tributaries in this system, creating debris fan constrictions and eddies that control the location and severity of the river’s more than 160 world famous rapids (Howard and Dolan 1980; Keiffer 1985; Melis 1997; Webb et al. 1989, 1999a). Large debris flows are quite rare, the most impressive recent ones being those in 1966 that scoured Nankoweap, Bright Angel and Crystal creeks, and dramatically reshaped Crystal Rapid. However, numerous smaller debris flows have occurred, including several at Lava Falls. Debris flows continue to reshape Grand Canyon’s smaller rapids because mainstream flows are not large enough to move the large boulders deposited during these events. Although no obstruction to navigation has yet occurred from these floods, one more good debris flow at a narrow location, such as Bedrock Rapid (Mile 131), might interrupt river travel.
Debris-fan complexes create eddies where, at higher flows, sand and finer grain alluvial sediments settle. Eddies, sand deposits, debris fans and shoreline environments support the river’s distinctive aquatic and terrestrial vegetation assemblages which, in turn, provide food and near-shore habitat for many fish and wildlife species (Stevens et al. 1995, 1997a, 1997b). These physical processes play a guiding role in terrestrial ecosystem development.
Flows:
See the USGS and GCMRC websites for an update of river flows. As a result of the U.S. Fish and Wildlife Service’s opinion in the GCD-EIS that low, constant flows in summer are best for native fish, the river was kept to nearly constant flow of 8,000 cfs during most of the summer in 2000. These flows stabilized shoreline habitats and allowed researchers to clearly document fish populations. Results of this massive change in dam operations are just becoming available, and may be obtained by contacting GCMRC at (520) 556-7094.
Sandbars:
Sand bars are a primary physical characteristic of the river, and are a resource that is much in demand by the more than 25,000 river runners who traverse the river each year. The condition of sand bars is under intensive scrutiny by the Northern Arizona Geology Department’s Sand Bar Erosion Studies Program, the U.S. Geological Survey, and the Colorado River Guides’ Adopt-a-Beach Monitoring Program. Collectively, these groups have concluded that the sand bars are rebuilt by high flows, such as the 1996 experimental flood, and then rather rapidly erode (Webb et al. 1999, and Patten and Stevens in press). The results of that experimental high flow are scarcely visible 5 years later, indicating the need for high flows every 5-10 years. Plainly visible in most of the wide reaches of the river is a terrace and flood-scour line created by the experimental 3-day 31,000 cfs flow in late summer. The beneficial effects of this smaller experimental flow are likely to dissipate within a year.
Water Quality:
Common physical and chemical parameters of water quality are routinely collected in the Glen Canyon reach, at Lees Ferry, and at the mainstream USGS gauges downstream. Several variables are continuously monitored in conjunction with the collection of all chemical samples, including temperature (degrees C), specific conductance (m S), pH, dissolved oxygen (mg/L), and turbidity (NTU). These data are available through the GCMRC and USGS websites in Flagstaff, and have not changed much in the past couple of decades. Bacteriology of the river is not currently monitored on a regular basis.
River water temperature has been consistently cold since 1970, and this temperature change is one of the most significant changes to the ecosystem arising from Glen Canyon Dam. Warm water temperature was undoubtedly important for development of most of the aquatic biota in the pre-dam river, and part of the justification for the constant flows program in the summer of 2000 was to warm the river for native fish. Also, hypothermia is a concern to modern river runners and a real threat to anyone in the water for more than a few minutes (Myers et al. 1999).
Water temperatures rose to near record levels in the summer
of 2000, as summertime flows were kept near 8,000 cfs. Water
temperature reached 67 degrees Fahrenheit at Diamond Creek,
a level not observed since the experimental flows of 1991.
Warmer flows may benefit native fish. To further protect
the fisheries in Grand Canyon, the Bureau of Reclamation
has proposed construction of a thermal control device to
the upstream face of Glen Canyon Dam. The TCD would allow
managers to remove warm, surface water from Lake Powell,
warming the downstream river by 6 to 10 degrees Fahrenheit,
depending on flows and season.
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