Glen Canyon Dam
The Glen Canyon Dam is the second largest dam on the Colorado River at Page, Arizona, USA. The dam's main purpose includes generating electrical power, storing water for the arid southwestern United States, and providing water recreation opportunities. The dam generates an average of 451 megawatts, which contributes 6% of the total electricity generated in Arizona. 13% of the electricity generated in Utah, in which most of Lake Powell resides, calculates to a figure that saves over 5 million metric tons of carbon dioxide . The dam has a maximum capacity of 1300 megawatts but only utilizes an average of 451 megawatts . Damming the Colorado River caused the Glen Canyon, which lies to the north of the dam, to become flooded and has subsequently created the large reservoir called Lake Powell. The Glen Canyon Dam is located upstream from the Grand Canyon National Park and has a 216m high concrete arch dam that controls the water flow from the upper to the lower Colorado River drainage basins. The dam controls a drainage basin of 281,671 km2 and also functions as a barrier for upstream-downstream migratory aquatic organisms. A 2280 megawatt coal-fired Navajo Generating Station is also located in the nearby area. Despite continuing opposition from many environmental groups, the dam, Lake Powell, and the Glen Canyon National Recreation Area remain popular tourist destinations. Continued population growth in the western and southwestern United States places more demands on the system of dams and reservoirs on the Colorado River for water, power, and recreational purposes which are important to the infrastructures and economies of the western United States.

The dam is part of the Colorado River Storage Project for the Upper Colorado Basin. It is located about 4.5 miles (7.2 km) south of the border between Utah and Arizona. According to the U.S. Bureau of Reclamation who operates the dam, "The project furnishes the long-time regulatory storage needed to permit States in the upper basin to meet their flow obligation at Lees Ferry, Arizona, (as defined in the Colorado River Compact) and still use their apportioned water." Construction of the dam began in 1956 by the industrial conglomerate, Merritt-Chapman & Scott. Although the dam was not dedicated until 1966, it was able to begin blocking the flow of the river in 1963.

Environmental opposition to dam site
The Sierra Club and other environmental organizations opposed the original plan for damming the Colorado River, including the construction farther upstream of the Echo Park Dam, which would have inundated part of Dinosaur National Monument. When the plan was modified, however, the Sierra Club dropped its objection to the Glen Canyon Dam. Its then Executive Director, David Brower, later called this decision one of the biggest mistakes of his career: "Glen Canyon died, and I was partly responsible for its needless death. Neither you nor I, nor anyone else, knew it well enough to insist that at all costs it should endure. When we began to find out, it was too late." In subsequent years the dam has continued to inspire deeply felt opposition. Eco-novelist and essayist Edward Abbey railed against the dam, and considered Glen Canyon the "living heart" of the Colorado River. In his 1975 novel, The Monkey Wrench Gang, the protagonists view the dam as an abomination, and fantasize about blowing it up. On March 21, 1981, Abbey was among the onlookers as a small group of Earth First! activists unfurled a 300-foot (91 m)-long, tapered sheet of black plastic from the top of the dam, making it look as though a gigantic crack had opened in the concrete. Since 1996, the Sierra Club has called for increasing the release of water, so that a more natural flow of the river can be restored and Lake Powell can be gradually drained. Thirty-one years after the dam's completion, Senator Barry Goldwater of Arizona, who originally supported the project, stated in an interview that he would be happier without the lake and expressed regret for voting in favor of its construction.

Important events in 1983, 1984
The winter of 1982-1983 provided a deep snowpack over much of the watershed behind the dam. This was followed by an exceptionally wet early spring. In May, there was a quick rise in temperatures. All of these factors combined to create perfect conditions for maximum runoff in the Colorado River basin. By early June, the river was rapidly filling Lake Powell. By late June, even with the powerhouse running at full flow capacity and opening bypass jet tubes (for a total flow of 44,000 cfs (1,246 m 3/s)), the sustained inflow of over 100,000 cfs (2,832 m 3/s) required using the spillways for the first time. Because this was the first test of the spillways, the spillways were shutdown after a few days of operation and were inspected for damage. This inspection showed serious damage from cavitation had already begun. Because the spillways are essential for dam safety, engineers decided to use the left spillway primarily, and keep the right spillway in reserve. The right spillway would only be used if the left spillway became too damaged to handle the required spill volume. To facilitate this mode of operation, an additional 8 feet (2.44 m) of freeboard was immediately added to the spillway gates. This modification prevented water from flowing over the top of the gates, effectively giving Lake Powell more storage capacity, and allowing control of spillway flow volume. This tactic proved effective and the flood was contained, with the high water level rising to within approximately 6 feet (1.8 m) of the dam's crest. The spillway flows were controlled to 20,000 cfs (566 m 3/s) for the left side and 4,000 cfs (113 m 3/s) for the right side. Even with the reduced flow levels, there were various indications that the spillway tunnels were being damaged by the flow, including breakdown of the "rooster tail" at the outflow works, and finally a noticeable red color present in the spillway output. After about six weeks of operation, the spillway gates were closed in early August. The tunnels were immediately inspected for damage, and the investigation confirmed that they had sustained serious damage from the high velocity flow, with the left side receiving the most damage. Typically, the tunnel lining had been seriously damaged and/or penetrated for hundreds of feet. In the left tunnel, in the area where maximum damage was sustained, 75% of the conduit's overall circumference was destroyed. This exposed the underlying soft sandstone layer to the high velocity flow and allowed significant erosion beyond the conduit's channel. This included a large hole approximately 35 feet (11 m) deep by 150 feet (46 m) in length. The followup investigation showed that the damage from cavitation could be reduced by installing an anti-cavitation system similar to the system first used on Montana's Yellowtail Dam. The repairs were completed on a short schedule to be ready in time for the 1984 runoff season. However, the spillways were not used in 1984, except for a short spillway test to verify that the design modification worked as expected. Passing this test was crucial, because a working spillway is an essential requirement for safe dam operation and it provided the necessary confirmation that the reservoir's operational rules were safe and prudent.

Structure, operations, and reservoir water allocations

The Glen Canyon Dam is 710 feet (216 m) high. The concrete arch dam has a crest length of 1,560 feet (475 m) and contains 4,901,000 cubic yards (3,747,000 m³) of concrete. The dam is 25 feet (8 m) wide at the structural crest and 300 feet (91 m) wide at the maximum base. Its height above the Colorado River is 583 feet (178 m).

The Glen Canyon hydroelectric powerplant, at the toe of the dam, consists of eight 155,500 horsepower (116.0 MW) Francis turbines. Total nameplate generating capacity for the powerplant is 1,296,000 kilowatts. Eight penstocks through the dam convey water to the turbines. The dam's hydroelectric plant generated 3,209,000,000 kilowatt-hours (11.55 PJ) of electricity in 2005. High volume flows are now periodically released to assist in re-arrangement of river beaches in the canyon, deemed necessary to prevent overgrowth of exotic plant species such as tamarisk and balance the needs of the human population with that of the environment.

Water allocation
Glen Canyon Dam releases 8.23 million acre feet of water every year. This meets the mandated 7.5MAF requirement for delivery at Lee's Ferry, per the Colorado River Compact. It also provides half of the allotment of Colorado River water that is supposed to flow to Mexico per the 1944 treaty with Mexico, with the lower basin providing the other half . Despite this prodigious amount of water being released, very little water is taken from the reservoir itself. Only the town of Page, and nearby Navajo Generating Station draw water directly from Lake Powell. By releasing the mandated amount of water downstream, this allows the upper basin to develop its water allotment (7.5MAF) without worrying about not meeting their legally required delivery minimums .

Environmental impacts
Because of the dam, there has not been the periodic flooding that would wash away and renew sand banks along the portion of the Colorado River that transits the Grand Canyon. Because of the stability of the sand banks, several non-native species of plants became established, adversely affecting the native wildlife. An environmental impact statement was completed in 1995, which concluded that some effort needed to be made to re-enact flooding events on the river. Public hearings were held in Salt Lake City, Denver, Phoenix, Flagstaff, Los Angeles, San Francisco, and Washington, D.C. More than 17,000 comments were received during the scoping period, reflecting national attention and the interest of people in the Western States. In accordance with the findings, a controlled flood was held in late March and early April 1996. At first, the controlled floods appeared to have a beneficial effect on the downstream ecosystem. New beaches were assembled for the rafting industry and many natural sandbars, upon which many plant and animal species are dependent, were partially restored. A sandbar is defined as an amount of sand remaining in the same place and having the same general shape through time while being able to flux dynamically with floods . Sandbars are usually formed in zones of low velocity that allows sediment to settle and accumulate. The are typically found along the upstream and downstream face of channels, in slow waters of an eddy center, and at still flow zones . Other less common depositional area include point bars and thin channel-margin sandbars. The most deposition for sandbars occurs during floods when alternate bars attach to the flood plain and begin to accumulate sand. The lack of high flooding episodes prohibits alternate bars form accumulating sand, which turns out to be an advantage for vegetation . The perennial, riparian vegetation has increased along the riverbanks for several reasons including the availability of free space, a more secured bank environment, and fewer large flooding disturbances. In 2006, the Bureau of Reclamation announced plans to develop another Environmental Impact Statement (EIS) on the implementation of a long-term experimental plan for operational activities at Glen Canyon Dam and other management actions on the Colorado River. The EIS continues efforts of the Glen Canyon Dam Adaptive Management Program created to protect resources downstream of Glen Canyon Dam, including the Grand Canyon, through adaptive management and experimentation. This EIS process implements the provisions of the settlement agreement recently executed between the United States and the Center for Biological Diversity and other environmental groups in the Center for Biodiversity et al. v. Kempthorne litigation regarding the operation of Glen Canyon Dam. In conformance with the National Environmental Policy Act, this EIS effort will include public involvement and scoping and will consider a range of options and evaluate their ability to address scientific understanding and resource protection objectives. Public scoping for the EIS was completed in January 2007 and a scoping report was issued in March 2007. Its implementation was in a state of suspension as of January 2009. The Bureau cites several reasons for the suspension, including litigation, extremely high levels of sedimentation from tributaries, and new findings with regard to the endangered humpback chub population. Its focus will shift to evaluation of recent experimental flows in light of the Endangered Species Act and the National Environmental Policy Act. In 1996, the Bureau of Reclamation found that 8% of the river's flow, almost 1,000,000 acre feet (1.2 × 10 9 m 3) worth hundreds of millions of dollars annually, disappears between the inflow to Lake Powell and the dam, due to a combination of evaporation and loss into the banks.