Glen Canyon Dam

The need for a dam The Colorado River is the single largest source of water in the southwestern United States and northwest Mexico; before massive dam projects tamed the river in the 20th century, its flow was far from dependable. Annual discharge from the Colorado River and its tributaries ranges from , and 10-year averages may fluctuate as much as . After this catastrophe, California and Arizona began to call for a dam to control the tempestuous river. In 1922, six U.S. states signed the Colorado River Compact to officially allocate the flow of the Colorado River and its tributaries. Each half of the Colorado River Basin—the Upper Basin, comprising Colorado, New Mexico, Utah and Wyoming—and the Lower Basin, with California and Nevada—was allotted of water annually, and Treaty relating to the utilization of waters of the Colorado and Tijuana Rivers and of the Rio Grande was signed in 1944 allocating to Mexico. The third lower basin state, Arizona, did not ratify the Compact until 1944 because it was concerned that California might seek to appropriate a portion of its share before it could be put to use. The total, , was based on only thirty years of streamflow records starting in the late 1890s. It was believed to represent the annual flow as measured at Lee's Ferry, Arizona (the official dividing point of the upper and lower basins), downstream of present-day Glen Canyon Dam. As it turned out, the early 20th century was one of the wettest periods in the last 800 years. The dependable natural flow past Lees Ferry is now believed to be about . Possible locations for this dam were debated for years, and the Bureau of Reclamation's first study for a dam at Glen Canyon was made in 1924, in addition to studies for locations at Black and Boulder Canyons lower on the Colorado, below Grand Canyon. These studies found that the lower Colorado sites had stronger foundation rock which might result in less reservoir seepage. The Glen Canyon site was so remote that delivering supplies and transporting workers there would be infeasible at the time. The first Glen Canyon proposal lay upstream of the Lee's Ferry dividing line, and would be considered the Upper Basin's water. With its substantial Congressional clout, California refused to allow the "virtual faucets" of a Colorado River dam "to be built in what amounted to hostile territory." With the Glen Canyon site out of the question, the initial need for a reservoir was realized in 1936 with the completion of Hoover Dam in Black Canyon, storing in the mammoth reservoir of Lake Mead. It was not able to weather the worst floods or droughts, and was filling with sediment at a rate that would render it useless in a few hundred years. But most importantly, Hoover only controlled the lower portion of the river. The Upper Basin states, whose rivers remained undammed, had no way to ensure they could fulfill their delivery obligation to the Lower Basin states while retaining enough water for their own use. Without storage reservoirs of their own, the Upper Basin states risked a "call" on the Colorado River during drought years: they would be forced to use less water in order to keep the river flowing to Lake Mead and California, the state with the most senior water rights. Colorado River Storage Project To provide water for the Upper Basin and ensure delivery to the Lower Basin, the Bureau of Reclamation proposed the Colorado River Storage Project, which would consist of a dam on the Colorado River at Glen Canyon, several dams on the Gunnison River and San Juan River, and a pair of dams to be built on the Green River, the Colorado's major upper tributary, at Echo Park and Split Mountain. The 1956 Colorado River Storage Project Act authorized the purposes of "regulating the flow of the Colorado River, storing water for beneficial consumptive use, providing for reclamation of arid and semi-arid lands, providing flood control, and generating hydropower." In 1963, Arizona's congressional delegation proposed these dams as part of the Central Arizona Project to accomplish these goals. The state of California opposed the project, as it would eliminate the "surplus" water in the Colorado (really the Upper Basin's yet unused supplies) it had gotten accustomed to using. The Bureau of Reclamation recognized a more serious problem. Construction of the Storage Project, and allowing the Upper Basin to develop its water supplies, would tip the whole Colorado River system toward a structural water deficit since the Colorado River's average flow is less than what was apportioned in the 1922 Compact. To make up for this deficit, the USBR incorporated these proposals with the "Pacific Southwest Water Plan" on January 21, 1964, in which power sales from Glen, Marble and Bridge (often called "cash register dams") would be used to fund a diversion of water from the wetter Pacific Northwest to the Colorado Basin. Environmental concerns (left) joining the Green River. The USBR proposed damming the Green a short distance downstream, behind the large cliff (Steamboat Rock) at right. The Echo Park dam would be inside the federally protected Dinosaur National Monument and would submerge of scenic canyons – a move that alarmed environmentalists. The environmental organization Sierra Club, led by David Brower, was the most vocal opponent of Echo Park Dam, and fought a protracted battle against the Bureau of Reclamation, on the basis that "building the dam would not only destroy a unique wilderness area, but would set a terrible precedent for exploiting resources in America's national parks and monuments". The Bureau of Reclamation favored the Echo Park site over Glen Canyon, because its narrow canyons and high elevation (more than , as compared to at Glen Canyon) would lead to less evaporation. It said that building Echo Park Dam and a "low" Glen Canyon Dam would save of water per year over a "high" Glen Canyon Dam (which was ultimately the version to be built). While studying the figures, Brower discovered that the difference should be no more than . Although it is unclear whether the discrepancy was due to a miscalculation or intentional manipulation, Brower said "it would be a great mistake [to rely on the Bureau's figures] when they cannot add, subtract, multiply and divide." Echo Park was considered a victory for the American environmental movement, but it only happened in exchange for a dam upstream at Flaming Gorge, and increasing the size of the proposed dam at Glen Canyon to replace the storage that would have been provided by Echo Park. A common misconception is that the environmentalists were given a choice between damming Echo Park and damming Glen Canyon, but the USBR "had always planned to build a dam at Glen Canyon, regardless of the outcome of the Echo Park debate". Floyd Dominy, commissioner of the Bureau of Reclamation, was a vital figure in pushing the project through Congress and convincing politicians to take a pro-dam stance, and to assuage rising public concerns. Dominy realized that the USBR had considerable political clout in Western states, due to the economic contributions of its water projects. Reisner wrote that "Dominy cultivated Congress as if he were tending prize-winning orchids  ... If some Senator was causing him trouble, money for his project could disappear mighty fast." With the necessary political support secured, the Colorado River Storage Project was authorized in April 1956, and groundbreaking of Glen Canyon Dam began in October of the same year. David Brower visited Glen Canyon shortly after the decision to build the dam, and "realized once he arrived that this was not a place for a reservoir". The Colorado River flowed gently across the bottom of the canyon, in sharp contrast to the roaring rapids upstream in Cataract Canyon and downstream in the Grand Canyon. After his groundbreaking 1869 expedition, John Wesley Powell had named Glen Canyon for its characteristics: "So we have a curious ensemble of wonderful features – carved walls, royal arches, glens, alcove gulches, mounds and monuments. From which of these features shall we select a name? We decide to call it Glen Canyon." In addition to its variegated rock formations, Glen Canyon supported a rich riparian zone habitat on the numerous low river terraces formed by the Colorado River, with as many as 316 bird species, 79 plant species and 34 kinds of mammals. In 1963, when construction on the dam was well underway, the Sierra Club published a book on Glen Canyon, The Place No One Knew, featuring photographs by Eliot Porter, and lamenting the loss of the canyon before most of the American public had a chance to visit, or were even aware of its existence. Though little known to most Americans before Porter's book, Glen Canyon had been visited by a handful of hikers and boaters (such as Powell's expedition), and some had even been interviewed by Brower. As said to Brower by writer Wallace Stegner, who had been to the canyon in 1947, "Echo doesn't hold a candle to Glen." Emboldened by Echo Park and desperate to prevent the Grand Canyon from reaching the same fate as Glen, Brower and the Sierra Club directed attention towards the proposed Bridge and Marble dams. The Sierra Club launched an extensive publicity campaign to sway public opinion against the plan; in response to the USBR's argument that new reservoirs would open up the Grand Canyon to recreational boaters as Lake Powell had, a full-page advertisement in the New York Times ran the slogan: "Should we also flood the Sistine Chapel so tourists can float nearer the ceiling?" Faced with public outcry, the Bureau abandoned its Grand Canyon dams, effectively terminating most of the Pacific Southwest Water Plan, in 1968. The coal-fired Navajo Generating Station was built near Page, to make up for the electric power that was lost with the cancellation of the dam project. The Sierra Club lost its IRS tax-exempt status a day after the advertisement was released due to its disruptive political activities. ==Construction==

Site preparations As early as 1947, the Bureau of Reclamation had begun investigating two potential sites, both located in the narrow lower reaches of Glen Canyon shortly upstream of Lee's Ferry. The site originally favored by the USBR was just upstream, but the final decision was to build the dam upstream because of stronger foundation rock and easier access to gravel deposits on Wahweap Creek. Because the dam site lay in a remote, rugged area of the Colorado Plateau – more than from the closest paved road, U.S. Route 89 – a new road had to be constructed, branching off from US 89 north of Flagstaff, Arizona, and running through the dam site to its terminus at Kanab, Utah. Because of the isolated location, acquiring the land at the dam and reservoir sites was not particularly difficult, but there were a few disputes with ranchers and miners in the area (many of the Navajo Nation). In the early stages of construction, the only way to cross Glen Canyon was a suspension footbridge made of chicken wire and metal grates. Vehicles had to make a journey in order to get from one side of the canyon to the other. When finished, the steel arch Glen Canyon Bridge was itself a marvel of engineering: at long and rising above the river, it was the highest bridge of its kind in the United States and one of the highest in the world. The bridge soon became a major tourist attraction. The March 1959 issue of LIFE reported that "motorists [were] driving miles out of their way just to be thrilled by its dizzying height." During the construction of the Glen Canyon Bridge, the USBR also began planning a company town to house the workers. This resulted in the town of Page, Arizona, named for former Reclamation Commissioner John C. Page. By 1959, Page had a host of temporary buildings, electricity, and a small school serving workers' children. As the city grew, it gathered additional features, including numerous stores, a hospital, and even a jeweler. The engineer in charge of the project would be Lem F. Wylie, who had worked on Hoover Dam and had previously designed six other USBR dams. Prior to and during construction, three separate grants were issued by the National Park Service to document and recover artifacts of historical cultures along the river. These went to University of Utah historian C. Gregory Crampton and anthropologist Jesse Jennings, and to the Museum of Northern Arizona. Crampton subsequently wrote several books and articles on his findings. The Museum of Northern Arizona funded an expedition by William Miller and Helmut Abt, in coordination with the Navajo Nation, to investigate historical artifacts. They discovered a petroglyph in the upper part of the canyon depicting the appearance of the Crab Nebula in 1054. River diversion In 1956, work began on the two diversion tunnels that would carry the Colorado River around the dam site during construction. Each of the tunnels was in diameter, with a combined capacity of ; the right-side tunnel was long and the left . The right tunnel would be used for carrying the Colorado's normal flow around the dam site, while the left tunnel, above the water, would only be used during floods. The lower reaches of the tunnels would later be used to form the lower ends of the dam's spillways. About of material would have to be excavated from the diversion tunnels. On October 15, 1956, President Dwight D. Eisenhower pressed a button on his desk in Washington, D.C., sending a telegraph signal that set off the first blast of dynamite at the portal of the right diversion tunnel. Drilling the tunnels through the porous Navajo sandstone abutting the dam site posed major problems for the excavation crews of the Mountain States Construction Company, which won the contract for the diversion tunnels in 1956. Transporting workers and equipment to the bottom of the canyon was extremely difficult. Initially, transport was done by barge from Wahweap Creek, but the fast current of the Colorado River could be dangerous. After a barge capsized, spilling tons of machinery into the river, a much safer cable-car system was installed. During excavation, the rock frequently broke apart or "slabbed" and collapsed into the tunnels, and metal bolts had to be drilled into the rock to secure it. The largest such event, on August 5, 1958, sent crashing down onto the upper portal of the left diversion tunnel. Then, right before construction began, about 750 workers organized a strike because of a wage reduction due to the completion of public facilities at Page. In December 1959, wages were raised by $4 a day, quelling the strikers. Concrete placement started on June 16, 1960, and started at a sluggish but growing pace. In 1962 the workforce topped out at nearly 2,500 employees laboring on the dam. Construction would ultimately claim eighteen lives and injure numerous other workers, but contrary to popular myth, no workers were buried alive in the concrete. Cement needed to make concrete for the dam came from the Phoenix Cement Company plant constructed for the purpose in Clarkdale, south of Flagstaff. A huge concrete plant capable of putting out 1,450 tons per hour was installed, and a pair of cableways with movable towers (with capacities of 50 and 25 tons respectively) spanned the canyon, carrying the concrete buckets to their final destinations on the steadily rising crest of the dam. The concrete was poured into modular high wooden blocks or "forms", the largest measuring up to by ; At the beginning of 1963, the dam was high enough to begin impounding water; huge steel gates were closed over the right diversion tunnel on January 21, and Lake Powell began to rise. A minimal flow of was allowed through the dam, to prevent the Colorado River from drying up completely. Filling Lake Powell With a capacity equal to almost two years' annual flow of the Colorado River, engineers were aware that Lake Powell would be difficult to fill, but more problems were encountered than expected. The original plan was to fill Lake Powell to above sea level, the minimum level necessary to generate hydroelectric power by late 1964, after which water would be released down to Lake Mead, with only the excess stored in Lake Powell. The spring runoff in 1963 was the lowest on record in ten years. By the beginning of 1964, Lake Powell had barely reached half the target level, and Lake Mead had seen a sharp decline. In March, Secretary of the Interior Stewart Udall ordered the filling halted and extra releases made to Lake Mead, to the consternation of the Upper Basin states. In May, Udall changed his mind yet again to lower releases, gambling that the spring runoff would be enough to raise Powell to minimum power pool by autumn, by which time power releases could begin, to prevent Lake Mead from falling below its minimum power pool. That gamble paid off, with Lake Powell barely inching over the mark on August 16, 1964. It took more than 17 years for Lake Powell to finally reach its full elevation of above sea level, which it crossed on June 22, 1980. One of the main reasons for this slow rise, in addition to the need to meet obligations to the Lower Basin, was the leakage of vast amounts of water into the porous Navajo Sandstone aquifer. Between 1963 and 1969, as much as leaked into the reservoir banks each year. The Bureau of Reclamation projected that once Lake Powell filled, the total bank storage would stabilize at approximately , and henceforth would fluctuate depending on water levels in the reservoir. The actual loss was , twice the initial prediction, but river flow data indicates that further leakage after 1980 has been negligible. According to a 2013 study by hydrologist Thomas Myers for the Glen Canyon Institute, the reservoir continues to lose about each year due to leakage. According to USBR data for water year 2015 (a year when Lake Powell did not experience a significant overall gain or loss in volume), Lake Powell lost a total of to evaporation and only to leakage.{{cite web ==Later history==

1983 floods During the El Niño winter of 1982–1983, the Bureau of Reclamation predicted an average runoff for the Colorado River basin based on snowpack measurements in the Rocky Mountains. Snowfall during April and May was exceptionally heavy; this combined with a sudden rise in temperatures and unusual rainstorms in June to produce major flooding across the western United States. With Lake Powell nearly full, the USBR did not have enough time to draw down the reservoir to accommodate extra runoff. By mid-June, water was pouring into Lake Powell at over . Even with the power plant and river outlet works running at full capacity, Lake Powell continued to rise to the point where the spillways had to be opened. Other than a brief test in 1980, this was the only time the spillways had ever been used. At the beginning of June, dam operators opened the gates on the left spillway, sending , less than one-tenth of capacity, down the tunnel into the river below. After a few days, the entire dam suddenly began to shake violently. The spillway was closed down for inspections and workers discovered that the flow of water was causing cavitation—the explosive collapse of vacuum pockets in water moving at high speed—which was damaging the concrete lining and eroding the rock spillway tunnels from the upper ends of the diversion tunnels, which connect to the bottom of the reservoir. This was rapidly being destroyed by the cavitation and it was feared that a connection would be made to the bottom of Lake Powell, compromising the dam's foundation and causing the dam to fail. Even this additional capacity was exhausted; discharges through the left spillway reached , and the right spillway was opened to . At Lee's Ferry, the Colorado River peaked at , which was and still is the highest water flow recorded there since the dam was built. On July 14, Lake Powell reached elevation, a level that has not been exceeded since. Just as it seemed inevitable that the dam would fail, inflows fell and the dam was saved. Upon inspection, it was found that cavitation had caused massive gouging damage to both spillways, carrying away thousands of tons of concrete, steel rebar and huge chunks of rock. Repairs to the spillways commenced as soon as possible and continued well into 1984. Air slots were installed at the bottom of each spillway to break up and absorb the shock of the bubbles formed by cavitation. In 1984, the Colorado River basin produced even more runoff than 1983, peaking at in early June. This time, the USBR had drawn down the reservoir enough that it absorbed most of the early high flows. Nevertheless, Lake Powell rapidly approached the top of the spillway gates and construction efforts were subsequently focused on the left spillway in order to get it in operation in time. On August 12, the left spillway gates were opened, releasing water at a rate of . The spillway was undamaged, proving the worth of the re-engineering and suggesting that Glen Canyon Dam will also be able to hold against future floods with the magnitude of 1983. Continuing debates Long after the Glen Canyon Dam was built and continuing to the present day, controversy remains between supporters of dam removal and those who believe it should be left in place. One of the earliest debates regarding the dam was its impact on Rainbow Bridge National Monument, whose high natural arch is the highest in North America, and is a sacred site to the Navajo people. The environmental lobby wanted the Bureau of Reclamation to keep Lake Powell at or below a level of , to prevent it from intruding into the monument. The Bureau of Reclamation proposed to build a barrier dam and pump system in order to keep water out of the monument. With the potential damage that would be caused to the remote environment, "the cure would be far worse than the disease." The proposal was fought over and litigated for years until it was permanently shelved in 1973. Glen Canyon Dam became the subject of influential literature, including Edward Abbey's novel The Monkey Wrench Gang (1975), which tells the story of a fictional group of environmentalists fighting against industrial developers in the American Southwest, their ultimate target being Glen Canyon Dam. The novel gained a cult following after its publication and established Glen Canyon Dam as a poster child of environmental destruction caused by dams. Abbey's book is discussed in Ecospeak: Rhetoric and Environmental Politics in America (1992) by Jimmie Killingsworth and Jacqueline Palmer, who write that Glen Canyon Dam became "the big symbol of all that blocked freedom in the interests of civilized progress." On March 21, 1981, the radical environmental group Earth First! staged an anti-dam protest by unfurling a tapered black sheet of plastic down the face of the dam, making it appear as if a gigantic crack had appeared in the structure – a direct re-enactment of a scene from Abbey's book. Authorities were unable to find the individuals responsible. In his comprehensive history of western water development, Cadillac Desert (1986), Marc Reisner criticized the political forces that resulted in Glen Canyon and hundreds of other dams being built in the 1960s and 1970s. Many of these projects had dubious economic justifications and hidden environmental costs, but the government agencies that built them – namely the Bureau of Reclamation and the U.S. Army Corps of Engineers – were more interested in maintaining their size and influence. Reisner writes that "in the West, it is said, water flows uphill towards money." In a 2011 interview, Floyd Dominy, the Reclamation Commissioner who had spearheaded the Colorado River Storage Project, maintained USBR's stance on the benefits of the dam project. Although Lake Powell loses water to evaporation and leakage, it continues to serve an important function capturing runoff during wet years, as "insurance" for droughts. During the 2000–2004 Colorado River drought, when the basin experienced its lowest five-year runoff on record, Lake Mead would likely have gone dry and the Lower Basin experienced massive cuts, were it not for releases from Lake Powell. Lake Powell and Lake Mead are currently operated under an "equalization" policy that governs releases from Glen Canyon Dam. In order to maintain hydropower generation at both Glen Canyon and Hoover Dams, the lakes must be kept at approximately the same level. By spreading out the water, evaporation is greatly increased. Since the year 2000, Lake Mead has steadily declined toward the critical level at which a shortage would be declared for the Lower Basin states. A plan called "Fill Mead First", which would drain Lake Powell in order to refill Lake Mead, has gained traction in recent years. Glen Canyon Dam would remain in place (as total removal of the structure would be prohibitively expensive), but would only store water in wet seasons when runoff exceeds the capacity of Lake Mead to hold it. Much of the opposition to this plan is along political lines: Lake Powell is legally considered the Upper Basin's water, and Lake Mead belongs to the Lower Basin. The Friends of Lake Powell have called this an attempt to steal water from the Upper Basin, to avoid a shortage in the Lower Basin. The Upper Basin has released 107% of its obligation from Lake Powell since 2000; therefore, falling levels in Lake Mead are a result of water overuse and waste in the Lower Basin states – a "structural deficit". There are also arguments for storing water in Powell: Lake Mead, with its much lower elevation and hotter climate, has a considerably greater evaporation rate than Lake Powell. In addition, a 1983 study by Larry J. Paulson of the University of Nevada showed that the cold water discharge from Glen Canyon Dam has led to a significant reduction of the water temperature, and thus evaporation, from Lake Mead. ==Design==

Dam and spillways and the electric switchyard to the right of the bridge. Glen Canyon's overall design was based on that of Hoover Dam – a massive concrete arch-gravity structure anchored in solid bedrock – with several significant changes. The engineers wanted the dam to rely predominantly on its arch shape to carry the tremendous pressure of the impounded water into the canyon walls instead of depending on the sheer weight of the structure to hold the reservoir back, as had been done at Hoover. The foundation rock at Glen Canyon consists of porous sandstone prone to spalling, in contrast to the stronger granite at the Hoover Dam site, forcing the Glen Canyon design to follow more conservative lines by greatly thickening the abutments, thus increasing the surface area through which the weight of dam and reservoir would be transmitted to the rock and relieving the pressure on the highly breakable cliffs. The two spillway tunnels are excavated through the canyon walls on each side of the dam. Twin radial gates, each wide and high, control the flow of water into the spillways. Together, the spillways can pass up to . Water storage and distribution With a capacity of , When Glen Canyon Dam was first built, the reservoir capacity was estimated at , but some of this has since been lost to siltation. Glen Canyon Dam's most vital purpose is to provide storage to ensure enough water flows from the Upper Colorado River Basin to the lower, especially in drought years. The 1922 Colorado River Compact requires annual delivery of to the Lower Basin states of Arizona, California and Nevada; the 1944 treaty with Mexico obligates the U.S. to allow at least for use in the Mexican states of Baja California and Sonora. Glen Canyon Dam must supply at least of this water; the remaining comes from other tributaries of the Colorado River. The required release from Glen Canyon is averaged over a 10-year period, so releases in each year may be higher or lower depending on the amount of runoff. In wetter years, the Bureau of Reclamation may decide to release extra water from Glen Canyon Dam if the level of Lake Powell exceeds the "equalization tier", an elevation determined by the difference in storage between Lake Powell and Lake Mead. Most of Lake Powell's inflow originates as summer snowmelt from the Rocky Mountains of Colorado, Utah and Wyoming. Releases are made over a water year of October 1–September 30, since the annual snowpack begins to accumulate in late autumn. On April 1 of each year, the Bureau of Reclamation releases its official forecast of the April–July (snowmelt season) runoff, and adjusts releases from Glen Canyon Dam accordingly to maintain Lake Powell at a safe level. An accurate forecast is vital to prevent uncontrolled spilling, which would waste water that could have been used for power generation. Although the snowpack typically reaches its peak and begins to melt in April, the picture can occasionally change unexpectedly and dramatically – either due to a hot and dry spring that evaporates snow before it can melt, or an extremely wet spring as occurred in May 1983. After the near disaster in 1983, the USBR has maintained a minimum of of flood-storage space in Lake Powell at the beginning of each year, to guard against unanticipated high runoff. 21st century drought Colorado River flows have been below average since 2000 as a result of the southwestern North American megadrought, leading to lower lake levels. In winter 2005 (before the spring run-off) the lake reached its then-lowest level since filling, an elevation of above sea level, which was approximately below full pool. After 2005, the lake level slowly rebounded, although it has not filled completely since then. Summer 2011 saw the third largest June and the second largest July runoff since the closure of Glen Canyon Dam, and the water level peaked at nearly , 77 percent of capacity, on July 30. and by April 22, 2022 Lake Powell was at in elevation – just of capacity. This marks the lowest water level for Lake Powell since it was filled in 1963. Peer-reviewed studies indicate that storing water in Lake Mead rather than in Lake Powell would yield a savings of 300,000 acre feet of water or more per year, leading to calls by environmentalists to drain Lake Powell and restore Glen Canyon to its natural, free-flowing state. Power generation removed The other principal goal of Glen Canyon Dam is hydroelectricity generation. It is the second-biggest producer of hydroelectric power in the Southwestern United States, after Hoover Dam. Revenues derived from power sales were integral in paying off the bonds used to build the dam and have also been used to fund other Bureau of Reclamation projects, including environmental restoration programs in the Grand Canyon and elsewhere along the Colorado River. For this reason, it has long been known as a "cash register" dam. The dam also serves as a primary peaking power plant and black start power source for the Southwest electrical grid. The power plant has a total capacity of 1,320 megawatts from eight 165,000 kilowatt generators. Each generator is driven by a 254,000 horsepower vertical-axis Francis turbine. The gross hydraulic head is . In 1990 temporary restrictions were put in place on dam operations, before the release of a final environmental impact statement (EIS). The minimum dam release was set to during the day and at night. Flood control releases are allowed to go higher, but must remain constant for the entire month. Because these criteria limit the flexibility of Glen Canyon Dam to meet grid demands, economic losses for the period 1997–2005 were estimated at $38 million to $58 million per year. Between 1980 and 2013, Glen Canyon Dam generated an average of 4,717 gigawatt hours (GWh) per year, enough for about 400,000 homes. The highest was 8,703 GWh in 1984, and the lowest was 3,299 GWh in 2005. Drought conditions in the 21st century have reduced the amount of hydropower available from Glen Canyon Dam. An unusual feature of the Glen Canyon power plant is the Kentucky bluegrass ==Environmental issues==

Because of its tremendous ecological effect on the Colorado River, the Glen Canyon Dam has been subject to decades of criticism from the environmental movement. Being located in a high desert climate amid porous geology, Lake Powell causes huge evaporation and seepage losses. The Glen Canyon Institute estimates that is lost from the reservoir in an average year. This amounts to 6 percent of the Colorado River's flow, an increasingly valuable amount of water in an arid land for both humans and the animals and plants that live along the river. Because of the dam, sediment deposited by the Colorado and its tributaries is slowly filling up the canyon, and projections put the useful life of the reservoir at 300 to 700 years. Before the dam was built, Colorado River temperatures ranged from over in the heat of summer to just above freezing in winter. Today, water released by Glen Canyon is a consistent throughout the year due to a thermal mass effect in Lake Powell. The water typically released from hundreds of feet below the lake surface through the penstocks is insulated from temperature fluctuations by the thick layer of water above it. Nikolai Ramsey of the Grand Canyon Trust describes the clearer, colder river as a "death zone for native fish", such as the endemic Colorado pikeminnow and humpback chub, which are adapted to survive in warm, silty water. The river water temperature in 1983 was significantly higher than normal, due to a large portion of the water having come from overflows of warmer surface water over the spillways of Glen Canyon Dam, rather than the colder lower levels which feed the penstocks. Because Glen Canyon Dam made possible an increased utilization of water from the Colorado River system, not enough water is left to flow to the delta in a normal year, and about of ecologically productive wetlands have disappeared. In 2014 an intentional "pulse flow" was released into the delta to restore some of these wetlands; the viability of such flows have been controversial, considering the already high demand for Colorado River water. ==Restoration efforts==

On March 26, 1996, the penstocks and two of the outlet works' bypass tubes at Glen Canyon Dam were opened to maximum capacity, causing a flood of to move down the Colorado River. This was the first of the Glen Canyon Adaptive Management Program "high flow experiments", a controlled effort to assist the recovery of the damaged riverine ecosystem by mimicking the floods that once swept through the canyons each spring. The flow appeared to have scoured clean numerous pockets of encroaching vegetation, carried away rockslides that had become dangerous to boaters, and rearranged sand and gravel bars along the river, and was initially believed to be an environmental success. However, in the following months it was discovered that the initial results were misleading. Subsequent releases in 2004, 2008, and 2014 were timed to take advantage of summer monsoon storms, and redistribute sediment carried into the Grand Canyon by the Paria and Little Colorado Rivers. The high-flow experiments do not change the total amount of water outflow from Lake Powell on an annual basis, but as a consequence hydro-electric power releases during the rest of the year must be reduced. Some organizations, such as Living Rivers, continue to believe that the dam has too large and severe of an effect on the river's ecology to make restoration efforts worthwhile. ==Recreation==

According to the U.S. National Park Service, Lake Powell is "widely recognized by boating enthusiasts as one of the premier water-based recreation destinations in the world." Despite its remote location, the Glen Canyon National Recreation Area, which surrounds the reservoir, receives more than three million visitors annually. Activities include boating, fishing, waterskiing, jet-skiing, swimming and hiking. Prepared campgrounds can be found at each marina, but many visitors choose to rent a houseboat or bring their own camping equipment, find a private spot somewhere in the canyons, and make their own camp (there are no restrictions on where visitors can stay). Aside from the bridges at either end of the lake, a car-and-passenger ferry between Halls Crossing and Bullfrog is the only way for vehicles to cross Lake Powell. More than 500,000 people tour the Carl Hayden Visitor Center at Glen Canyon Dam each year. The Bureau of Reclamation provides guided tours of the dam; stringent security measures have been in place since the September 11 attacks. The base of the dam can also be reached via boat from Lee's Ferry. Because of the cold, clear water released from Lake Powell, the stretch of the Colorado River between Glen Canyon Dam and Lee's Ferry has become an excellent rainbow trout fishery. Trout are not native to the Colorado River system; they were stocked in the river below Glen Canyon Dam after the dam was built. Other non-native fish such as smallmouth bass, striped bass, largemouth bass and black crappie were planted in Lake Powell to provide sport fishing opportunities. Like many U.S. lakes and reservoirs, Lake Powell has an active problem with zebra and quagga mussels, invasive bivalve species originating in eastern Europe. Mussels are most commonly transferred from lake to lake attached to the hulls, and inside the bilge area of boats. Lake users are required by law to clean, drain and dry their vessels, both before and after taking a trip to Lake Powell. Mussel infestations tend to clog the hydroelectric intakes at the Glen Canyon Dam, as well as the propellers and exhaust pipes of boats, requiring expensive de-contamination. Their impact on the lake ecology appears to be low, or even beneficial due to their providing a food source for fish. ==See also==