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Wednesday, September 7, 2016

Hoover and Glen canyon dam USA

On September 30, 1935, with a crowd of 10,000 people looking on in 102-degree heat, President Franklin Roosevelt dedicated what was then the world’s biggest dam. Situated on the Nevada-Arizona border, 30 miles southeast of Las Vegas, and stretching 1,244 feet across the Black Canyon, Hoover Dam took five years to construct. Built to control flooding along the Colorado River and provide water and hydroelectric power for California and the Southwest, the dam spurred tremendous growth in that part of the United States. Learn more about this engineering marvel, including why its name once sparked controversy, what happened when it was the target of an alleged German bomb plot and if any bodies are buried in its concrete.

Glen Canyon Dam is a concrete arch-gravity dam on the Colorado River in northern Arizona, United States, near the town of Page. The 710-foot (220 m) high dam was built by the U.S. Bureau of Reclamation (USBR) from 1956 to 1966 and forms Lake Powell, one of the largest man-made reservoirs in the U.S. with a capacity of 27 million acre feet (33 km3). The dam is named for Glen Canyon, a colorful series of gorges now flooded by the reservoir; Lake Powell is named for John Wesley Powell, who in 1869 led the first expedition to traverse the Colorado's Grand Canyon by boat.
A dam in Glen Canyon was studied as early as 1924, but these plans were initially dropped in favor of a dam in Black Canyon (the Hoover Dam, completed in 1936). By the 1950s, due to rapid population growth in the seven U.S. and two Mexican states comprising the Colorado River Basin, the Bureau of Reclamation deemed necessary the construction of additional reservoirs.However, USBR faced opposition when it proposed the Echo Park Dam in Utah's Dinosaur National Monument, which the nascent environmental movement saw as a legal threat to the status of protected lands. After a long fight, USBR agreed not to build the dam in Dinosaur, but only if the environmentalists did not oppose the proposed dam in Glen Canyon.
Since first filling to capacity in 1980, Lake Powell water levels have fluctuated greatly depending on water demand and annual runoff. Operation of Glen Canyon Dam helps ensure an equitable distribution of water between the states of the Upper Colorado River Basin (Colorado, Wyoming, and most of New Mexico and Utah) and the Lower Basin (California, Nevada and most of Arizona). During years of drought, Glen Canyon guarantees a water delivery to the Lower Basin states, without the need for rationing in the Upper Basin. In wet years, it captures extra runoff for future use. The dam is also a major source of hydroelectricity, averaging over 4 billion kilowatt hours per year.The long and winding Lake Powell, known for its scenic beauty and recreational opportunities including houseboating, fishing and water-skiing, attracts millions of tourists each year to the Glen Canyon National Recreation Area.
In addition to its flooding of the scenic Glen Canyon, the dam's economic justification was highly questioned;with what critics saw as unnecessary damage to the wilderness, it became "a catalyst for the modern environmental movement," and was one of the last mega-dams to be built in the United States.Critics of the dam cite the huge  from Lake Powell and harm imposed on the ecology of the Grand Canyon, which lies downstream; environmental groups continue to advocate for the dam's removal. Water managers and utilities argue that the dam is a major source of renewable energy and provides a vital defense against severe droughts.


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. One, just 4 miles (6.4 km) upstream, was originally considered the superior site, but the final decision was to build the dam 16.5 miles (26.6 km) upstream because of stronger rock and easier access to the gravel mining area at the confluence of Wahweap Creek with the Colorado. The dam site lay in a remote, rugged area of the Colorado Plateau, more than 30 miles (48 km) from the closest paved road, U.S. Highway 89, and a new road had to be constructed, branching off from Highway 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). Much of the land acquired for the dam was through an exchange with the Navajo, in which the tribe ceded Manson Mesa south of the dam site for a similar-sized chunk of land near Aneth, Utah, which the Navajo had long coveted.
Aerial view of a river cutting a canyon through a rocky plateau.
Glen Canyon damsite from the air in November 1957, prior to construction of the Glen Canyon Bridge
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 225-mile (362 km) journey in order to get from one side of the canyon to the other. A road link was urgently needed, in order to safely accommodate workers and heavy construction equipment. The contract for building the bridge was awarded to Peter Kiewit Sons and the Judson Pacific Murphy Co. for $4 million and construction began in late 1956, reaching completion on August 11, 1957. When finished, the steel arch Glen Canyon Bridge was in itself a marvel of engineering: at 1,271 feet (387 m) long and rising 700 feet (210 m) above the river, it was the highest bridge of its kind in the United States and one of the highest in the world. By 1959, the bridge itself was a major tourist attraction and "motorists [were] driving miles out of their way just to be thrilled by its dizzying height."
Workers moved to the dam site beginning in the mid-late 1950s; the construction camp started out as a haphazardly organized trailer park that grew with the workforce. During the construction of the Glen Canyon Bridge, 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. It was intended to serve a maximum population of eight thousand, accounting for the workers' families; the peak workforce would eventually exceed 2,500 in the busiest phases of construction.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.

River diversion

Architectural plans for the Glen Canyon Dam and ancillary structures
Architectural plans for the Glen Canyon Dam and ancillary structures
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 41 feet (12 m) in diameter, with a combined capacity of 200,000 cubic feet per second (5,700 m3/s); the right-side tunnel was 2,740 feet (840 m) long and the left 2,900 feet (880 m). The right tunnel would be used for carrying the Colorado's normal flow around the dam site, while the left tunnel, 33 feet (10 m) 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 182,000 cubic yards (139,000 m3) 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 5,200 cubic yards (4,000 m3) crashing down onto the upper portal of the left diversion tunnel.
Material dug out of the tunnels and the dam abutments on the canyon walls was used to build the two cofferdams to divert the Colorado River, which were complete in February 1960. The upper cofferdam was 168 feet (51 m) high, and it alone could store several million acre-feet of water to protect the dam site from flooding in the event that inflows exceeded the capacity of the diversion tunnels. On February 11, 1959, the right diversion tunnel was completed and began to carry the flow of the Colorado. The left tunnel was finished over three months later on May 19, 1959, slightly behind schedule.