Glen Canyon Dam - Frequently Asked
Q. Which is taller, Glen Canyon Dam or Hoover Dam? Which is larger, Lake Powell or Lake Mead?
A. Hoover Dam, which stands 726 feet (221 meters) tall, is 16 feet (4.9 meters) taller than Glen Canyon Dam which stands 710 feet (216 meters) tall. When full, Lake Mead, which holds 28,537,000 acre-feet (35,200 million cubic meters) of water, is larger than Lake Powell which holds 26,215,000 acre-feet (32,336 million cubic meters) of water when full.
Q. How long will Glen Canyon Dam last?
A. The dam itself could be in place indefinitely. However, it is currently estimated that the lake will fill with silt in 700 to 1,000 years. This might be considered the effective life of the project..
Q. Was anyone buried in the concrete during construction of Glen Canyon Dam?
A. During construction of Glen Canyon Dam, 18 workers died in various kinds of accidents. All of the bodies were recovered and accounted for and no one was buried inside the dam. Because of the construction techniques used to build the dam, it would have been impossible for a worker to be buried in the concrete. The enormous quantity of concrete used in the dam could not be placed all at one time. Instead, the dam was constructed as a series of concrete blocks, some as large as 60 by 210 feet (18 by 64 meters). It took many buckets of concrete, dumped one at a time, to fill each block. When each bucket load was released, there were many workers waiting to smooth out the concrete with special equipment. Once the concrete was spread out within each block, the maximum depth of fresh concrete that a worker could have fallen into was only 12 inches (30 centimeters).
It would not have been possible for a worker to become trapped and buried in such a shallow depth of concrete.
Q. Why is it so cold inside the dam?
A. The water that flows into Lake Powell originates as snowpack high in the mountains of Colorado. This icy water flows into the lake where it is so deep it cannot be warmed by the sun. As a result, the temperature of the water deep below the lake’s surface is about 44 degrees F (7 degrees C) year-round, keeping the dam cold inside. The temperature inside the dam remains fairly constant at 50 degrees F (10 degrees C) year-round.
Q. What is the purpose of the large rock bolts in the canyon walls?
A. Hundreds of rock bolts were installed by high scalers to reinforce the canyon walls and prevent rock slabs from falling. The high scalers were suspended from long cables attached to windlasses on the rims of the canyon walls and drilled holes from 45 to 75 feet (14 to 23 meters) into the Navajo sandstone, inserted the bolts with expansion anchors, then forced concrete grout around them to secure them within the walls.
Q. How much water seeps through the dam?
A. The sandstone walls of Glen Canyon contain natural fractures that allow water from Lake Powell to seep through them and into various tunnels in the dam. About 2,600 gallons per minute (9,841 liters per minute) seep into the dam, which is not a large amount of water for a structure this size. All water that leaks into the dam is routed through measurement weirs, then diverted to troughs at the base of the dam and is discharged to the river below. Dams are designed to handle seepage; no dam or foundation for a dam is absolutely impervious to seeping water. Seepage is controlled to prevent erosion. Seepage is not only tolerable, but normal.
Q. How much water from Lake Powell is lost into the atmosphere by evaporation?
A. About two to three percent of the lake’s water evaporates into the atmosphere each year.
Q. How much water was absorbed into the rock of the canyon walls during the initial filling of Lake Powell?
A. About 13.4 million acre-feet (16,500 million cubic meters), or almost one third of the reservoir’s capacity.
Q. What are the benefits of using hydroelectric power?
A. Hydroelectric power is a clean, renewable form of energy that does not pollute the air, land, or water. Hydroelectric powerplants have low failure rates, low operating costs and are very reliable. In addition, the use of hydropower reduces our dependence on other more environmentally polluting energy sources. The hydropower plant provides system reliability for the overall regional power grid and is the primary unit to assist in a "black start" of the power system following a blackout failure. Under that scenario, hydropower facilities generate the power that is necessary to restart the system, which includes other power generating sources such as coal, natural gas, and nuclear facilities.
Q. Can the electricity produced at Glen Canyon Powerplant be stored for use at a later time?
A. One drawback to electricity is that it cannot be stored for future use. Consequently, a utility company must have the capacity to provide power instantly to meet public needs. But since power demand varies greatly during the day and night, this job becomes quite complex. Since hydroelectric generators can be started and stopped almost instantly, hydropower is more responsive than other energy sources in meeting changing power needs.
Q. How many kilowatt-hours of power does Glen Canyon Dam's powerplant produce annually?
A. Approximately 4.5 billion kilowatt-hours.
Q. How much coal would it take to produce that same amount of power?
A. It would take 2.5 million tons of coal or 11 million barrels of oil each year to generate the same amount of power (based upon an approximate conversion rate of 580 kilowatt-hours per barrel of oil and 1,822 kilowatt-hours per ton of coal).
Q. Why does the lake look so blue and the river below the dam look so green?
A.Lake Powell’s deep blue color results from the fact that it is a relatively clear body of water that reflects the color of the sky. The river below the dam looks green due to the feathery algae called cladophora that thrives in the river. This algae forms the basis of a highly productive food chain and is an important source of nutrients for many species living below the dam.
Q. What is the large crane on top of the dam used for?
A. The main purpose of the gantry crane is to periodically withdraw the penstock fixed-wheel gates from beneath Lake Powell’s surface and place them on the dam crest where they can be inspected. The large crane can lift 165 tons (162 metric tons) with its big hoist and 25 tons (24.6 metric tons) with its smaller one.
Q. How do vehicles get to the bottom of the dam?
A. There is a two mile (3.2-kilometer) dam-access tunnel through the east wall of Glen Canyon that begins on the mesa. The tunnel was blasted prior to construction of Glen Canyon Dam in order to transport heavy equipment and supplies to the canyon floor. The holes in the wall are adits or “windows” that were initially blasted out for the purpose of allowing excavated rock to be dumped to the river banks below. The adits now allow for circulation of air in the tunnel. The dam-access tunnel is not open to the public.
Q. How is the Colorado River controlled?
A. The Colorado River is administratively controlled by numerous statutes, compacts, decrees, and a treaty with Mexico, collectively referred to as the “Law of the River.” In 1970, the Criteria for Coordinated Long-Range Operation of Colorado River Reservoirs was prepared in accordance with these formal regulations to form the rules by which the Colorado River reservoir system is operated. An operating plan is prepared each year to guide the operations of the Colorado River reservoirs.
Q. What percent of the nation’s food is grown with Colorado River water?
A. About 25 percent of the nation’s food is grown on approximately two million acres of land irrigated by Colorado River water.
Q. Are there any federally-listed endangered species living in the Grand Canyon below Glen Canyon Dam?
A. Yes. Federal endangered species living in the Grand Canyon below the dam include the humpback chub, razorback sucker, southwestern willow flycatcher and the Kanab ambersnail.
Q. Will Glen Canyon Dam be de-commissioned and Lake Powell drained?
A. There are no plans to de-commission Glen Canyon Dam and drain Lake Powell. Congress created Glen Canyon Dam and only Congress can remove it. Lake Powell provides vital long-term carry-over storage that is critical to the Upper Colorado River Basin states, especially in times of drought. Lake Powell helps sustain continued economic development in the rapidly growing Southwest by providing water for municipal and industrial use, agriculture, recreation, generation of electricity, and environmental needs. For these reasons, each year through appropriation legislation, Congress repeatedly says “no” to the proposal to drain Lake Powell.