Glen Canyon Dam
Glen Canyon Dam, 710 ft (216 m) high, 1,560 ft (475 m) long, NE Ariz., on the Colorado River. The key unit of the U.S. Bureau of Reclamation's Colorado River storage project, it is one of the world's largest concrete dams (larger in bulk, though not in height, than Hoover Dam). The dam, completed in 1963 and dedicated in 1966 after completion of its power-generation facilities, regulates the flow of the upper Colorado and its tributaries and produces hydroelectricity (since 1964). The dam sharply reduced the seasonal flow of the Colorado downstream, dramatically altering the ecology of the river in the Grand Canyon. Changes in water releases have been experimented with in an attempt to ameliorate the effects of the dam.
Lake Powell, formed by the dam, extends 186 mi (299 km) upstream into S Utah. The lake was named after the American explorer John W. Powell, who mapped and named the canyon in 1870. This lake is the nucleus of the Glen Canyon National Recreation Area (see National Parks and Monuments, table). Downstream is the Glen Canyon Bridge, 1,271 ft (387 m) long and 700 ft (213 m) high, one of the world's longest and highest steel-arch bridges.
"Glen Canyon Dam." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (June 29, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/glen-canyon-dam
"Glen Canyon Dam." The Columbia Encyclopedia, 6th ed.. . Retrieved June 29, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/glen-canyon-dam
Glen Canyon Dam
Glen Canyon Dam
Until 1963, Glen Canyon was one of the most beautiful stretches of natural scenery in the American West. The canyon had been cut over thousands of years as the Colorado River flowed over sandstone that once formed the floor of an ancient sea. The colorful walls of Glen Canyon were often compared to those of the Grand Canyon, only about 50 mi (80 km) downstream.
Humans have long seen more than beauty in the canyon, however. They have envisioned the potential value of a water reservoir that could be created by damming the Colorado. In a region where water can be as valuable as gold, plans for the construction of a giant irrigation project with water from a Glen Canyon dam go back to at least 1850.
Flood control was a second argument for the construction of such a dam. Like most western rivers, the Colorado is wild and unpredictable. When fed by melting snows and rain in the spring, its natural flow can exceed 300,000 ft4(8,400 m4) per second. At the end of a hot dry summer, flow can fall to less than 1% of that value. The river's water temperature can also fluctuate widely, by more than 36°F (20°C) in a year. A dam in Glen Canyon held the promise of moderating this variability.
By the early 1900s, yet a third argument for building the dam was proposed—the generation of hydroelectric power. Both the technology and the demand were reaching the point that power generated at the dam could be supplied to Phoenix, Los Angeles, San Diego, and other growing urban areas in the Far West.
Some objections were raised in the 1950s when construction of a Glen Canyon Dam was proposed, and environmentalists fought to protect this unique natural area. The 1950s and early 1960s were not, however, an era of high environmental sensitivity, and plans for the dam eventually were approved by the U. S. Congress. Construction of the dam, just south of the Utah-Arizona border, was completed in 1963 and the new lake it created, Lake Powell, began to develop. Seventeen years later, the lake was full holding a maximum of 27 million acre-feet of water.
The environmental changes brought about by the dam are remarkable. The river itself has changed from a muddy brown color to a clear crystal blue as the sediments it carries are deposited behind the dam in Lake Powell. Erosion of river banks downstream from the dam has lessened considerably as spring floods are brought under control. Natural beaches and sandbars, once built up by deposited sediment , are washed away. River temperatures have stabilized at an annual average of about 50°F (10°C). These physical changes have brought about changes in flora and fauna also. Four species of fish native to the Colorado have become extinct, but at least 10 species of birds are now thriving where they barely survived before. The biotic community below the dam is significantly different from what it was before construction.
During the 1980s, questions about the dam's operation began to grow. A number of observers were especially concerned about the fluctuations in flow through the dam, a pattern determined by electrical needs in distant cities. During peak periods of electrical demand, operators increase the flow of water though the dam to a maximum of 30,000 ft4 (840 m4) per second. At periods of low demand, that flow may be reduced to 1,000 ft4 (28 m4) per second. As a result of these variations, the river below the dam can change by as much as 13 ft (4 m) in height in a single 24-hour period. This variation can severely damage riverbanks and can have unsettling effects on wildlife in the area as, for example, fish are stranded on the shore or swept away from spawning grounds. River-rafting is also severely affected by changing river levels as rafters can never be sure from day to day what water conditions they may encounter.
Operation of the Glen Canyon Dam is made more complex by the fact that control is divided up among at least three different agencies in the U. S. Department of the Interior, the Bureau of Reclamation , the Fish and Wildlife Service , and the National Park Service , all with somewhat different missions. In 1982, a comprehensive re-analysis of the Glen Canyon area was initiated. A series of environmental studies called the Glen Canyon Environmental Studies were designed and carried out over much of the following decade. In addition, Interior Secretary Manuel Lujan announced in 1989 that an environmental impact statement on the downstream effects of the dam would be conducted.
The purpose of the environmental impact statement was to find out if other options were available for operating the dam that would minimize harmful effects on the environment , recreational opportunities, and Native American activities while still allowing the dam to produce sufficient levels of hydroelectric power. The effects studied included water, sediment, fish, vegetation, wildlife and habitat , endangered and other special-status species, cultural resources, air quality , recreation , hydropower, and non-use value (i.e., general appreciation of natural resources ).
Nine different operating options for the dam were considered. These options fell into three general categories: unrestricted fluctuating flows (two alternative modes); restricted fluctuating flows (four modes); and steady flows (three modes).
The final choice made was one that involves "periodic high, steady releases of short duration" that reduce the dam's performance significantly below its previous operating level. The criterion for this decision was the protection and enhancement of downstream resources while continuing to permit a certain level of flexibility in the dam's operation.
A later series of experiments was designed to see what could be done to restore certain downstream resources that have been destroyed or damaged by the dam's operation. Between March 26 and April 2, 1996, the U.S. Bureau of Reclamation released unusually large amounts of water from the dam. The intent was to reproduce the large scale flooding on the Colorado River that had normally occurred every spring before the dam was built.
The primary focus of this project was to see if downstream sandbars could be restored by the flooding. The sandbars have traditionally been used as campsites and have been a major mechanism for the removal of silt from backwater channels used by native fish. Depending on the final results of this study, the Bureau will determine what changes, if any, should be taken in adjusting flow patterns over the dam to provide for maximum environmental benefit downstream along with power output.
[David E. Newton ]
Elfring, C. "Conflict in the Grand Canyon." BioScience (November 1990):
Udall, J. R. "A Wild, Swinging River." Sierra (May 1990): 22–26.
"Glen Canyon Dam." Environmental Encyclopedia. . Encyclopedia.com. (June 29, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/glen-canyon-dam
"Glen Canyon Dam." Environmental Encyclopedia. . Retrieved June 29, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/glen-canyon-dam