Glen Canyon Dam, a concrete arch dam on the Colorado River in the American state of Arizona, is viewed as carrying a large amount of risk, most notably due to siltation. The Colorado and San Juan rivers deposit large volumes of silt into Lake Powell, slowly decreasing its capacity. The sediment will eventually build up against the dam and could affect its safe operation and lead to its failure. The dam, anchored in unstable Navajo sandstone (sometimes said to be "solidified sand dunes"), nearly failed in 1983 as the result of a flood on the upper Colorado River that led to extended use of its tunnel spillways. The spillways, designed for short-term use, soon underwent cavitation and began to fail. Emergency installation of flashboards and other efforts narrowly averted total failure. Siltation, concrete degradation, spillway operational problems, and unstable dam abutments are all key factors that affect the safe operation of the dam. It is estimated that a breach of the dam would produce a floodwave that would overtop the Hoover Dam. The dam's useful lifespan has been estimated by some to be 85 to 100 years, and was described as "America's most regretted environmental mistake" by David Brower, then-head of the Sierra Club. Different estimates by the Bureau of Reclamation and others suggest a lifespan of between 500–700 years. Overview Glen Canyon Dam is a concrete arch dam on the Colorado River in northern Arizona in the United States, just north of Page. The dam was built to provide hydroelectricity and regulate the flow of the river from the upper Colorado River Basin into the lower. The Lake Powell reservoir is the second largest artificial lake in the country, extending upriver well into Utah. The dam is named for Glen Canyon, a colorful series of gorges, most of which now are buried under the reservoir. Construction of Glen Canyon Dam began in 1956 and was finished in 1966. When the reservoir filled, the dam began to deliver a steady, regulated flow of water downstream and generate a cheap, plentiful supply of electricity. In 1983, major floods nearly destroyed the two tunnel spillways that could have led to the dam's collapse, but disaster was averted by a close margin. By taming floods that were characteristic of the Colorado River, the dam has led to major physical and ecological changes in the lower river. Controversy continues over the positive and negative