Dams are designed as impenetrable barriers -- strong enough to resist the horizontal pressing force of millions, or even trillions, of gallons of water that would otherwise send the structure sliding down a river valley. Because dams restrict the flow of water downstream, they create artificial lakes upstream. Water released from these reservoirs can be used to generate electricity. Hydroelectric dams like the Glen Canyon Dam along the Colorado River supply almost one-fifth of the world's electricity by using plentiful, renewable river water to spin turbines that power generators. By smoothing out the natural, seasonal variation in water flow, these dams also provide a reliable water supply to regional populations, help reduce the risk of floods and droughts, and provide recreational opportunities, particularly in their upstream reservoirs.

By many measures, life in the desert southwest has been improved by the damming of the Colorado River. But the presence of the dam and the regulation of water flow have greatly reduced the amount of sediment in the river and the annual variation in water temperature. The large volumes of water from periodic releases tear away at the downstream beaches. The beaches erode more and more because there are no springtime floods to replenish the sands. Some native fish species are unable to adapt to the unusually cold water released by the dam, or to pass the huge blockades to return to upstream spawning grounds. Upstream, entire canyon ecosystems are destroyed under the reservoir as the canyon is filled with sediment and water. Also, the build-up of sediment in the reservoir eventually reduces the electricity-generating capacity of the dam.

Assessing the value of dams today is a complex matter. Studies designed to look closely at the effects of damming have considered the question of whether opening dams to allow more frequent controlled flooding could rejuvenate a river's ecosystem. Following a federally mandated environmental impact study completed in 1995, and with some public input, the U.S. Secretary of the Interior signed a Record of Decision that established a program to monitor and analyze the effects of dam operations on downstream resources in the Grand Canyon and to recommend adjustments intended to preserve and enhance conditions. Since that time, several hydroelectric dams across the country have been decommissioned and dismantled, restoring natural river flows, but it is still too early to gauge the long-term implications for their respective ecosystems.

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