Assessing the value of dams is a complex matter. Damming can serve many of society's needs, especially those of a thriving community or region. For example, water released from reservoirs that form upstream from a dam can be used to generate electricity. By smoothing out the natural, seasonal variation in water flow, dams also provide a reliable water supply to regional populations, help reduce the risk of floods and droughts, and provide recreational opportunities.
However, there are also many reasons not to build dams. For example, the presence of a dam and the regulation of water flow greatly reduce the amount of sediment carried by the river. Such sediment creates riverside beaches and coastal wetland features like marshes that provide habitats for many animals and plants. In addition, some native fish species are unable to adapt to the unusually cold water released by the dam, or to pass these huge blockades to return to upstream spawning grounds. Upstream, the artificial lakes that dams create can destroy entire canyon ecosystems when the canyon fills with water and, ultimately, too much sediment. The pressure to preserve rivers in their natural, or wild, state has led to the decommissioning of several large dams and has thwarted plans in many states to build new dams.
In the early 1980s, population growth in Denver and the surrounding areas created a need for additional water supply. However, a proposed dam that would have enabled the city to store water for longer-term use met with opposition from local citizens, fishermen, and environmental groups. In 1990, President George H. W. Bush vetoed the plan for Two Forks Dam. As an alternative to the dam, opponents urged residents, businesses, and the city itself to conserve the water they already had.
Water-conservation initiatives like those described in this video segment have been tried successfully in Denver and elsewhere. Although the pressures of population growth and short supply continue — and are likely to continue for some time — it's clear that, dams or no dams, conservation is a necessary component of any overall water management strategy.
To learn more about the effects of dams on river ecosystems, check out The Grand Canyon: Conservation and Development.
To learn more about what people can do to reduce household water use indoors and outdoors, check out Conserving Water at Home.
To learn more about the physics, engineering, and social aspects of dams, check out Hoover Dam and The Dam Challenge.