The first time I laid eyes on Glen Canyon Dam (image below, looking downstream) was in the summer of 1969, during my migration from northern Montana to southern Arizona. It happens that U.S. Route 89 crosses an arch bridge just on the downstream side of the dam. Even to my naive eyes, untutored in the ecology and politics of the Southwest, this 710 foot tall concrete arc looked jarringly out of place in the redrock canyonlands of southern Utah and northern Arizona.
The dam was the object of controversy from its inception. Supporters pointed to the hydroelectric power it would provide, to the impoundment of water for agricultural irrigation, and to flood control, especially during Spring runoff. But environmental groups strongly opposed it, on two fronts -- upstream, it would flood miles of precious and beautiful canyons, ultimately creating a reservoir (Lake Powell, satellite image below) that is 186 miles long, an average 25 miles wide, and 132 feet deep, fed primarily by the Colorado, Escalante and San Juan River watersheds. And downstream, the habitat in the Grand Canyon would be inescapably altered by (a) the introduction of much colder water, since the dam releases water from the lower (and colder) depths of the lake, and (b) the removal of the seasonal floods' scouring and replenishment of sand bars along the river bottom, to be replaced by moderating controlled releases from the dam (which would allow non-native species to invade that delicate ecosystem).
This is a vastly oversimplified summary of the debate, which raged between agricultural interests in four states, whitewater touring companies, naturalists, energy-hungry cities, lake recreation advocates, and of course the designers and contractors who would profit handsomely from the dam's construction. See this link for a more thorough presentation of the issues.
Long story short, the dam was built, floods were controlled, electricity was generated, water was diverted for farming and for urban use via the Central Arizona Project (originating at Lake Mead) -- resulting in the predicted alterations to Grand Canyon ecology, and one further ripple of consequence -- the Colorado River is sucked dry by the time it reaches the U.S.-Mexico border. It no longer flows to its historic mouth at the Sea of Cortez (the Gulf of California).
Environmental groups ranging from the moderate (the Sierra Club) to the radical (Earth First!) have denounced this desecration of the desert Southwest to no avail. Writer Edward Abbey famously proposed a number of (hypothetical) means for destroying the dam. Even those who were initially persuaded to support it, later came to regret that choice -- including David Brower and Senator Barry Goldwater. Sad, sad, sad.
This story is only one part of a much larger pattern. There are well over 8000 major dams in the United States (see map below). Many of them no longer serve a useful function. Further, many of them actively interfere with the migratory routes of native salmon and other species, threatening their extinction. On a tiny scale, steps have been taken to remove a few of these dams. It is my contention that we should remove most, if not all of them, and allow nature to reclaim her own rhythms. What about floods, you ask? How about this -- have a little common sense and don't build your house or farm in a flood plain. Would you plant a house atop a dormant volcano, hoping that it wouldn't erupt? Would you build on a known tectonic fault line? Many people do -- denial (in the buyer) and greed (in the seller) run deep in the human psyche. Whatever happened to forethought?