POSTED JAN 16, 2003

Colorado conundrum
Colorado River water. Everybody wants some. Ever-expanding California is using way beyond its share. The desert sin-'n-sun capital, Las Vegas, is booming. And it needs precious river water for those stupendous fountains. The water comes from the huge dams blocking the Colorado River. Despite a treaty between the United States and Mexico, barely a trickle reaches the river's mouth in the Sea of Cortez.

Economically, the Colorado River dams are critical to the heavily-populated desert Southwest, and nobody is seriously considering taking them down.

Construction at Glen Canyon Dam, 1963. Environmentalists warned that the dam, by blocking the Colorado River, would forever alter the ecology of the Grand Canyon. Glen Canyon ain't coming down anytime soon, but other dams are - an estimated 63 dams in 2002 alone. Image: U.S. Department of the Interior

It's a different story with "little, dinky dams." Estimated to number 2 million (one measure of their ubiquitousness is the fact that nobody knows the exact number), they are coming under attack for economic, safety, and environmental reasons.

And they are coming down. American Rivers, an environmental group, says 63 small dams were scheduled for removal in 2002, part of a trend toward removing dams that have outlived their usefulness.

Rivers are, by definition, moving water, and dams interfere with that flow - and with river ecology. Yet despite the many presumed environmental benefits of removal, the real motivation is usually dollars and cents. Old dams need repair, and that's expensive, especially when the dam produces little or no benefit.

We can't help it. The Why Files is obsessing about dams. What do they do to rivers? What happens when they are removed?

How come dam?
Dams are built for many reasons:

To control floods.

To make lakes for recreation.

To store water for drinking, industry or irrigation

To raise river levels for navigation (consider the dam-studded Mississippi and Missouri Rivers, which are essentially barge canals at this point).

To store water for hydroelectric power (in earlier times, to power grain- or saw-mills.)

Many dams have overlapping justifications. The Glen Canyon dam, built primarily for water storage and hydroelectric power, created Lake Powell, a tourist attraction in the arid Southwest. (It also starved the Grand Canyon of water and sediment, causing biological and geological changes to the national park.)

A dam-buster caught red-handed on Koshkonong Creek, Wisconsin. Courtesy Emily Stanley.

What dams do
Whatever their purpose, dams regulate water flow. And that can cause a series of upstream and downstream problems.

Upstream and downstream:

Constant water level prevents sediment from consolidating, turning banks mucky.

Dissolved oxygen may fall, harming fish and shellfish.

Water may warm, injuring or killing fish that need cold water.

Upstream of the dam:

Wildlife habitat is flooded.

Fish are blocked from migrating and spawning (although the decline of the salmon is blamed largely on dams in the Northwest, steelhead, striped bass, sturgeon and alewife have similar spawning habits).

Silt in the dam impoundment can damage or destroy fish spawning grounds.

Downstream of the dam:

By regulating river flow, dams destroy habitat for organisms adapted to rising and falling water.

River stay inside their banks, so floodplains no longer get deposits of fertile sediment .

Dams block sediment flow, causing many changes downstream:

The river bottom becomes more rocky, as sediment no longer fills gaps between larger stones.

Beaches on the nearby ocean are starved of sand normally carried in the river, a particular problem in Southern California.

The same starvation can affect wetlands. The many dams on the Mississippi-Missouri rivers, for example, have deprived wetlands in the Gulf of Mexico of sediment, exposing coastal Louisiana to devastating hurricanes.

Oldies, not goodies
Still, the impetus for dam removal comes not from environmental damage, but from the simple fact that dams are getting long in the tooth: 25 percent of America's 2 million dams are older than 50 years.

Many of these codger dams have problems: They may have cracks. Water may have undermined the foundation. They may be so full of sediment that they cannot store water. They may have been built for a purpose that no longer exists. Or they may endanger swimmers or canoeists, who can get trapped and drown in "scour holes" that appear downstream of dams.

The dam-removal process often begins when a state inspector looks at a dam and insists on repairs. These often turn out to cost far more than removal, so repair can only be justified if the dam provides significant economic benefits. If a dam was built, for example, to power a grain mill that is long gone, or is supplying only a small amount of hydroelectricity, who would want to pay a million bucks to keep it going, when it could be ripped out for $50,000 or $100,000?

The Amistad Dam, on the Rio Grande at the border between Mexico and the United States, is used for flood control, water storage and electricity generation. It also completely rewrites the ecology of the area. Image from Amistad Dam Project

Dam bottom line
Angela Bednarek, now at Colombia University's Earth Institute, queried state dam safety officers about the motivations for removals, and learned that safety and liability trumped environment. In Pennsylvania, where she focused her research, the message was clear: owners were worried about lawsuits, not fish habitat. Although she knew that dam safety officers would naturally stress safety, she notes that they are responsible for approving removals and thus in a good position to know the situation.

At first, Bednarek was surprised by the bottom-line emphasis. "I was an ecologist interested in river restoration, so I assumed the removals were occurring for environmental reasons. But after I thought about it, I realized that would be rare for that to drive the process completely; it would come down to balancing costs and benefits."

Unlike the cost of removal or repair, environmental advantages are tough to enter into cost-benefit calculations. Still, she says, "It's exciting that environmental reasons are playing a role, and starting to play more of a role in many states."

Internationally, the effects of dam construction are rather different. The World Commission on Dams, in its 2000 report, said large dams had created enormous social dislocation:

The negative social impacts reflect a pervasive and systematic failure to assess and account for the range of potential negative impacts on displaced and resettled people as well as downstream communities. Estimates suggest that some 40-80 million people have been displaced by dams worldwide while the livelihoods of many more living downstream were affected but not recognized. Mitigation, compensation or resettlement programs were often inadequate.

Here in the United States, however, the fate of many small dams is being sealed by the fact that they are no longer needed. And that could be good news for the environment.

Generalities, shmeneralities. Can you give me some specifics?

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Terry Devitt, editor; Sarah Goforth, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2002, University of Wisconsin, Board of Regents.