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Ask the Conservationist

October 2009: Dams—the Good and the Bad

A kayaker in Chicago wants to learn more about dams and how they impact the environment: What's good and bad about dams?

Read what our conservation scientist in Illinois has to say, and when you're done reading, send us your questions on any conservation subject for one of the Conservancy's 720 staff scientists. (Note: We regret that we can only answer one or two questions each month and that we cannot answer the others offline.)
Cara Chowaniec of Chicago, Illinois, writes:

“My kayak group would like to understand more about what's good and bad about dams. I know never to pass over a dam, but for what reasons were they initially installed and how can they be retrofitted or removed? What's their impact on our environment?"


Doug Blodgett

Dams are built to provide hydropower, water supply, flood protection, navigation, recreation or a combination of these — all important resources for people. Since dams differ from river to river — and even from dam to dam on the same river — it's difficult to make blanket statements about their environmental impacts, but there are a few ways that a dam can threaten its surrounding ecosystem:

  • When you construct a dam, you create a pool behind it. That usually inundates terrestrial areas with water, flooding valuable wildlife habitat.
  • Dams can restrict movements of fish and other aquatic organisms.
  • Dams alter the natural flow patterns — like floods and periods of low, stable water — to which our native plant and animal communities have adapted.

Dams also can change important natural habitats along the river or in its floodplain. The Illinois River, for example, has a relatively flat floodplain bounded by steep bluffs that were carved out by big floods in the past. The abundant shallow water habitats that resulted are what we'd call highly productive areas — they support a lot of biological diversity. Damming a river can permanently flood the shallow water habitats, devastating the native plants and animals that live there.

Dams and Pollution

Despite these negative impacts on the environment, keeping a dam intact can benefit an ecosystem. The Clean Water Act, passed in 1972, was a landmark piece of legislation that vastly improved the health of our waterways by helping to eliminate toxic pollutants and runoff. Prior to the bill's passage, rivers were too easily polluted by toxins, heavy metals and other industrial byproducts.

What does this have to do with dams? As the dams slow the flow of water, sediment tends to drop out and collect behind dams. In dams built before today's stringent pollution control, that sediment may contain heavy metals or other toxins. If we removed those dams now, it could unleash harmful materials downstream.

Even so, the long-term ecological benefits of removing dams usually outweigh those of leaving them in place.

Removing Dams

So, in most cases, dams can be removed for the benefit of nature. Conservation scientists in Oregon did just that in 2008, when they used 200,000 pounds of explosives to remove two miles of levees in the Klamath Basin Conservation Area.

More dams are being removed in Illinois and around the country. Conservationists are also working to build fish passage structures in remaining dams. Through programs such as the Illinois River Basin Restoration Program and the Navigation and Ecosystem Sustainability Program, the Conservancy is working with the Army Corps of Engineers, the Illinois Department of Natural Resources and others on both dam removals and fish passage structures.

Even though structures like dams can be damaging, we've learned that nature is resilient, and we are committed to restoring lakes and rivers in Illinois and around the world.


About the Conservationist

Doug Blodgett is the Nature Conservancy's river conservation director in Illinois.

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