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Mitigation of the Impacts Caused by Dams

     The Earth’s Surface is covered by about 70% water.  Approximately 3% of that water is fresh water, the rest being salt water.  To put this in perspective take a liter of water – with an eyedropper pick up one drop and place it in your open hand by the liter container.  That precious, fragile drop taken from the liter is, in proportion to the world’s water, all the freshwater available for human use, for the maintenance of optimal levels of populations of all freshwater and terrestrial species, and for ensuring the proper functioning of the freshwater systems in our planet.  In the USA only 2% of the country’s 5.1 million km of rivers remain free flowing and undeveloped, with more than 85% of inland waters artificially controlled.[1]  The total number of dams in the U.S. is conservatively estimated to be 60,000.[2] 

     Dams impact watersheds in many ways.  Dams remove water needed for healthy in-stream ecosystems.  Dams may cause downstream stretches to alternate between no water and powerful surges that erode soil and vegetation, and flood or strand wildlife.  These irregular releases destroy natural seasonal flow variations that trigger natural growth and reproduction cycles in many species.[3]  The flow of the water system is interrupted by dams which confuse migrating fish.  They also change the temperature variation of the system and the dissolved content of the water.  Upstream from dams, ecosystems are drowned by the water built up behind the dams.  These structures block the pathways of many migrating fish up and down the river.  The obstruction causes sediments to be deposited behind the dam.  This can affect fish habitat and also concentrate toxic substances.  The deposition at the dams reduces nutrient flows to places down stream and can even affect the deposition of material at the mouth of the river.  Dams and their construction can interfere with protected areas such as wetlands or endangered species, which results in the mitigation of the impact on the environment.  These projects are regulated by the Federal Environmental Protection Agency through Section 404 of the Clean Water Act.

     Section 404 of the Clean Water Act establishes a program to regulate the discharge of dredged and fill material into waters of the United States, including wetlands. Activities in waters of the United States that are regulated under this program include fills for development, water resource projects (such as dams and levees), infrastructure development (such as highways and airports), and conversion of wetlands to uplands for farming and forestry.

      The basic premise of the program is that no discharge of dredged or fill material can be permitted if a practicable alternative exists that is less damaging to the aquatic environment or if the nation's waters would be significantly degraded. In other words, when you apply for a permit, you must show that you have: taken steps to avoid wetland impacts where practicable; minimized potential impacts to wetlands; provided compensation for any remaining, unavoidable impacts through activities to restore or create wetlands.

     Regulated activities are controlled by a permit review process. An individual permit is usually required for potentially significant impacts. However, for most discharges that will have only minimal adverse effects, the Army Corps of Engineers often grants up-front general permits. These may be issued on a nationwide, regional, or state basis for particular categories of activities (for example, minor road crossings, utility line backfill, and bedding) as a means to expedite the permitting process[4].

    Mitigation in this context is defined as: A process of minimizing or compensating for damages to natural habitats, caused by human developments. These activities are designed to decrease the degree of damage to an ecosystem. They may include restoration, enhancement, or creation. According to the Clean Water Act, mitigation is a sequential process that includes avoiding impacts, then minimizing impacts, and lastly, compensating for impacts.[5]  Mitigation can be as simple as changing the location of the planned activity on the site or as difficult as building new areas, such as wetlands, to compensate for the area being destroyed.  However, when discussing dams, moving their location is usually not an option and it will still affect the area of concern. 

     Although dam builders and operators are forced to mitigate some of the impacts of their existence the mitigation is not always worthwhile and can sometimes be worse then doing nothing.  The most common mitigation measures taken in the US is to release more water from the reservoir then would be the case of the dam were operated only to maximize power or water storage.  The US Federal Energy Regulatory Commission now requires the operators of many privately owned hydrodams in the US to release instream flows as a condition of renewing their federal dam licenses.[6]  The releases are supposed to help recreate the natural changes in water height that occurred before the dam was built. 

     Another type of dam mitigation is to control the temperature of the water being released.  This is done by fitting dams with devices that allow the water to be drawn from different places in the reservoir.  This helps to keep the down stream temperature of the water at the normal temperature for the current time of year or in some instances to maintain a cold or cool water system down stream.  However, this action cannot replicate the natural seasonal variations in temperature if the reservoir becomes too low.

     Hatcheries are also widely used as a mitigation technique.  Since the late 1940s the US government has spent hundreds of millions of dollars on hatcheries to mitigate the impacts of dams on Pacific salmon.[7]  Although hatcheries attempt to replenish the fish populations they are only treating the symptoms and not the cause.  Hatcheries do nothing for the destruction of habitat of the fish.  Therefore most of the released fish do not survive.  Hatcheries also have limited genetic stock.  When these fish are released into the wild they generally degrade the genetic stock of the wild fish.  Fish in hatcheries are generally kept in which densities allowing the easy spread of disease which can then be transmitted to the wild fish upon release.

    These techniques are not used individually but in combination with one another depending on the environment.  For example:

The Hungry Horse Mitigation Program, sponsored by Montana Fish, Wildlife and Parks, began in 1992 to address fish losses associated with the construction and operation of Hungry Horse Dam. The dam isolated approximately 38 percent of the FlatheadLake drainage and changed the physical and biological characteristics of the lake and river. The program's goals are to restore and reconnect critical habitat, reduce the negative interactions between native and non-native fish, and improve dam operations for native trout recovery.

The Flathead River system in Northeast Montana is a regional stronghold for migrating westslope cutthroat trout, part of Montana's natural heritage. Installation of the dam completely blocked fish migrations from FlatheadLake to the South Fork Flathead River upstream. In order to improve fish passage to critical spawning and rearing habitat, the program initiated several culvert replacement projects. These combined projects re-opened 16 percent of the available spawning and rearing habitat to migratory fishes in the reservoir system, and monitoring surveys have shown significant increases in adult and juvenile fish upstream of each passage improvement site. The program is also using innovative natural channel restoration techniques to improve native fish habitat throughout the upper Flathead River drainage. In one instance, improvements to Emery Creek included removing sections of a logging road that had distorted the natural meandering of the stream, causing habitat degradation and creating barriers to fish migration. The improvements enhanced fish habitat and restored a two-mile section of channel to aid the spawning and rearing habitat for native trout.

Dam operations had also created unnatural flow and temperature fluctuations in the Flathead River downstream of Hungry Horse Dam. In 1996, a temperature control structure was installed on the dam to correct the problem. It allows dam operators to take water from the appropriate depth in the reservoir so the water flowing through the dam turbines matches the natural, seasonal temperature pattern in the river. As a result, normal temperatures were restored in the Flathead River downstream of the dam, helping to increase favorable stream and habitat conditions for fish.[8]

This shows how a program incorporates multiple techniques to provide an environment more close to the preexisting environment.  Some of these techniques are common such as the temperature control structure; however, other techniques such as the removal of logging roads are examples of site specific solutions. 

     Sometimes companies will propose restoring one area of a river or even a different river in exchange for building a new dam or renewing licenses.  This is the case in western North Carolina.  Duke Power operates 11 dams in the area.  Permits to operate the 11 dams expire over the next two years. To get new permits, Duke is required to provide recreational and environmental compensation over the 30- to 40-year life of the new permits.[9]  As part of their environmental compensation Duke has proposed the removal of the Dillsboro dam on the TuckasegeeRiver.  Representatives with the U.S. Fish and Wildlife Service and the N.C. Department of Environment and Natural Resources have agreed to waive other environmental mitigation in exchange for removing the dam.[10]  This shows the diversity in mitigation techniques available for dams.

     However, issues can arise where there are no mitigation techniques available except moving the project to another location or even having to abandon the project altogether.  In the mid 1960’s The Tennessee Valley Authority (TVA) began construction of a dam on the Little Tennessee River.  During the years of construction of this project the Endangered Species Act (ESA) was passed by Congress.  The ESA and its implementing regulations prohibit any person from harassing, harming, pursuing, hunting, shooting, wounding, killing, trapping, capturing or collecting any listed threatened or endangered species. This broad prohibition has been interpreted to prohibit even ordinary land use activities such as farming or forestry or site development if the result of such activities would be significant habitat modification or degradation where it actually kills or injures wildlife by interfering with essential biological functions such as feeding, breeding or sheltering.[11]  The ESA was applicable to the TVA dam construction even though construction began before the passage of this act. 

     The ESA halted the construction in the early 1970’s because an Ichthyologist discovered a new species of fish called the Snail Darter (Percina tanasi).  This fish was thought to only live in the area of the Little Tennessee River which was to be flooded by the dam.  The Darter would become extinct if the project were aloud to continue.  In the late 1970’s more populations of the Snail Darter were discovered in other regions of the country which caused the darter to be changed from an endangered listing to a threatened listing.  Through this change in listing, court decisions and new legislation the dam was eventually finished.

     The ESA only affects dams when a listed species is discovered.  Discovery of listed species usually occurs when dams are in the development process.  Part of this process includes the involved parties applying for one of the previously mentioned permits under Section 404 of the Clean Water Act.  The parties also have to file for the appropriate permits for the state they are in.  The permits are enforced by the EPA, The Army Corps of Engineers and the appropriate state agencies.  These permits will spell out what mitigation techniques will be used to compensate for the lost ecosystems, habitats and wildlife.  Section 404 also provides penalties for failure to comply with permit requirements.  The penalties will include but are not limited to “a civil penalty not to exceed $25,000 per day for each violation. In determining the amount of a civil penalty the court shall consider the seriousness of the violation or violations, the economic benefit (if any) resulting from the violation, any history of such violations, any good-faith efforts to comply with the applicable requirements, the economic impact of the penalty on the violator, and such other matters as justice may require.[12]

    The mitigation techniques and problems previously mentioned are not the only resources out there.  Mitigation of environmental impacts is complicated regardless of the project and takes creative ideas.  Dams face more complex issues because they deal with major watersheds and multiple ecosystems on a much larger scale the most other projects.  However, this should not be an excuse for the lack of concern for these systems.  Although mitigation cannot completely replace the loss, when done properly it will lessen the impact of the structure on the river and its surrounding environment. 

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Selected Bibliography

1. EPA  Friday, March 4th, 2005

2. Ramsar Convention Bureau, Gland,

Switzerland 23 January 1998, Dwight Peck, Ramsar.

3. Olsen, W. Kent; Hydroelectric Power Resources of the United States, FERC 1988.

Natural Rivers and the Public Trust, (1989)

4. Pace University,,

            White Plains, New York Design ©2000 Baseline Institute, Lafayette, Colorado

            Revised 3/1/2005

5. Burzon, Julia K., leAnne M. Rohrberg;  

  ; June 12, 1999

6. McCully, Patrick; Silenced Rivers: The Ecology and Politics of Large Dams; Zed

 Books, New York 2001

7. Northwest Power and Conservation Council;

  ; 851 S.W. Sixth Avenue,

            Suite 1100 Portland, Oregon  97204

8. Johnson, Becky; Feds begin review of Duke’s dams Proposal to remove Dillsboro

 Damn dominates discussions;

9. Ryan, Patrick W, Galen Schuler; The Endangered Species Act - A Primer;

   1998 Perkins

            Coie LLP, Seattle, Washington

10. Section 404 of the Clean Water Act Title 23 chapter 26 Subchapter IV Section 1344

Subsection sMay 18, 2009


[1] Ramsar Convention Bureau, Gland, Switzerland23 January 1998, Dwight Peck, Ramsar.

[2] Olsen, W. Kent; Hydroelectric Power Resources of the United States, FERC 1988. NaturalRivers and the Public Trust, (1989)




[6] McCully, Patrick; Silenced Rivers: The Ecology and Politics of Large Dams; Zed Books, New York 2001

[7] McCully, Patrick; Silenced Rivers: The Ecology and Politics of Large Dams; Zed Books, New York 2001


[9] Johnson, Becky; Feds begin review of Duke’s dams Proposal to remove Dillsboro Damn dominates discussions;

[10] Johnson, Becky; Feds begin review of Duke’s dams Proposal to remove Dillsboro Damn dominates discussions;


[12] Section 404 of the Clean Water Act Title 23 chapter 26 Subchapter IV Section 1344 Subsection s

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