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Effects of Dams on Aquatic Ecosystems Published by the Illinois Department of Natural Resources |
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The effects of dams on aquatic systems have been known for many years. However, only recently have the general public become more aware of the issue. Evaluations of dam projects are beginning to consider not only the benefits of a dam, but also the substantial ecological and economic costs of dams. This has led to an increasingly visible public debate surrounding both large and small dam projects. In Illinois, DNR studies indicate that dams affect all aspects of stream ecosystems, including water quality, habitat quality, flow regime, and food source, causing severe local impacts on upstream aquatic communities. Stream quality is determined by evaluating the fish community using a rating system ranging from "A" (highest rating) to "E" (lowest rating). Areas directly above dams consistently rate in the "D" range. Free-flowing area downstream typically have "A" and "B" ratings for the Fox River and "C" ratings for the Des Plaines River. Sportfish populations are significantly diminished above dams due to water quality, habitat, and forage limitations. Invertebrate communities such as insect larvae and mussels are also severely impacted. Reduced current upstream of dams causes increased deposition of sediments and organic material creating poor habitat for bottom-dwelling organisms, important components of the aquatic food chain. The slow water upstream of dams also provides favorable conditions for algal blooms. Algal blooms cause wide swings in dissolved oxygen concentrations — the primary source of dam-related water quality problems. On the Fox River, segments above several dams have been placed on the EPA's water quality impaired list due to the problems related to summer algal growth. In addition to these local effects, dams block fish migration affecting entire river systems. DNR studies have found that dams on small tributaries block spawning migrations of sensitive sucker species and sportfish such as smallmouth bass. These dams also prevent recolonization of fish and and mussels (which "hitch a ride" on fish during the larval stage) following natural events such as floods and droughts or human disturbances. In Northeastern Illinois, where there have been severe water quality problems, fish and mussel species have been lost in many areas. Numerous dams prevent migration back into historically degraded sections of the Fox, Des Plaines and DuPage Rivers, despite recent improvements in water quality. As a result, dams present a major impediment to aquatic ecosystem restoration efforts in these watersheds. In recent years, dam removal has become a commonly used restoration tool. American Rivers has reported 465 successful dam removals across the country. In Wisconsin more than 70 dams have been removed. Their studies have shown that dam removal is one of the most cost-effective stream restoration techniques and is almost always cheaper than repair or rebuilding options. As an alternative to removal, other solutions have been used to address dam problems. Fish passage has been implemented on many coastal river dams for salmon passage with varying success. Passage structures for Midwestern fish, which require lower water velocities than salmon, are less well developed and researched. Fish ladders also do not address the habitat and water quality problems upstream of the dam. Other alternatives such as bypass channels, roughened ramps, dam lowering, and/or combinations of these three techniques have been used. In most cases these solutions are more expensive and less effective than dam removal. In Illinois, dam issues are just beginning to be addressed. One dam has been removed on a small Fox River tributary. Alternatives are being evaluated on a number of other dams throughout Northeastern Illinois. Re-connection of rivers and streams through dam removal and modification will improve the ability of these systems to handle the stresses associated with the large population increase projected for the Chicago Area. Initial public reaction to dam removal/modification proposals have been mixed. Obviously, not all dams can be removed. However, as part of our ecosystem management approach, we are beginning to consider the significant impacts dams have on our river resources and are evaluating alternatives to address these problems. |
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