This article was researched and written by a student at the University of Massachusetts, Amherst participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
Since pioneer days, dams have played an important role in the development of the United States. At first they provided mechanical power for grist mills, flax mills, and saw mills, as well as water storage for people and livestock. Later, dams provided hydroelectric power, flood control, irrigation, navigation, recreation and scenic beauty. The National Inventory of Dams reports that there are approximately 79,000 dams in the United States that are over 1.8 meters (6 feet) high, with 8,100 over 15.2 meters (50 feet) high. But the number of dams of all sizes is most likely close to 2 million, and many of them are over 100 years old. Despite the long history and importance of dams in the United States, there is a growing trend to remove them and return the waterways to their natural states.
The deterioration of aging dams creates a financial burden for dam owners and safety hazards to those living downstream. According to the National Performance of Dams Program at Stanford University, 1,595 significant hazard dams are within one mile of a downstream city. There are 3,300 unsafe dams in the United States and since 1999 there have been 129 dam failures. In addition to safety issues, power companies that own small dams are faced with diminishing profits and many dam owners do not want the expense of repair and maintenance of crumbling structures.
In addition to these issues, the federal government requires that dam owners provide appropriate fish passage in situations where dams impede the spawning migration of fish going to, or coming from the ocean. In these situations, rather than installing fish ladders or fish lifts, dam removal may be the most financially and ecologically appropriate decision. There is also growing interest in restoring continuity to riverine habitats--habitats that provide colder, healthier, more oxygenated aquatic ecosystems. For these economic and environmental reasons, dam removal is becoming more prevalent across the United States. However, it is a complex process and has environmental, economic, and social implications.
Dam removal studies have shown that each dam is unique and that many factors affect the results of removal. One of the most important factors is the quantity and type of sediment that has collected behind the dam. Prior to removal, the sediment must be tested for toxins and dredging is sometimes recommended before the dam is breached. When a dam is removed, sediment is carried downstream and the fine particles can be deposited up to many kilometers away from the dam site. A carefully planned water draw-down can ameliorate some of the sediment transport problems.
After dam removal, a new water channel will form in the impoundment behind the dam and the banks will slump before the new channel stabilizes, making it wider and shallower. This increases sediment transport to downstream areas. In addition, storms occurring before the banks stabilize could mobilize even more sediment.
Sediment deposition downstream occurs along stream banks, stream bottoms, backwater and floodplain areas, and affects biota in a variety of ways. Studies show that mussels, in particular, are negatively impacted. Population size is reduced, and some species might be extirpated. It can take years, if not decades, for mussels to rebound. To mitigate harmful impact, some states require that endangered species be moved to appropriate habitat prior to dam removal. On the other hand, macroinvertebrates such as caddisfly and mayfly larvae can re-establish their populations within a year. This is important for survival of fish communities that feed on these organisms.
Upstream, the warmer, still-water reservoir ecosystem that existed before the removal of the dam suddenly becomes a colder, running-water environment and the resident reservoir fish disappear immediately. In the case of carp, a non-native invasive species, this may be welcome news to environmentalists and anglers. Other welcomed news is that dam removal is a boon to migratory fish. Research consistently shows that migratory fish such as salmon, shad, river herring and striped bass readily utilize areas upstream of dam removal sites. On the other hand, Wisconsin dam removal studies show that resident fish, such as smallmouth bass, did not immediately begin to use new stream channels in the impoundment areas. Even though a cold, running-water environment was established similar to upstream and downstream areas, the bass did not use the new channel until appropriate habitat was formed. Substrate and other characteristics such as cover provided by woody debris, need to be in place prior to colonization by resident fish.
In high latitudes, some dams have provided impediments to downstream movement of ice during winter months. In some instances, removal of the dams can cause changes to the ice regime, causing jams, riverbed scouring, and possible flooding. One example was in Augusta, Maine, where ice jams occurred during the two winters following the removal of the Edwards Dam. The impact of dam removal on the ice regimes of rivers is rarely assessed prior to dam removal, yet there are measures that can be taken to mitigate negative impacts if recommendations are made in time.
Establishment of vegetation along the river banks is another environmental variable. A concern expressed by residents facing local dam removal is that the newly exposed impoundment surface will be an unattractive mud flat. However, past dam removals show that when the dam is removed in the growing season, vegetation establishes quickly, even within a matter of weeks. Grasses and other small plants arrive first, aiding in bank stabilization. To thwart invasions by exotic species, seeding native species is recommended in certain situations, but is not always successful. It should be expected that plants highly dependent on water environments along the banks of the impoundment may die once the waterline moves. Also, it takes decades of normal plant succession before trees become established in newly exposed areas These factors affect the ecosystem as well as the aesthetics of the landscape.
There can be deep emotional attachment to the landscape created by dams. People generally enjoy open-water vistas and recreation that impoundments provide. Fishing, boating, bird watching, water skiing, and snowmobiling are some of the activities that take place on reservoirs. In towns where a dam has been in place for many decades, personal and public histories and identities have developed around these bodies of water. These attachments are embedded in the life fabric of many local citizens and therefore, when dam removal is introduced to a community, an emotional response is to be expected.
Influencing public opinion can be a lengthy and difficult process. Many groups are involved in dam removal decisions; local residents, citizens groups, local government, state government agencies, federal government agencies, businesses and environmental groups. Opponents to dam removal raise concerns such as the effect on property values, recreation, and town identity, while proponents of dam removal raise concerns such as public safety, the cost of maintaining an aging dam, and water quality problems created by dams. Divisiveness is heightened when the idea of dam removal feels sudden, when a decision is rushed, and when outsiders, including state agency and environmental group personnel, are involved.
Unfortunately, a decision about dam removal made in the ‘court of public opinion,’ is often based on inaccurate and incomplete information. Research indicates that individuals may use shortcuts to develop opinions such as agreeing with someone similar to them or well liked by them, agreeing with the social norm, or just being stubborn in order to be consistent with previous positions. A community is best served when scientific information about the benefits and drawbacks of dam removal is used as the basis of decision making. However, effectively disseminating information requires time, creativity and sensitivity to the needs of the community. Most dam removal proceedings are fairly lengthy, formal, quasi-judical reviews of environmental assessments that consider social, economic and environmental isses.
Ultimately, dam removal requires adjustment periods for both the ecosystem and humans involved. Given the increased ecological dynamics associated with sea level rise and climate change, we can expect an increased focus on the issues of dam removal.
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