Conclusions
Numerous studies have shown that dams have significant impacts on almost every part of the ecology of rivers and their surrounding environs. While there are some recreational, and environmental, benefits that may accrue (such as more waterfowl), most impacts are damaging to ecological values and can be costly, or in some cases impossible, to mitigate with the dam in place. Preliminary results from documented dam removals suggest that dam removal does restore ecological values in catchments, and probably coastal areas as well. However, removing dams is not simply a matter of removing the dam structure and waiting for the fish to return. For any given river, there will be a unique set of circumstances that will determine the range and extent of environmental benefits that may accrue from the removal of a dam, and also any potential adverse effects. Considerable forethought and planning must be put into any removal project and this must particularly deal with sediment mobilization and mitigating its impacts. Without this, endangered fish or invertebrate stocks may run the risk of extinction occurring in the short-term, before medium to long-term benefits can help restore populations. Considerable attention to channel formation and bank stabilization in formerly impounded reaches is likely to be required, and typically this should include extensive works and riparian planting to prevent erosion of impounded sediments.
Conflicting ecological goals make dam removal a difficult issue insofar as ecologists are concerned. One example that illustrates this is the controversial idea of removing Glen Canyon Dam and emptying Lake Powell. The relative importance of a population of peregrine falcons that have become established since the formation of Lake Powell may supersede lost instream ecological values for some biologists. Other biologists may argue that saving one species is less important than losing 5 species of fish native to the impounded reach, or the loss of vast areas of the Colorado River delta and their attendant biota.
Even when desirable ecological goals are agreed upon by ecologists, these are seldom accepted by all stakeholders, which can generate intense controversy. Proposals to remove dams from the lower Snake River, for example, have recently foundered primarily due to socio-economic and political reasons. The decision to abandon dam removal came despite independent scientists agreeing that the available evidence shows that dam removal is the only option likely to restore populations of salmon and steelhead in the Snake River (PATH, 1999). The management decision was made possible because of ambiguity and uncertainty in data from studies of fish survivorship in affected rivers, as well as a lack of certainty about the results of dam removal. Uncertainty expands the range of options that can be considered by management, and consequently makes possible the selection of options that may not work. This situation suggests that proponents of dam removal need to better document the real impacts resulting from dam removals so that more certainty about the benefits of removal can be achieved. By reducing uncertainty, management options can be constrained to a narrower range of possibilities that are more likely to result in the successful attainment of ecological goals. Likewise, opponents of dam removal should theoretically also reduce uncertainty over the success of alternative methods, to better demonstrate that they will work. However, the burden of evidence is likely to remain on dam removal proponents because the prevailing socio-economic and political climate seems to favor the status quo.
The removal of dams from rivers is a relatively recent topic of study. Clearly, there exists a need for far more research into the results and processes that occur when a dam is removed in order to better determine how swiftly ecological values can be restored. There is also a need for restoration efforts to incorporate other factors (such as land-use practices, fishing intensity, riparian conditions etc.) that adversely impact on the ecological health of river systems. The choice of management goals is beyond the scope of this paper, but if the goal is to restore populations of native instream biota, then dam removal presents a viable option. It is probably the most effective way to restore the manifold ecological links that contribute towards functional ecosystems and, consequently, restoring populations that have adapted to life in those ecosystems. Even though dam removal is likely to be the most effective tool for river restoration, it should be noted that multiple factors harm riverine ecosystems. Only an integrated approach to river system management will achieve desired environmental outcomes.
Acknowledgments
I would like to express my gratitude to the many fisheries scientists who generously provided me with information about dam removals in the USA. In some cases this information was "hot off the press" and I am grateful that they were so forthcoming with their results and observations. In particular I would like to thank Mike Wicker (North Carolina Fisheries and Wildlife Service), Joe Hightower (North Carolina State University), Wayne Jones (North Carolina Wildlife Resources Commission), Michael Hill and Rick Long (Florida Fish and Wildlife Conservation Comission), Steve Brooke (American Rivers, Maine Field Office), Susan Davies (Maine Department of Environmental Protection), Matthew O’Donnell (Maine Department of Natural Resources), David Marshall (Wisconsin Department of Natural Resources), and Luther Aadland & Tim Schlagenhaft (Minnesota Department of Natural Resources). I would also like to thank NIWA for allowing me to use their library to get references not readily available elsewhere. Many thanks go to Dr. Crile Doscher who provided lots of information on the Elwha River dam removal controversy, and for supervising this essay. Finally, I would like to thank my wonderful fiancé Karen Rounds for helping to translate ‘Dolphinese’ into something closely approximating the English language.