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CM . . Other adverse effects may include reductions in wetland habitat or groundwater recharge, as well as shifts in species abundance and distribution. DL 2001). . When a reference condition is considered together with a stressorresponse relationship, the maximum potential benefit of a particular dam removal can be determined. Biota respond to the physical removal of the barrier, as well as to changes in water chemistry, habitat, and flow regime. Stanley Ward The potential for recovery of various taxa following dam removal varies markedly, depending in part on their ability to colonize and thrive in new habitats. Bednarek T J . . E Future efforts to assess the risks associated with potential sediment contamination should focus particular attention on current and former human activities within the watershed, as well as on the total volume and particle size distribution of sediment within the impoundment. Because many streams and rivers are impaired by more than one kind of stressor, a coordinated effort is clearly needed. E Dam removal usually causes many abiotic factors to change simultaneously (e.g., flow, sediment transport, water temperature), thereby hampering the identification of causal pathways that govern observed responses. JE KH Stanford As used above, effects are the observed changes in various ecological attributes, and endpoints are the broader environmental values or management goals that give context and meaning to the observed effects. Ultimately, the ability to predict ecological responses to dam removal from a knowledge of existing dam effects could be greatly improved by studying a broader range of dam sizes and types, especially smaller dams. Ward Natural analogs to small dams: Similarities and differences, Table 1. Myers CA JA Bushaw-Newton For instance, a lower threshold in depth or hydraulic residence time has to be exceeded before stratification begins, and once the upper threshold for this relationship is exceeded, no further changes in stratification occur. 2000); Mad River, California (Winter 1990), Neuse River, North Carolina (Bowman 2001); and Clearwater River, Idaho (Shuman 1995). Other impacts (e.g., those due to sediment transport) are perhaps best evaluated in the context of natural disturbance regimes. For example, beaver (Castor canadensis) dams often cause large changes in aquatic habitat types and biogeochemical cycles as well as moderate changes in sediment transport, but they usually have smaller effects on downstream flow regimes. Valdez E Even when contaminants are absent, we need to know how much sediment can safely be released, and during what seasons, to minimize downstream impacts. . Several important ecosystem features (e.g., pattern of floodplain inundation and habitat characteristics) are strongly affected by hydrology, which in turn depends on channel morphology, so the restoration of these ecological attributes follows the time scale of channel changes. LA Hayes 2001). In contrast, waterfalls probably have negligible effects on most ecosystem characteristics, but they can be potent barriers to the upstream movements of fish. Hart . Dam removal may prove to be a particularly useful method for reducing some forms of ecosystem impairment, but it needs to be considered as part of a broad, watershed-scale management plan (Stanford et al. They observed no significant upstreamdownstream differences in dissolved oxygen, temperature, or most forms of nitrogen (N) and P, either before or after dam removal. Overbeck If dam removal is to become an effective method of river restoration, we must be able to predict the potential benefits of any proposed removal. Warner SW . Note that if the stressorresponse relationship is nonlinear, then the potential benefits of dam removal vary in a complex way depending on dam and watershed characteristics. Power Pizzuto Although initial colonization may be rapid, population recovery in the former impoundment and downstream reaches ultimately depends on restoration of habitat conditions (e.g., temperature, substrate, topography, large woody debris) that are strongly influenced by channel morphology, flow regimes, and riparian vegetation. Likewise, the development of mechanistic models describing ecosystem structure and function has not yet advanced to the stage where they can be readily used to predict stressorresponse relationships. For example, if the stressorresponse relationship is similar to curve 2, then the removal of dams with stressor levels below the lower threshold may yield relatively small ecological benefits. Richter Biotic responses to dam removal have often been large and rapid. DB Several other factors limit our ability to draw more robust conclusions: Magilligan SC Poff TL Some of these adverse responses to dam removal are probably transient, however, and might be considered analogous to the short-term impairment of human performance that often occurs during the recuperative period following surgery. . Frissell RK Schmidt Webster Avery J Likens . For example, simple scaling considerations may facilitate the prediction of dam effects on some ecosystem attributes as a function of dam and river characteristics, such as the effect of dam height on fish blockage. 1999), or the enhancement of downstream water quality by aeration and temperature modification (Higgins and Brock 1999). Thorp Figure 2 explores how various ecosystem attributes (e.g., flow regime, sediment transport, biotic migration) may be affected by different types of human-made dams and natural analogs of small dams. Pejchar Stanley March Master's thesis, Fisheries Investigations of Newnan's Lake, Florida Game and Fresh Water Fish Commission, Physical-biological coupling in streams: The pervasive effects of flow on benthic organisms, The Manatawny Creek Dam removal: Species and community characteristics, Overview of reservoir release improvements at 20 TVA dams, Effects of dam removal on Dead Lake, Chipola River, Florida, Proceedings of the Annual Conference of Southeastern Association Fish and Wildlife Agencies, Effects on fish populations of removal of a dam on Manatawny Creek (Schuylkill River drainage, Pottstown, Pennsylvania), Paper presented at the Annual Meeting of the American Fisheries Society; 1923 August 2001, A theoretical study of river fragmentation by dams and its effects on white sturgeon populations, Kettle River Dam removal: Impacts of sediment on downstream mussel populations, Paper presented at a meeting of the Freshwater Mollusk Conservation Society; March 2001, The Manatawny Creek Dam removal: Project overview and geomorphic characteristics, Changes in the habitat and fish community of the Milwaukee River, Wisconsin, following removal of the Woolen Mills Dam, North American Journal of Fisheries Management, Temperature effects of dams on coldwater fish and macroinvertebrate communities in Michigan, Long-term changes in regional hydrologic regime following impoundment in a humid-climate watershed, Journal of the American Water Resources Association, Alteration of North American streams by beaver, Riparian ecology and management in the Pacific coastal rain forest, Fish habitat restoration following dam removal on a warm water river, American Fisheries Society, North Central Division, Rivers and Streams Technical Committee Symposium Proceedings: The Restoration of Midwestern Stream Habitat; 45 December 1990, Fragmentation of riverine systems: The genetic effects of dams on bull trout (, Productive capacity of the Betsie River watershed for steelhead smolts, Kennebec River diadromous fish restoration annual progress report2000, Paper presented at the Midwest Region Technical Seminar on Removal of Dams, Association of State Dam Safety Officials, A river might run through it again: Criteria for consideration of dam removal and interim lessons from California, Impounded Rivers: Perspectives for Ecological Management, Effects of dam removal on river form and process, How dams vary and why it matters for the emerging science of dam removal, The natural flow regime: A paradigm for conservation and restoration of river ecosytems, Benthic invertebrate community change following dam removal in a small Wisconsin stream, Engineering Approaches for Lake Management, Lake outlets and the distribution of filter-feeders: An assessment of hypotheses, Potential responses of riparian vegetation to dam removal, Environmental considerations for assessing dam removal alternatives for river restoration, Regulated Rivers: Research and Management, Sediment problems associated with dam removalMuskegon River, Michigan, Hydraulic Engineering: Proceedings of the 1991 National Conference of the American Society of Civil Engineers, Breaching of a small irrigation dam in Oregon: A case history, Influence of beavers on stream fish assemblages: Effects of pond age and watershed position, A general protocol for restoration of regulated rivers, Phosphorus transport before and after dam removal from a nutrient-rich creek in southern Wisconsin, A geomorphic perspective on nutrient retention following dam removal, Short-term changes in channel form and macroinvertebrate communities following low-head dam removal, Journal of North American Benthological Society, Simulated long-term temperature and dissolved oxygen characteristics of lakes in the north-central United States and associated fish habitat limits, [USEPA] US Environmental Protection Agency, Guidelines for Ecological Risk Assessment, Laboratory assessment of the role of a larval pheromone and natural stream odor in spawning stream localization by migratory sea lamprey (, Canadian Journal of Fisheries and Aquatic Sciences, Quantification of wood habitat in subtropical coastal plain streams, Small dams as barriers to freshwater mussels (Bivalva, Unionoida) and their hosts, The 1996 controlled flood in Grand Canyon. K J Some studies have relied on qualitative observations rather than quantitative measurements of responses. HI Given the widespread occurrence of beaver dams in North America prior to European settlement (Naiman et al. Auble To apply this risk assessment framework, it is important to understand how the effects of dam removal differ across a range of dam and watershed characteristics, and to recognize that the shape of the stressorresponse relationship varies with different ecological effects and endpoints. DW Prestegaard Marzoff DB Chapra Burroughs Lawrence RE Braatne This basic framework can be used to evaluate the effects of dam removal by considering dams as stressors and dam size (or another measure that accounts for dam, river, and watershed characteristics) as a measure of stressor level. . Robin Davis offered outstanding editorial assistance. . J . Petts JE We are currently quantifying these relationships across a range of dam and river types in the Mid-Atlantic region, with the ultimate goal of using this approach to prioritize dam removals so that restoration benefits are maximized. . Determining whether dam impacts are reversible not only requires a focus on the processes that contribute to ecological recovery following dam removal, it also depends on how the concept of reversibility is defined. In most watersheds, however, successful river restoration will require a focus on more than just the problems created by dams.

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