There are clear cases for which the decision to remove a dam is virtually unquestioned except perhaps for its historical significance, said dam having outlived its usefulness. However, most proposals for dam removal are controversial because such decisions must evaluate benefits of retaining the dam against benefits of removing it. Certainly, these are complex decisions because of the conflict between ethical, economic, and legal aspects.
Dam removal has become increasingly popular, particularly as regards enhancement of populations of anadromous fishes--salmon and steelhead in the western U.S. and shad, herring and other species in the eastern U.S. Figure 13 illustrates the dams that have been removed in the lower 48 states (Figure 14 expands the eastern U.S.) from 1936 to 2013 on the basis of data tabulated by American Rivers [1]. These dams have been removed largely to improve the ecological conditions of river systems and to allow migratory fish to pass unimpeded for spawning in the upper reaches of rivers. Dam removal is commonly cited as a way of increasing stocks of imperiled fishes (Chesapeake Quarterly: Those Dammed Old Rivers [2]), allowing them to spawn in rivers that have been inaccessible to fish migration for as much as a century. Others argue that dam removal from coastal rivers is only part of the solution for fisheries improvements. Although numbers of anadromous fish may increase, the size of individual fish may not as long as fishing pressure remains the same (Oregon State University: Department of Fisheries and Wildlife [3]). This has been argued in the case of Elwha Dam removal in the Olympic National Park watershed (Crosscut: Elwha dams: Will bringing down NW dams really help salmon? [4]). In some cases, the benefits will accrue to native Americans who formerly depended on fishing for their nutrition and livelihood.
Arguments have also presented for esthetic improvements as the result of dam removal. An example of this approach is the argument for the removal of Hetch Hetchy (O'Shaughnessy) Dam on the Tuolomne River in the Sierra Nevada Mts. of California (Restore Hetch Hetchy [7]). This dam and its reservoir flooded a canyon much like Yosemite just to its south beginning in 1923, even though Hetch Hetchy was included in Yosemite National Park in 1890 by President W.H. Harrison. President Wilson, in 1913, signed the Raker Act that allowed San Francisco to dam the valley. Proponents of this dam removal argue for ecological improvements as well as access to once-spectacular scenery eliminated as the result of the water project. Hetch Hetchy provides some 20 percent of hydroelectric power generation to San Francisco as well as significant water supplies. Nonetheless, proponents of the dam removal argue that impacts on water or electrical power availability would be minor (Hetch Hetchy Today [8]).
One of the issues accompanying dam removal is the potential impact of the large volume of sediment that accumulated behind some dams over time. This so-called "legacy sediment" is commonly very fine-grained and contains stored nutrients and organic matter, among other possible pollutants. When dams are destroyed, efforts must be made to avoid a large flux of this sediment downstream as the newly released river cuts down to its natural base level. This requires careful engineering and significant funding. Merritts and Walter (2010) have suggested that most rivers and streams in eastern Pennsylvania and Maryland run through floodplains and levees of legacy sediment, not natural river valleys, created by the plethora of small dams built to impound water for hydropower in the past. And, even now, some large dams serve as a buffer against sediment transport that would create broad mudflats and high turbidity in coastal bays such as the Chesapeake Bay. Conowingo Dam in Maryland is one such structure that is estimated to trap as much as 50% of the sediment carried by the Susquehanna River before it empties into the Chesapeake Bay. There are concerns that this dam only has a bit more than a decade of useful life before it is filled to the brim with sediment, thus imperiling Chesapeake ecosystems (Langland, USGS, 2012). For an example of the engineering and costs of removing a dam, check out the San Clemente Dam Removal & Carmel River Reroute Project [9] in central California.
Many dam removal projects are proposed, but await funding from federal sources—Congress must appropriate funds. Projects on the Snake River and the Klamath River in the west remain controversial, but at least in the case of the Klamath, the Department of the Interior has recommended removal to allow salmon to reinhabit this river. Figure 9 illustrates the consequences of waiting for funding of dam removal using the Olmstead Dam example.
Links
[1] https://www.americanrivers.org/threats-solutions/restoring-damaged-rivers/dam-removal-map/
[2] http://www.chesapeakequarterly.net/V10N4/main3/
[3] https://fw.oregonstate.edu/
[4] https://crosscut.com/2014/05/elwha-dam-damnation-siff-film-fish-yves-chouinard
[5] https://www.americanrivers.org/
[6] http://www.americanrivers.org/
[7] https://hetchhetchy.org/
[8] http://www.hetchhetchy.org/hetch_hetchy_today
[9] https://www.sanclementedamremoval.org/
[10] https://www.americanrivers.org/threats-solutions/restoring-damaged-rivers/dam-removal-map/?gclid=CjwKCAjw-ITqBRB7EiwAZ1c5U9fpPcUJsSYeIgubaOdmK_gLp_C82fidn-TfbrMFAaRpnyVmJ5ONUhoCZt0QAvD_BwE