Română
English
https://www.scientificamerican.com/article/climate-change-complicates-the-whole-dam-debate/
With California now on track to have the rainiest year in its history-on the heels of its worst drought in 500 years-the state has become a daily reminder that extreme weather events are on the rise. And the recent near-collapse of the spillway at California's massive Oroville Dam put an exclamation point on the potentially catastrophic risks.
More than 4,000 dams in the U.S. are now rated unsafe because of structural or other deficiencies. Bringing the entire system of 90,000 dams up to current standards would cost about $79 billion, according to the Association of State Dam Safety Officials. Hence, it has become increasingly common to demolish problematic dams, mainly for economic and public safety reasons, and less often to open up old habitats to native fish. About 700 dams have been taken down across the U.S. over the past decade, with overwhelmingly beneficial results for river species and ecosystems.
Now, however, a new study in Biological Conservation takes the science of dam removal in an unexpected direction. Although acknowledging that reopening rivers usually leads to "increased species richness, abundance and biomass," a team of South African and Australian authors argues that in some cases threatened species may actually benefit from keeping existing dams intact.
The idea for the study arose because both South Africa and Australia are now experiencing "an incredibly dry period," says co-author Olaf Lawrence Weyl, a specialist in endangered fish with the South African Institute for Aquatic Biodiversity. Many native fish species have been "pushed right back into headwater streams" by competition with alien fishes, typically introduced for sportfishing and by "dewatering of entire rivers" to supply irrigation water for crops, Weyl adds. In some cases, the streams and reservoirs backed up behind dams are the only remaining aquatic refuges for endangered species.
Australia's prolonged drought has been especially intense in the country's southwest. Even before modern development, many streams there naturally dried up for much of the year, confining endemic fishes to a few disconnected pools. Two species actually evolved to burrow into the mud and become dormant in the dry season, a strategy called estivation. But since the 1970s median stream flow has declined by half, with further declines predicted over this century. Even the estivating fishes have suffered sharp losses-but they have managed to hang on, along with other endemic species, in artificial ponds created by dams. Removing these dams "for economic reasons without proper evaluation of their potential ecological value," the co-authors warn in the new study, "may therefore cause a major loss of vital refuge habitat." In some cases, removal of dams could also open upstream habitat to introduced sport fish, which typically kill or otherwise outcompete native species.
"This isn't a call to stop dam busting, or a one-size-fits-all scenario," Weyl explains. "What we are saying is that there are sites in some regions, particularly dry regions, where dams could have important conservation value, and that needs to be taken into account when planning removal of dams."
Many U.S. scientists and conservationists have responded to the new study with one eyebrow raised. The argument "sounds plausible for the particular cases they're talking about," says Jeffrey Duda, a research ecologist for the U.S. Geological Survey's Western Fisheries Research Center who was not involved with the new work. But in the U.S. most dam removals occur in less arid regions, where "very few cases meet the criteria in the article." The co-authors "are very careful and specific to acknowledge that the vast majority of dams have negative impacts on the environment," adds Shawn Cantrell, Pacific Northwest director for Defenders of Wildlife, a conservation nonprofit organization. Cantrell was not involved in the new study.
Moreover, if climate change is sometimes a reason to preserve existing dams, it can also make the argument for removing other dams more urgent, says Scott Bosse, a former fisheries biologist who is now Northern Rockies director for nonprofit American Rivers. Last year a federal judge tossed out a proposal for the continuing operation of hydropower dams in the Columbia River Basin, saying the plan failed to take adequate account of climate change's "potentially catastrophic impact" on populations of salmon and steelhead (sea-run, or anadromous, rainbow trout), which are protected under the Endangered Species Act. Removing the dams, Bosse says, would reopen "some of the coldest, highest-elevation spawning habitat for anadromous fish"-a move that could be critical to their survival.
But the new study is correct, says Bosse, who was not part of the research, in saying some dams can also serve as essential barriers to invasive species. The Hungry Horse Dam in northwestern Montana, for instance, keeps lake trout-introduced for sportfishing-out of the south fork of the Flathead River, preserving "our best native trout habitat," Bosse says. "Everything upstream is bull trout and westslope cutthroat trout," native species that have lost much of their original habitat to logging, grazing, mining and introduced nonnative species. Likewise, dams in the Great Lakes region play a critical role in keeping invasive sea lampreys out of potential spawning grounds. "Not only are some dams not coming out," Bosse says, "there are barriers being installed to protect habitat. Right here in Montana in any given year, we probably see five or 10 such projects, and it's similar across the West." In such cases, the U.S. Forest Service typically collaborates with a state fish and game department to create an artificial waterfall on a headwater stream or to erect a six- or 10-foot concrete dam, "just high enough to prevent brook trout or brown trout from invading upstream habitat."
But this practice involves a a trade-off, a 2009 study in Biological Conservation warned: Isolating native fish with protective barriers also risks cutting them off from old migratory routes, preventing natural gene flow from one population to another and increasing the likelihood of extinction. Moreover, anglers may simply introduce a favorite nonnative fish above a barrier because they think it looks like a good place to fish. The 2009 study warned against the danger of making decisions about a dam or barrier installation subjectively, based on "personal philosophy or simply what has worked elsewhere."
For Weyl and the other co-authors of the new "dam busting" study, that seems to be the bottom line-the debate about dams has always been prone to ideological or exclusively economic arguments. The greater challenge, they suggest, is to come to terms, dam by dam, with the ecological nuances.
