Most of the 14,000-plus dams that dot New England’s rivers and streams – almost all built more than a century ago – once provided power to nearby mills. While these mills no longer exist, the dams remain, blocking migratory runs of fish to their spawning grounds, trapping nutrients that otherwise would be natural fertilizer for floodplains, and disrupting the natural flow regime that triggers seasonal flooding events in and along the rivers.
“The era of dam building in the U.S. is over for lots of environmental and economic reasons,” said Frank Magilligan, a geography professor at Dartmouth College, “but the problems they cause still remain.”
Fortunately, now that the public is recognizing the environmental harm these aging dams are causing, some are being removed. Magilligan has examined the effects of dam removal in the region and has found that as rivers are reconnected, both social and environmental benefits soon follow.
Between 1990 and 2013, an average of 12 dams per year were removed in New England, which accounts for about one-fifth of all the dams removed in the nation during that time. While many larger dams are removed for environmental reasons, Magilligan said that the smaller dams are more often removed because of liability and safety concerns. Often no one knows who even owns smaller dams anymore, and new landowners may find that it is cheaper to remove a dam than to repair it.
It doesn’t take long for the benefits of dam removal to be observed. While salmon may not return quickly, the habitats critical for fish-egg nests soon reappear. In addition, resident fish benefit from having more space to maneuver and access to headwaters and tributaries from which they had previously been excluded.
“With climate change, stream temperatures are going to increase, so these headwater upland areas are cold-water refuges for fish,” said Magilligan. “Stream temperatures are cooler in well-forested inland areas, which make upstream portions of the watershed more resilient to climate change.”
The Dartmouth professor’s research shows, however, that there are some “social downsides” to dam removal. He said that some people are attached to their nearby dams, and dams are an important historic emblem of the industrial history of the region. Some people also fear that the trapped sediments may be contaminated or that removal of dams may cause flooding. While polluted sediments are a real concern that is usually addressed by state agencies, concerns over flooding are usually unwarranted.
“These old mill dams weren’t built for flood control, so removing them shouldn’t increase flooding,” Magilligan said. “The fear of flooding is warranted when an unsafe dam remains in place, because if it fails, then you’ll have a giant wall of water coming downstream unabated.”
So how do we prioritize dam removal?
Magilligan feels the best approach is first to remove those dams that are closest to failing, then to think about what he calls watershed geometry – identifying those dams that will liberate the greatest amount of free-flowing river. “Once we do something to repair or remove those that are most at risk, we should then focus on removing those that will achieve the greatest amount of ecological benefit. That’s the future,” he said.