In a sedimental
The LDDs may not get the headlines, but they certainly got the numbers. Stanley puts "the best guesstimate " at 2 million in the United States (along with about 75,000 dams taller than 6 feet). In Wisconsin, she says, there is one dam for every 24 kilometers of river, for a total of 3,800 dams.
Image from Maine Atlantic Salmon Commission & American Rivers
Over the past few years, American Rivers has documented the removal of 600 to 700 dams, but Stanley calls that number "really deceptive." In Wisconsin alone, she says, between 800 and 900 dams have apparently been abandoned. In the Northeast, she adds, "Dams used to be like stairsteps along rivers, but they have been dismantled or blown out by floods, or the logging industry blew them up when they were done with them. There have been a lot more removals than we realize."
Once upon a time, you removed a dam with a few sticks of dynamite. Now, you do it with lawyers, public relations, committees, and red tape. Still, the impetus for removal remains, says Stanley. "The landscape has these aging structures that are not doing anything, and we have to balance the ecological cost against community benefits."
Sediment, she said
Sediment can suffocate plants and animals on the river bottom. It can also, says Hart, contribute to "changed patterns of flooding or erosion, or bridge abutments being scoured differently."
With colleagues at the Patrick Center for Environmental Research, Hart has extensively studied the ecological effects of dam removal at the Manatawny Creek in Pennsylvania (see "An Integrative.... " in the bibliography). Along with Wisconsin, Pennsylvania is a leader in the study and practice of dam removal.
The Manatawny study, Hart says, disproved some of the assumptions governing dam removal. While the impoundment had lots of sediment, it was coarse, not fine, so the river did not turn to "chocolate milk" when the dam was breached. Also refuted was the idea that dams must have a big impact on dissolved oxygen, water temperature, and river flow.
In fact, dams where water normally just flows over the top may have minor impact, says Hart. "These little run-of-the-river dams don't really make the river a lot wider, if water is going over the crest all the time anyway, they don't have that big an effect. It's nothing like taking out a big hydropower or flood-control dam."
In general, he says, the height of the dam, combined with how long water remains in the impoundment, are both good indicators about the intensity of ecological effects.
Silt to the hilt?
When impoundments contain more sediment, predicting the fate of that sediment is a major task for advocates and students of dam removal. De-dammed rivers often quickly erode sediments deposited in the impoundment, triggering a series of changes. "The slope of the channel steepens, the river cuts down. This incision [cutting] destabilizes the banks, and they slough off," says Grant. 'That, in turn, contributes even more sediment. The channel moves back and forth across its former deposit and eventually excavates some or all of the sediment it deposited in the past."
Each case is different, he says, and "we don't have the case lore to build up a strong empirical prediction." With sediment, he says, "the big questions are where it's going, how long it will take to get there, and what it will do when it gets there."
T. Mayer, Northwest Indian Fisheries Commission.
Still unclear is whether slugs of sediment flowing down the river will make any difference in the long run. "We don't know," says Grant. "There are reasons to believe it's fairly transitory, but if it does stick around for several weeks or months, it will affect things." Foraging fish, for example, won't see as well, and may have trouble finding dinner. But that drawback may be offset if predatory fish cannot see them. "It's not clear which is better if you are a fish," Grant says.
When the dam was removed, Hart says, "the PCB took off downstream, and now it's one of the biggest Superfund sites in the nation. We don't want that to happen with a small dam removal." (To avoid a rerun of Fort Edwards, researchers draw cores of sediment for contamination testing.) And while river restorationists ardently promote the demolition of dams that are serving no purpose, two facts stand out: some dams may be doing little ecological damage, and the ecological effects of removals are a bit elusive.
©2002, University of Wisconsin, Board of Regents.