Perhaps more than any other ecological no-no out there, dams enrage environmental activists. Legend has it that John Muir, founder of the Sierra Club, died of a broken heart after the O'Shaughnessy Dam in Yosemite National Park was built despite his group's protests. These activists argue that you can't redirect millions of gallons of water--even for such worthy causes as flood control or renewable-energy projects--without having at least some deleterious effect on the local environment. But documenting long-term changes to ecosystems along rivers is complex, so such conclusions have been difficult to test.

A recent study of Swedish rivers published in Science, however, has succeeded in quantifying the extent to which biodiversity can be choked off by dams. Researchers at Umea University counted different species of trees, shrubs and herbs at some 90 sites along rivers that had been dammed. Some of the Swedish dams are nearly 70 years old, which enabled the team to examine how ecosystems change over decades. In addition, the group surveyed species along pristine rivers in Sweden--hard to find in an era when the majority of rivers around the world are controlled by dams.

Christer Nilsson, who led the Umea team, recalls that "when I began my career, engineers told me that everything would recover" after dams were constructed. "[We have] now shown that different things happen." Nilsson and his colleagues Roland Jansson and Ursula Zinko demonstrated that in some areas, certain types of trees and shrubs did recover, especially along small, so-called run-of-river impoundments. But in total, the number of plant species fell by 15 percent, and the size of the habitat along the riverbank also decreased. Near larger storage reservoirs, the researchers found that the number of species within a given area dropped by about 50 percent.

More surprising to Nilsson were the long-term trends in these ecosystems. After a dam was built, the diversity of plant species rebounded only during the first 20 or 30 years before tapering off. Nilsson attributes the subsequent scarcity of new species to either a gradual depletion of seeds over the decades or a slow deterioration of the habitat.

Studies such as this one should figure prominently in the ongoing debate about whether and how to maintain aging networks of dams throughout the world. One option being considered in the U.S. is the periodic opening of certain dams. Last year's uncorking of the Glen Canyon Dam and the resulting flood in the Grand Canyon, intended to revitalize riverbanks and wildlife, were ecologically "trivial," according to Jack Stanford of the University of Montana's Flathead Lake Biological Station. "But from a sociological standpoint, it was huge," he says. That brief flood could be the first drop in a very large bucket to restore the ecology of dammed rivers, in which the primary concern is endangered animals, particularly fish.

In an effort to protect salmon populations, the Clinton administration has been pushing for the removal of two dams along the Elwah River in Washington State; at press time, the proposed 1998 budget contained $32.9 million for the project. The U.S. Army Corps of Engineers is studying the possibility of breaching four dams along the Snake River and lowering the reservoir behind the John Day Dam on the Columbia River as part of a plan to protect salmon runs. Even the Glen Canyon Dam has been targeted by a number of environmental groups, including the Sierra Club, that are arguing for its removal.

Opponents of such plans protest that dams are vital to the livelihood of the West. Lewiston, Idaho, for example, is an inland port along the Snake River. Without the current system of dams, jobs in the area shipping goods to market would dry up.

Dismantling dams would take years of construction work. And the payoff could take decades or more, even with extensive environmental rehabilitation. Dutch Meier of the U.S. Army Corps of Engineers points out that the removal of the dams on the Snake River could very likely reveal "scoured, denuded hillsides with entirely changed ecosystems." Meier adds: "Just because you pull the plug on the tub and make the water go away doesn't mean you won't leave a bathtub ring."

--Sasha Nemecek