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Environmental Impact of Hydroelectric Dams

Rivers around the world are being tamed by massive hydroelectric dams, with high-profile projects under construction in Laos and China and several proposed for the Mekong River in Southeast Asia. Researcher Guy Lanza of the University of Massachusetts Amherst is working to limit the environmental impact of these projects, which he says often deliver a legacy of economic hardship and health problems instead of prosperity for people living near the dams and downstream.

“After dam construction, there is an immediate drop in water quality that destroys useful fish populations and poses a threat to livestock and humans,” says Lanza, an aquatic biologist and microbiologist who consults for the environmental organization International Rivers. “Converting river systems into lakes also creates more habitat for the snails and mosquitoes that carry malaria, dengue fever and schistosomiasis, leading to an increase in the number of cases of these diseases.”

Lanza recently critiqued the Nam Theun 2 Hydroelectric Project, a 1,200 foot wide expanse across the Nam Theun River in Laos that will be completed in December 2009. Dam gate closure and reservoir filling will begin in June 2008, and power from the dam will be exported to Thailand as part of the Laotian government’s plan to generate export revenue by building more than 30 dams by 2020.

After reviewing the NT2 Environmental Assessment and Management Plan, Lanza worked with International Rivers, and together their efforts helped to convince the Nam Theun 2 Power Company to remove some of the biomass, in the form of fallen trees and leaves, prior to filling the reservoir instead of simply leaving it behind to rot. The experience with Nam Theun 2 has highlighted the importance of clearing biomass from future dam projects, a requirement the Laotian government is reportedly considering.

Lanza says, however, that the Nam Theun 2 biomass clearance plans, which include cutting and burning biomass from part of the reservoir area, may not prevent significant water quality problems. “Burning biomass adds air pollutants, including carbon dioxide, ozone and other greenhouse gasses, and toxic substances such as mercury,” he says. “Burning will also release mercury to the soil and greatly accelerate the release of nutrients such as nitrogen and phosphorus from the biomass.”

After burning, nutrients from the ash would trigger and support the sudden growth of excess bacteria and algae in the water as the reservoir fills, triggering a cascade of water-quality problems, including greatly reduced dissolved oxygen, fish kills, the formation of toxic metabolites by cyanobacteria and the release of toxic gasses and metals such as hydrogen sulfide and mercury from reservoir sediments.

Leaving the biomass behind would also be problematic, says Lanza, since rotting vegetation would increase greenhouse gas emissions from the reservoir, use the available oxygen in the water, cause fish kills and result in water that was unsuitable for drinking and irrigation.

Lanza started his career as an aquatic ecologist for the Smithsonian Institution in 1971, living in Thailand and doing the detective work to predict the ecological impacts of a proposed hydroelectric dam on the Mekong River. That research led to two new discoveries: a new species of schistosome parasite (the Mekong schistosome), and a new species of snail that transmits the Mekong schistosome. He says it was obvious then that water quality would decrease, and the shores of lake behind the dam would provide the ideal habitat for the snails and insects that carry disease.

In 1996, Lanza reviewed the environmental impact report for the Nam Leuk Hydropower Project in Laos and visited the site after the dam was complete, which reinforced his concerns and confirmed his predictions.

“After the completion of Nam Leuk, there was a sharp drop in the oxygen content of water in the lake and blooms of cyanobacteria that release toxins that are deadly to livestock and can cause liver cancer in humans,” says Lanza. “The data show that water-quality problems eliminated useful species of fish that people depend on for food and livelihood, replacing them with less desirable species.”

Villagers downstream of the project were experiencing water-quality problems, and the Nam Leuk reservoir provided expanded habitats for the snails and mosquitoes that carry schistosomiasis and malaria, threatening a rise in the number of cases of these diseases.

“Midstream dams are again being proposed for the Mekong River, and we are finding that disease-causing schistosomes are much more prevalent in this area than we originally thought. This must be considered when developing future environmental assessment and management plans,” says Lanza.

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