According to a new study, coastal ecosystems suffer when hydroelectric dams are built upstream.

Mangrove forests, wetlands and other estuarine habitats are already facing the threat of rising sea levels. Now, new research suggests these ecosystems are disrupted by upstream dam construction.

For the new study -- published in the journal Science Advances -- scientists studied four rivers in the Mexican Pacific states of Sinaloa and Nayarit, two dammed and two mostly unobstructed.

The Santiago and Fuerte rivers both feature large hydroelectric dams that produce large amounts of electricity. The two dams block 95 percent of the flow in each river. San Pedro and Acaponeta remain 75 percent unobstructed, making them relatively free-flowing. All four rivers follow parallel paths from the mountains to the Pacific and run through similar terrain.

As the new research revealed, more than a million tons of sediment are blocked by the two hydroelectric dams. Sediment and fresh water are the life blood of coastal ecosystems. Since the Santiago and Fuerte became obstructed by dams, the rivers' annual rate of coastal land loss has increased 21 hectares.

Coastal lands in the mouths of the San Pedro and Acaponeta remained stable during the same time period.

"Similar processes of damming rivers and controlling water flows are destroying estuaries and coasts in many parts of the world," lead study author Exequiel Ezcurra, a professor at the University of California, Riverside, said in a news release. "Despite the huge implications for ecosystem conservation, the process of coastal degradation as a result of large dams has not been well-studied or quantified using a strict comparative approach."

Wetlands provide a range of important ecological and economic benefits. In addition to providing protections against flooding and storm surges, estuarine habitats support commercial fish stocks. Estuarine habitats also shelter a large variety of unique plant and animal species, as well as help sequester carbon.

For the latest study, scientists estimated the economic value of the services lost when a river is dammed at $10 million annually.

"The benefits of ephemeral jobs generated around the construction of the dam need to be weighed against the long-term costs the dam will cause to local livelihoods," said co-author Octavio Aburto-Oropeza, professor Scripps Institution of Oceanography.

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Demo outside World Bank offices in Beirut over dam project
Beirut (AFP) March 4, 2019
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