Upriver Dams Doing Damage to Coastal Ecosystems

(CN) – Dams erode tropical coastlines at a rate that may outweigh the environmental benefits of clean hydroelectric energy, according to a study published Wednesday in the journal Science Advances.

A team of researchers led by Exequiel Ezcurra at University of California, Riverside, compared four rivers originating in Mexico’s Sierra Madre mountain range, flowing parallel to each other into the Marismas Nacionales or the Ahome wetlands. The Fuerte has been dammed for 61 years and the Santiago has been dammed for 23 years. The Acaponeta and the San Pedro, meanwhile, flow free. 

Estuaries, wetlands and lagoons, the fertile environments where freshwater and saltwater mix, teem with wildlife. They sequester carbon, protect coastlines from big storms and provide jobs and subsistence through fishing. They also act as a nursery for the ocean, a place where a wide variety of species rear young before living lives at sea. And the authors of the study say those contributions – and their precipitous loss – are not usually calculated as part of the cost of building dams.

Where the rivers are dammed, the coastlines shrank at a rate between 13 and 35 football fields per year since 1975, the team found. Coastlines at the mouth of the Acaponeta River were stable during the same period, while the coast of the San Pedro grew by absorbing more river sediment at a rate of between 8 and 12 football fields per year.

The diversity of plant species also plummeted on the coasts of the dammed rivers and the impact on coastal fisheries was severe, the team found. More than three times the fishing boats were able to work the undammed San Pedro estuary than made a living fishing the dammed Santiago estuary. And the San Pedro yielded 658 tons of shrimp, oysters and fish during the 2007-2008 season, while fishermen on the Santiago that same year were able to catch only 35 tons.

Coastal mangrove forests are some of the world’s most carbon-rich environments, sequestering an average of 1,000 tons of carbon per hectare of forest. The team estimated that dams on the Santiago River had caused the release of 6,000 tons of carbon per year.

Most of the damage comes from the way reservoirs behind the dams on the two rivers trap over one million tons of sediment that would normally flow downstream to replenish estuary soils, feed sandbars and deltas and grow the coastline. But problems also arise from altering the flow of rivers, converting natural pulses created by rain and runoff to the smaller, steady flow that passes through dams. Reducing the amount of water passing a dam also increases the salinity of estuaries downriver, which harms wildlife and ruins drinking water.

Trapping sediment threatens the existence of the complex lagoon system of the Marismas Nacionales, which was created by thousands of years of pulses of sediment-rich river water – a process called coastal accretion, the team found.

“Reductions in sediment loads from sediment trapping in reservoirs may reverse the historic accretion of the lagoon systems, opening the way to the erosion of sandbars and coastal ridges and, potentially, to the destruction of the coastal wetland ecosystems,” the researchers wrote.

Dams are often seen as reliable producers of clean energy. But the team stressed that rosy view may stem from a lack of science showing the true environmental effects of dams.

“The damages a hydroelectric project can cause in the coast and the lower part of tropical basins, in terms of loss of mangrove services and estuarine productivity, may add a significant amount to the environmental costs of a dam and are rarely calculated,” researchers wrote.

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