Researchers at Scripps Institution of Oceanography at UC San Diego and UC Riverside found that inland river dams can have highly destructive effects on the stability and productivity of coastal and estuarine habitats.
The researchers analyzed downstream ecosystems from two dammed and two unobstructed rivers in the Mexican Pacific states of Sinaloa and Nayarit. They found dramatic coastal recession along the mouths of the obstructed rivers, including within vital ecosystems like mangrove forests, which provide protection from storms, commercial fishery habitats, and belowground carbon storage.
The rivers run roughly parallel to each other through similarly-developed land into large coastal lagoon systems. The Santiago and Fuerte river dams provide hydroelectric power for the region, but withhold 95 percent of the river flow. Meanwhile, the San Pedro and Acaponeta rivers are relatively free-flowing and undammed with over 75 percent of the rivers remaining unobstructed.
Over one million tons of sediment is trapped in the dams along the Fuerte and Santiago rivers each year, resulting in striking coastal recession at the rivers’ mouths. This sediment would normally make its way downstream, where it would accumulate along the coast and allow ecosystems like mangroves to grow. Since damming, the Santiago and Fuerte rivers showed an annual loss of coastal lands of up 21 hectares (about 40 American football fields). In contrast, coastline surrounding the San Pedro and Acaponeta river estuaries did not recede, but rather, remained stable or showed sediment growth over the same time periods.
“Similar processes of damming rivers and controlling water flows are destroying estuaries and coasts in many parts of the world,” said Exequiel Ezcurra, lead author on the paper and a professor at UC Riverside and adjunct professor at Scripps Oceanography. “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.”
Coastal recession has widespread economic impacts on the region, including loss of fisheries habitat, reduced coastal protection from storms, decreased biodiversity, and loss of estuarine economic functions, as well as increased release of carbon previously stored in coastal sediments. The researchers tallied the economic consequences of these losses at well over $10 million USD annually, with $1.3 million of that coming from the region’s natural capital in fisheries alone.
“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 Octavio Aburto-Oropeza, co-author and professor at Scripps Oceanography.
The study was published March 13 in the journal Science Advances.
Co-authors Sula Vanderplank from San Diego State University and Lorena Villanueva of UC Riverside, both botanists, also found that this instability directly impacts the floral biodiversity of the coasts. Many of the species disappearing from dammed estuaries are found nowhere else and have high conservation value. The estuarine sandbars of the undammed San Pedro and Acaponeta rivers had significantly greater levels of species-richness than the Fuerte and Santiago rivers.
Hydroelectric dams are championed as sources of renewable energy and low-emission alternatives to fossil fuels. However, the damage that a hydroelectric project can cause on the coast and in the lower part of tropical river basins, in terms of loss of mangrove services and estuarine productivity, may add a significant amount to a dam’s environmental costs, which are rarely calculated.
“This study sheds light on the need to take into account the environmental and economic impacts of hydroelectric dams on ecosystems along coasts and basins,” said Aburto-Oropeza. “We need to fully account for the effects that damming upriver has on the entire region.”
This research was funded by the David and Lucile Packard Foundation, UC MEXUS, and the Leona M. and Harry B. Helmsley Charitable Trust. Aburto-Oropeza and Ezcurra are both Pew Fellowship recipients.