In a blow to the image of hydropower as a source of green energy, scientists have found that damming rivers can indirectly dump more carbon dioxide into the air than the electricity they generate saves in fossil fuel emissions.

In a study published in Science Advances, researchers led by Octavio Aburto-Oropeza, a marine ecologist at Scripps Institution of Oceanography in California, US, looked at four rivers along Mexico’s western coast, north of Mazatlan.

Two of these rivers, the Santiago and the Fuerte, are dammed. The other two, the San Pedro and the Acaponeta, still run free. All four feed into large estuaries lined with mangrove forests.

Using satellite images dating back to 1975, Aburto-Oropeza and his colleagues compared the mangrove forests on dammed rivers to those on the undammed ones.

On the undammed rivers, they found, the mangrove forests were either stable or expanding. On the dammed ones, however, they are shrinking, rapidly.

“We are losing tens of hectares a year,” Aburto-Oropeza says. “It’s very dramatic.”

The difference, he explains, stems from that fact that even though the dams are many kilometres inland, they trap sediments that would normally make their way to the estuaries — sediments needed to keep the mangrove forests from being slowly consumed by encroaching ocean water.{%recommended 8292%}

One major consequence of this is the demise of fisheries dependent on mangroves to provide habitat for juvenile fish and crabs. There are also substantial effects on species diversity in the estuaries of dammed versus undammed rivers, with sandbars in the two undammed rivers featuring 26 to 33 plant species, while sandbars in the dammed ones only featured four to eight.

As mangrove forests die, they also release carbon dioxide back into the atmosphere, thereby contributing to global warming.

Mangroves sequester carbon in several ways, Aburto-Oropeza says. Some is contained in their trunks, roots, and other biomass. More is accumulated in the sediments trapped by their roots. Thus, when mangroves are lost, the biological “machine” responsible for sequestering carbon disappears, as well as large amounts of previously sequestered carbon.

“Some of these mangroves have been growing on top of themselves for 3000, 4000, or even 5000 years,” he says.

All told, he adds, in the estuaries in question, this might add up to between 900 and 3400 tonnes of carbon per hectare.

That’s enough to require a rethinking of views on hydropower, especially on rivers like the ones Aburto-Oropeza and colleagues studied, where water flows are seasonal, with significant power generation only during the rainy season.

“Some of these [hydropower] plants work very few months during the year,” he says.

And, he says, his finding understates the total effect, because it ignores impacts upstream from the estuary.

“These hydroelectric plants are contributing to the climate change problem,” he says.

Amy East, a geologist at the US Geological Survey’s Pacific Coastal and Marine Science Centre in Santa Cruz, California, US, is impressed.

“It’s an interesting study,” she says. It’s long been known, she adds, that the sediment-trapping properties of dams can have profound downstream impacts.

“A common effect is the ‘hungry water’ result,” she explains, “whereby sediment-starved water eats away at the riverbed downstream of the dam, causing the widespread erosion of sandbars.”

Sediment starvation is also known to cause river deltas to subside and become uninhabitable.

But there had been little previous attention to its effects on estuaries and ecosystems, or on the economic consequences to fisheries.

“I’m not aware of another study that so comprehensively assesses these three angles together,” East says.

Normally, Aburto-Oropeza adds, cost-benefit analyses relating to dam building focus only on the area near the dam itself, including the soon-to-be-submerged river valley.

“What we did is to follow the impacts all the way to the river mouths,” he says.

It’s a reminder of the 1970s environmental slogan TANSTAAFL, borrowed from a science fiction novel by Robert A. Heinlein and short for “There Ain’t No Such Thing As A Free Lunch.”

In this case it suggests that even the greenest of technologies can have negative side-effects that must be taken into account when seeking the most environmentally friendly option possible.

That said, there may be ways to build hydropower systems that do not starve rivers for sediments, Aburto-Oropeza says.

One such approach might be the construction of “run of the river” hydro projects in which water is diverted upstream and piped to a lower-elevation power plant, without ever fully blocking the flow of the river.

“It seems that the technology already exists where you [can generate hydroelectric power] but allow sediments to continue moving down the river,” Aburto-Oropeza says.

But unfortunately, he notes, “in developing countries there is still this idea that dams are the best things to generate electricity and pollute less”.