Wherever a hydroelectric dam is built, you’re pretty sure to find activists up in arms.

In Honduras, local human-rights advocates have put their lives on the line to voice opposition to dams under construction that will affect their communities. In the tropical forest zones from the Amazon to Indonesia, environmentalists decry the threat to species and habitats, and also the carbon emissions that result when plant life is submerged by dams. And officials in Massachusetts have stirred controversy by making Canadian hydro imports a centerpiece of the state’s clean-energy plans.

Yet for all the concern about adverse environmental and human impacts, hydro is also continuing to grow as one of the world’s major sources of electric power.

It produces about one-sixth of the world’s electricity – making it the largest renewable in the global mix. And with governments under pressure to address the threat of climate change, it’s seen as a low-carbon source that will retain a prominent place in the world’s energy mix for decades to come.

The next era for hydro, however, won’t look like the past. While proponents have long lauded big dams as both a renewable and cost-effective power source, the environmental controversies aren’t going away, and increasingly concerns such as biodiversity are shaping the industry alongside traditional economics.

How the tensions play out remains to be seen, but hydropower experts including some environmentalists see the potential for hydro to carve out a future where power production and habitat protection coexist.

“If we take a system approach, we can really ensure that we design energy systems that will keep the things we value, like ecosystems and the environment, protected,” says Jeff Opperman, Director and lead scientist of the Great Rivers Program of The Nature Conservancy.

“There will be things that will be lost,” he adds. “A big dam on the river is going to have impacts, it’s going to change the flow and fragment the river. But if they’re well designed and part of an intelligent system, we can hope to do a much better job.”

'A lot of benefits'

The United States is among the testbeds for hydro’s future. River-generated power is expanding, including a capacity increase since 2005 big enough to power more than 650,000 homes. But a big focus is on managing the environmental impacts – through everything from developing more fish-friendly turbines to retrofitting existing dams rather than building new ones. (Many reservoirs in the US and globally have been created for purposes other than electric power.)

And in some cases, hydro’s impacts are being minimized through “run of river” projects that have little effect on a river’s flow.

Even if it means occasionally producing less power than old-style projects would, the possibilities for hydropower to be an ecologically sustainable source of renewable energy are growing, says Michael Sale, executive director of the Low Impact Hydropower Institute (LIHI), an organization that aims to reduce the environmental impact of hydropower.

“Now almost every hydropower project can be compatible with the environment,” Mr. Sale says. “They may have to give up some energy when doing that, but hydropower delivers a lot of benefits when it’s done right.”

Already, he says, the US has the most highly developed hydropower regulatory system in the world – developed in recent decades due to a growing awareness of environmental impacts.

By LIHI’s reckoning, some 130 of America’s roughly 2,000 hydropower projects are certified as environmentally sustainable. The organization rates dams according to eight criteria, including water quality protection, upstream fish passage, and safeguarding threatened and species.

Greenhouse controversy

Despite the promise that Sale and others see, the backdrop for hydropower is still one of controversy. Dam-created reservoirs often destroy floodplain forests or displace communities. And, although many nations tout “clean” hydropower as a significant part of their response to climate change, critics say they aren’t taking into account the large quantities of methane gas that dams create. Methane, a potent greenhouse gas, is released by plant life submerged or trapped by dams.

To top it off, dam construction is often cited as the main reason freshwater ecosystems have declined faster than any other category of ecosystem over the past century of industrialization.

So, where the industry presents the picture as one of vast potential for expanding global power generation, a group of around 300 non-governmental organizations presented a manifesto during last year’s Paris climate conference requesting that large hydropower dams be excluded from the potential solutions for fighting climate change.

The document cited research estimating that the methane released from reservoirs accounts for 4 percent of human-caused climate change. The problem is greatest in tropical climates. “In some cases, hydropower projects are producing higher emissions than coal-fired power plants generating the same amount of electricity,” the appeal said.

The critique came as about 3,700 new dams are being planned or built worldwide – including in tropical regions from Asia to South America. The Amazon basin already has around 191 dams, and almost 250 more are in various stages of planning or construction.

It may take quite a while for the tensions over hydropower to be resolved.

Where the sharpest critics argue that wind and solar are the preferred renewables, backers say hydro done right can remain a cost-effective and responsible part of the answer to climate change.

Small is beautiful – sort of

Many factors can influence the size of a dam’s environmental footprint. Smaller dams are often assumed to be better than larger ones, but even that often depends on where the dam is placed and how the technology interacts with the local environment, experts say.

“There is this tendency to assume that small is low impact, and that’s reflected in policies,” says Mr. Opperman. “But the truth is that a [small] dam in the wrong place can have pretty serious impact.” Instead, he says location can often be the most important factor.

Run-of-river dams are generally considered the best kind of hydropower for the environment because they don’t require reservoirs for water storage. These projects are so named because they divert a portion of a river into a pipe and run it downhill in the direction water flows naturally. Gravity then pushes the water into turbines to generate electricity.

“I do think dams are more efficient and effective the smaller and more decentralized they are, but probably even better than size is whether dams are run of river or not,” says Lea Kosnik, an economist and hydropower expert at the University of Missouri.

Not every topography can accommodate the run-of-river approach. But some nations have found a sweet spot that allows hydro to be the main power source without jeopardizing local ecosystems.

Norway, a country populated by mountains and coastal fjords, gets about 99 percent of its electricity from hydropower. The government there responded to concerns from environmentalists by promoting small-scale projects of 10 megawatts capacity or less, and run-of-river technologies.

Even in nations where few run-of-river projects are feasible, not every hydropower dam has to cause inordinate amounts of environmental damage. As new technologies are developed, like fish-friendly turbines, it’s likely that more dams can become sustainable.

With this in mind, the Nature Conservancy has collaborated with the Army Corp of Engineers to reduce the impact of the dams already in existence. Their aim is to lower the impacts hydropower has on the environment by changing the way dams operate, helping to restore downstream ecosystems, and removing dams in places that have aging infrastructure that can’t be rehabilitated.

On the Bill Williams River in Arizona, for example, the Nature Conservancy altered the quantity and timing of water flows to mimic the river’s natural rhythms.

A tale of two rivers

In some places, making small adjustments to existing dams or choosing a few to remove will have a big impact on local ecosystems.

On both the Yangtze River in China and the Penobscot River in Maine, changes were made so that ecosystems could flourish without reducing the amount electricity provided.

In the case of the Yangtze, that meant cancelling plans for a new dam’s construction.

“It was the last free flowing stretch of the Yangtze and it had hundreds of species, an impressive number of which are found nowhere else, so it’s really important that the ecosystem remain untouched,” says The Nature Conservancy’s Opperman.

The government settled on a plan that tapped hydropower while preventing irreversible damage to the river’s fish habitats.

“We were able to point out that other dams in the area, with just small changes to the way those dams were operated, could actually make up for the energy that would be lost by not building the new dam,” says Opperman. “We used their science to point out that the dam was going to be in a place that didn’t make sense.”

In the Penobscot River basin, a similar scenario played out back in 1999, when federal agencies joined with local conservation organizations and the Penobscot Indian Nation for ecosystem restoration.

Ultimately, two dams on the main river were removed, and a fish passage was constructed on the third dam, opening access to migratory fish. And once again, energy production held steady.

“We happened to have a situation that allowed for the increase of energy at certain sites where it was less ecologically significant in exchange for taking out big dams,” says Laura Rose-Day, Executive Director of the Penobscot River Restoration Trust.

She says the project worked because stakeholders took the specific geography of the area into account.

“One of the messages people take from the Penobscot is that the formula is to increase energy in some places and remove dams in others, and everything will work out fine. And it did,” says Rose-Day. “But it just so happened that the geography worked. It really depends on the right formula for the right place.”

While most of the dams being built today are constructed to the detriment of local ecosystems, such tailored approaches could go a long way to mitigate that harm, experts say. What remains to be seen is how much, and how soon, governments take greater account of the environment when planning projects.

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For hydropower to claim its place among environmentally friendly sources of renewable energy like wind and solar, experts say new approaches will be needed.

“We need to take a serious fresh look at how hydropower interacts with natural systems,” says Ms. Rose-Day. “People need to realize that the old model of all or nothing doesn’t work for hydro in the long-term.”