A 2015 view of Nevada’s Hoover Dam, which provides electricity in addition to controlling the flow of the Colorado River. Drought-related reductions in water levels have led engineers to install more efficient turbines at the dam. (Bonnie Jo Mount/The Washington Post)

Long before wind and solar, water was the nation’s top renewable energy source. Going back some 100 years, the United States built enormous dams — like the Depression-era Hoover Dam in Nevada — to produce tremendous amounts of energy.

We have so many such dams that hydropower last year remained our fourth largest source of electricity overall and our single largest renewable source, providing 6 percent of Americans’ electricity. Yet it’s rarely talked about and lacks the excitement attached to other renewables. That’s in part because dams are controversial and can have major environmental consequences, affecting wildlife and altering local ecosystems. New ones also are expensive to build.

“A lot of people, when they think about hydro, they don’t think that there’s much growth opportunity,” said Jose Zayas, who directs the Wind and Water Power Technologies Office at the Department of Energy. “We wanted to really quantify the benefits of hydro.”

A new report from Zayas’s office takes a sweeping look at the state of U.S. hydropower and finds big potential. Based on the work of roughly 300 experts, the report published Tuesday concludes that substantial growth is possible in the sector, considerably upping the percentage of electricity that could come from non-carbon emitting sources.

According to the report, the nation’s current hydropower has an electricity-generating capacity of about 101 gigawatts (or billion watts of instantaneous power generation) that could grow by about 50 percent by 2050. That equates to nearly 50 more gigawatts, which would add a few percentage points to hydropower’s overall contribution to U.S. electricity.

One key reason is the number of existing dams that are not producing any electricity. “We have over 80,000 dams in our country, of which only 2,000 have power production,” Zayas said.

Some could be equipped for generation, even as older facilities could be made much more efficient, the report notes.

Bob Irvin, president of American Rivers, a group founded to protect the nation’s rivers, said he agrees that the country has the capacity to increase the amount of electricity generated by hydropower but that it must be done thoughtfully and cautiously.

“How can we go forward with hydropower, recognizing that it does have adverse impacts, in terms of blocking free-flowing rivers and harming wildlife? It’s certainly better than burning fossil fuels, but it’s not without its own issues as far as climate change,” Irvin said, noting that the reservoirs created by dams can emit methane and alter local plants and wildlife. “There are definitely adverse impacts that have to be considered. … Whenever we build a dam, we destroy a river in one form or another.”

Tara Moberg, a senior freshwater scientist at The Nature Conservancy, which provided input to the new report, said the group does see potential for more hydropower but only if it is done in a sustainable way.

“[Hydropower] infrastructure has powered our growth for the past century, but it has come at a cost,” she said, underscoring that dams have changed river flows, blocked migratory fish and affected water quality in surrounding areas. “Those impacts have had ripple effects on the communities that depend on river resources.”

Moberg said the hydropower of the future must look different than the hydropower of the past. For instance, there have been innovative agreements in some areas, such as on the Penobscot River in Maine, to couple the removal of older dams with enhancements to increase total electricity generation at other dams in the same area.

The Energy Department’s definition of hydropower includes not only dams, which generate electricity when the flow of water turns enormous turbines, but also so-called hydroelectric “pumped storage.” With the latter, large volumes of water are pumped to a higher location where they have the potential to later flow downhill again, generating hydroelectric energy. The storage facility essentially operates like a battery, ready to provide power when needed.

Current U.S. hydropower is mostly based on dams, which produce just under 80 gigawatts. Yet the new report concludes that the future could also see today’s 22 gigawatts of pumped storage increase considerably. In the Tesla era, when energy storage itself is having a major boom, it’s an attractive option.

Finally, the report projects the possible installation of new hydropower facilities in some environments. Environmental considerations would be paramount, of course.

Not addressed are other possible ways of one day getting energy from water, such as marine or riverine hydrokinetics — letting water flow turn turbines without the involvement of huge dams — or wave and tidal energy. These technologies are still a bit too far off, Zayas said.

The benefits of a hydropower expansion would be considerable, the government researchers found: fewer greenhouse-gas emissions, fewer health costs from air pollution and far less water consumed in the process of cooling down power plants.

The expansion of hydro would also further the expansion of its partners, wind and solar, the report contends.

“It’s a really flexible generation source, significantly more flexible than say fossil, coal, significantly more flexible than nuclear, and even possibly more flexible than natural gas,” Zayas said. Plus, the intermittency of wind and solar requires the availability of other electricity sources that can switch on fast to take up the slack when the wind fails or when it isn’t so sunny.

Despite the potential for gains highlighted in the report — which itself makes no specific policy recommendations — the hydropower industry’s growth isn’t guaranteed. Additional funding, research and technological improvements will be required. Not to mention a belief that increasing hydropower is a worthwhile endeavor, particularly given a changing climate.

“The future of hydropower is not in building new dams. It’s in re-powering existing dams, adding power generation to those dams that don’t have it and upgrading and improving the dams that have hydropower in them,” Irvin said. “That’s the kind of future we ought to be looking at, where we can invest in responsible hydropower, while making sure we don’t destroy any of the remaining rivers we have.”