Contributed photo by the Tennessee Valley Authority / The Norris Dam in Anderson County was one of the first hydroelectric dams built by TVA in the 1930s

WASHINGTON — The industry that operates America's hydroelectric dams and several environmental groups announced an unusual agreement Tuesday to work together to get more clean energy from hydropower while reducing the environmental harm from dams, in a sign that the threat of climate change is spurring both sides to rethink their decadeslong battle over a large but contentious source of renewable power.

The United States generated about 7% of its electricity last year from hydropower, mainly from large dams built decades ago, such as the Hoover Dam, which uses flowing water from the Colorado River to power turbines. In the Tennessee Valley, TVA operates 29 power-generating dams throughout the Tennessee River system, some of which date back to TVA's creation in 1933, and a pumped-storage facility atop Racoon Mountain in Chattanooga. TVA derives about 10% of its power from its hydroelectric stations.

While these hydro facilities don't emit planet-warming carbon dioxide, the dams themselves have often proved ecologically devastating, choking off America's once-wild rivers and killing fish populations.

So, over the past 50 years, conservation groups have rallied to block any large new dams from being built, while proposals to upgrade older hydropower facilities or construct new water-powered energy-storage projects have often been bogged down in lengthy regulatory disputes over environmental safeguards.

The new agreement signals a desire to de-escalate this long-running war.

In a joint statement, industry groups and environmentalists said they would collaborate on a set of specific policy measures that could help generate more renewable electricity from dams already in place, while retrofitting many of the nation's 90,000 existing dams to be safer and less ecologically damaging.

The two sides also said they would work together to accelerate the removal of older dams that are no longer needed, in order to improve the health of rivers. More than 1,000 dams nationwide have already been torn down in recent decades.

The statement, the result of two years of quiet negotiations, was signed by the National Hydropower Association, an industry trade group, as well as environmental groups including American Rivers, the World Wildlife Fund and the Union of Concerned Scientists. Another influential organization, The Nature Conservancy, listed itself as a "participant," signaling that it was not prepared to sign the full statement but would stay engaged in the ongoing dialogue over hydropower policies.

Bob Irvin, president of American Rivers, which has long highlighted the harm that dams cause to the nation's waterways, said that growing concern over global warming had caused some environmentalists to reassess their long-standing opposition to hydropower.

"The climate crisis has become a lot more acute and we recognize that we need to generate carbon-free energy whenever and wherever we can," Irvin said. "And we do see that hydropower has a role to play there."

Irvin emphasized that his group would still oppose any effort to build new dams on rivers. But that still left plenty of room for compromise.

Energy experts have said that adding more hydropower could provide a useful tool in the fight against climate change. While wind turbines and solar panels are becoming more widespread, they don't run all the time, and hydroelectricity can offer a backstop as utilities clean up their electrical grids.

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File photo / TVA workers check the alignment of a turbine shaft at the top of the guide bearing in the TVA hydroelectric plant at Watts Bar Dam in Rhea County.

In theory, there's potential for the United States to get much more energy from running water. An in-depth study by the Department of Energy in 2016 found the nation could increase its hydropower capacity by 50% without building any large new dams.

Today, less than 3% of the nation's 90,000 dams generate power. There are numerous smaller dams built for irrigation or flood control that could be retrofitted with turbines to produce electricity.

"We're not talking about the Hoover Dams of old," said Jose Zayas, a former Energy Department official who oversaw the study. "There have been some big technological advances that now let us produce more energy in a much more sustainable way."

Some companies are designing new turbines that allow fish to pass safely through, while others are looking at ways to reduce oxygen depletion in the water caused by dams.

One particularly promising approach is to build more facilities known as pumped hydro storage, an old technology that involves connecting two reservoirs of water, one at a higher altitude than the other. When there's surplus electricity on the grid, these facilities use that power to pump water from the lower reservoir to the higher one. When electricity is needed, such as during lulls in wind or solar power, the water flows back downhill, spinning a turbine to generate electricity.

Although many grid operators are now installing large arrays of lithium-ion batteries for this type of storage, batteries can typically only store 4-6 hours' worth of electricity. A pumped-hydro facility could potentially store power for much longer periods of time, allowing utilities to juggle even more solar and wind energy.

The downside is that these massive, billion-dollar pumped-storage facilities face steep regulatory hurdles, and can attract opposition even when they don't require large new dams. While energy companies have proposed or applied for federal approval to build 50 gigawatts worth of pumped-storage projects — roughly 30 times the capacity of all the batteries connected to the grid today — hardly any new pumped storage has been built since 1995.

"Investors tend to be wary of these projects, because there's a lot of regulatory risk," said Lee Bailey, managing director of the U.S. Renewables Group, a private equity fund.

As part of the new agreement, environmental groups and industry said they would collaborate to help expand the pumped-storage market, exploring lower impact off-river technologies and new policy incentives. The groups also said they would work together to make the regulatory process for upgrading and removing dams more predictable.

The groups also agreed to lobby for policies to repair, or in some cases take down, the thousands of aging dams around the country that are in danger of collapse. In May, rain-swollen flooding breached two dams in central Michigan, forcing thousands of nearby residents to flee their homes.

Heavier downpours fueled by climate change are putting many dams at increased risk of failure. Experts have estimated it could cost tens of billions of dollars to repair and upgrade the 15,500 dams nationwide classified as high hazard.

Achieving many of these goals will be difficult, requiring significant regulatory changes at both the state and federal level, as well as major new sources of funding. Many of the nation's dams serve a vast array of purposes, such as producing electricity, controlling floods, irrigating crops and creating reservoirs for boaters. Taking down older dams or upgrading existing ones can often be a complicated process that requires balancing numerous competing interests.