The Chesapeake Bay stretches out beyond a boarded-up house on Hoopers Island, Md. (Michael S. Williamson/The Washington Post)

William C. Baker is president of the Chesapeake Bay Foundation.

America’s largest estuary is on the mend. It has been a long road for the Chesapeake Bay. The Clean Water Act of 1972 reduced industrial pollution to the bay and all of the nation’s waterways. In 1984, President Ronald Reagan singled out the Chesapeake for special help, pushing for funding to start a cleanup.

The bay’s recovery, still fragile and with much work to be done, promises to be one of the world’s greatest environmental success stories. But today, instead of seizing the opportunity, the Trump administration is doing everything in its power to stop progress.

It started with a proposal to eliminate all federal funding for the Chesapeake Bay Program, the federal-state collaboration put in place by the Reagan administration. Thankfully, Congress restored that funding, a sign that even in these politically divisive times, the Chesapeake enjoys bipartisan support.

But more recent deregulatory efforts by the Trump administration could be devastating to the Chesapeake’s recovery — and to countless lakes, rivers and streams throughout the country.

Two administration proposals illustrate the problem. The first attempts to weaken Corporate Average Fuel Economy (CAFE) and tailpipe emissions standards. The second would replace the Clean Power Plan . While never implemented because of litigation, the Clean Power Plan would have established the first-ever attempt to limit carbon pollution from U.S. power plants. These proposed rollbacks have been widely criticized for prioritizing corporate profits over global health. Less publicized is the specific harm to water quality.

Air pollution not only poisons our lungs and heats our planet but also eventually ends up in our water. Scientists say about one-third of the nitrogen in the bay comes from air pollution. Excess nitrogen spurs a biological chain reaction that produces “dead zones” of low oxygen spanning several cubic miles each summer in the Chesapeake. Massive fish kills are still common. From Lake Erie to Florida, similar mass poisonings happen throughout the United States, caused by nutrient runoff from land and by air pollution.

Much of the airborne nitrogen reaching the Chesapeake is in the form of nitrogen oxides from power plants, cars and trucks, and industrial sources. Nitrogen oxides can drift for hundreds of miles before falling to the ground and into our waterways. These clouds of pollution also contribute to smog, which harms the lungs of children, senior citizens, and people with asthma and heart conditions.

To understand this phenomenon, look just north of the Chesapeake to Lancaster, Pa., a hot spot of pollution to the bay. Its streams are badly polluted by farm runoff, as well as inordinate amounts of nitrogen oxide from upwind power plants. The Lancaster metropolitan area is among the worst in the nation for soot and smog pollution, according to the American Lung Association’s most recent “State of the Air” report. What doesn’t end up in Lancaster residents’ lungs ends up in local streams. Together with nutrient pollution from local manure and fertilizer, the stew of bad water flows directly into the Chesapeake.

There is some good news. Thanks in part to federal Clean Air Act regulations, air pollution in the United States has been declining. Two years ago, the University of Maryland Center for Environmental Science published a landmark study concluding that those air pollution rules were a major cause of cleaner water in streams in rural western Maryland, and likely the entire Chesapeake. The cause-effect link between clean air and clean water is that strong. The study pointed specifically to 1990 amendments to the Clean Air Act signed by President George H.W. Bush that required coal-fired power plants to install pollution-control systems.

The study predicted that the bay would continue to improve if the federal government followed through on regulations to further reduce air pollution. Indeed, in crafting the historic cleanup plan for the bay, the Environmental Protection Agency relied on CAFE standards and the Clean Power Plan. The agency pledged to support those new air regulations in its role as a partner to the cleanup. Now, the Trump administration wants to weaken them.

The Chesapeake Bay Foundation and its members oppose these regulatory rollbacks, which would damage human health and increase pollution to local rivers, streams and the Chesapeake Bay. They make no sense.

The Trump administration could score an environmental success right in its backyard. The Chesapeake Clean Water Blueprint — the regional cleanup plan — is cooperative federalism at its best. Even then-EPA Administrator Scott Pruitt admitted as much. Studies prove the blueprint would pay huge dividends, including mitigating global climate change. The plan is working. Saving the bay can be a great American success story and a global model for environmental improvement.