The two streams trickling through the Hollin Hills neighborhood in Northern Virginia have long been part of the charm of the woodsy community, where homes were built without fences in what was meant to be a shared sense of harmony with nature.

But after years of polluted storm water runoff rushing in from nearby gutters, the streams are eroding — and so is the feeling of togetherness in an enclave of $1 million houses that has become part of a larger debate over how to fix damaged streams in the Chesapeake Bay region.

A normally cheery online neighborhood forum has crackled with angry exchanges about a Fairfax County plan to deal with the polluted waterways, with one side anxious about the ragged stream beds and the other worried their historic neighborhood will be overrun by a massive government project that will destroy dozens of towering trees.

Each group has accused the other of fearmongering, leading to a “real bitterness,” said Monique Derfuss, who operates a gong meditation and yoga studio near one of the streams and is against the county plan. “Some people give me the stink eye because they know . . . I’m part of this group.”

Since 2010, when the Obama administration enacted federal water quality requirements for the bay, 142 miles of streams have been repaired in its watershed, with about half of that work occurring in the District, Virginia and Maryland, according to the Environmental Protection Agency.

A billion-dollar industry has emerged as local governments work to stay below EPA limits for urban runoff that allow them to qualify for storm water permits and that help determine federal funding to states in the Chesapeake Bay watershed.

But environmental scientists say it is unclear whether the high-cost projects are worth the investment. The work typically uses heavy machinery to clear old trees and plant new ones around re-engineered streams that contain boulders, wood and vegetation meant to absorb harmful pollutants.

In some cases, such projects may be hurting surrounding wildlife unnecessarily, some experts say.

“You modify the system so much that you risk transforming a stream ecosystem into something else. And the question becomes: Is that good?” said Solange Filoso, an aquatic biologist at the University of Maryland’s Center for Environmental Science who advocates for smarter stream restoration designs and a greater focus on the sources of urban runoff.

“These restorations are not so reliable that they justify changing a stream ecosystem so dramatically for a result that is not 100 percent guaranteed,” Filoso said. “I think that we may be losing a lot more than we’re gaining.”

Most stream restorations are geared toward state and federal mandates for reducing the amounts of nitrogen, phosphorus and sediments — byproducts of urban runoff — draining into the bay every year.

Filoso said there has been a tendency for projects to be done near the bottom of watersheds, based on the assumption that they will filter out greater quantities of pollutants, allowing local agencies to claim they are closer to meeting those mandates.

Some of the “wetland complexes” created by the restorations appear to be successful at absorbing nitrogen, she said, but are not as effective at keeping phosphorus and small particulates of solid waste from entering the bay.

“They’re trying to create little filters at the end that can solve all the problems in the watershed,” Filoso said. “It’s not happening.”

Thomas Jordan, a senior scientist at the Smithsonian Environmental Research Center, said a fair amount of guesswork is involved in the effort. He cited a $1 million project on his center’s property in Anne Arundel County that initially caused the water to turn a rusty color — because of iron leaching out of rehydrated soil — and, later, appeared to be no more effective at removing pollutants than a beaver dam farther downstream.

“And the beavers do that free,” he said.

Jordan said growing urgency about the bay has allowed projects to go forward without conclusive evidence as to the best approach.

“They’re going to try some stuff, where it looks like it might work,” he said. “The rationale is: ‘We need to do something now.’ ”

Rod Simmons, a plant ecologist who works for the city of Alexandria, said the cumulative damage to the region’s tree canopy because of stream restorations over the years should be alarming to local government officials.

He and other critics say the government should instead prioritize “low-tech” repairs that rely more on dead trees and other vegetation in the area to fortify streams.

“As a society, we get out of sorts over the loss of one acre of Amazonian rainforest, but we’re doing the same thing right here,” Simmons said. “You can’t replant a forest, as much as these guys like to say you can. They put young trees back in and some seed mixture . . . but that hardly replaces the biodiversity that was there.”

In Annandale, a $1.7 million Fairfax County restoration of Indian Run required dozens of trees to be cut down. After nearly a year of construction, the now-widened waterway is fortified with large boulders. It winds through a clearing where young trees will be planted to replace the older ones.

But homeowner Laura Anderko said a family of barred owls that nested there seems to have disappeared. And polluted water still comes gushing down a long concrete chute from a nearby school parking lot during heavy storms.

“They’re not fixing the source of the problem,” said Anderko, a Georgetown University School of Nursing and Health Studies professor who focuses on the public health effects of climate change. “If we’re doing this in the name of the environment, then we should be very thoughtful about how we’re going about it.”

Local and state officials in the Washington region say the stream restorations have been vital in storm water management programs that include upgrading infrastructure, monitoring for illegal dumping and reimbursing homeowners who plant vegetation to prevent runoff.

Several jurisdictions have reaffirmed their commitments to such projects in recent weeks, after an EPA official in charge of the Chesapeake’s cleanup called “the maximum daily loads” of pollutants allowed in the bay before it is considered unhealthy — known as TMDLs — “aspirational.”

“The Bay TMDL is not simply aspirational, it’s enforceable, and it’s not simply informational, it’s integral to our success,” Ben Grumbles, secretary of Maryland’s Department of the Environment, said in a statement.

Both Maryland Gov. Larry Hogan (R) and Virginia Gov. Ralph Northam (D) say they’re considering legal action to force the EPA to be more aggressive about bay cleanup efforts. Northam’s proposed budget also allocates $182 million to the Stormwater Local Assistance Fund, which helps finance stream restorations.

Fairfax County, the region’s largest jurisdiction, has been the most active on stream restorations, completing 71 since 2010, with seven under construction. The one planned for Hollin Hills has generated the most controversy.

That $2.6 million project began after Elisabeth Lardner — a resident who oversees park maintenance for the neighborhood — noticed a homeowner’s sewer line had become exposed and was leaking raw sewage into the eroding stream bed.

Lardner, whose landscape architecture firm worked on park redesigns for Fairfax that have included stream repairs, asked the county to revive a dormant 2004 plan to restore the two streams in her neighborhood.

Fairfax officials readily agreed, seeing an opportunity to report another storm water cleanup project to state officials while eliminating a safety hazard caused by a deepening gulch in one of the streams.

“To us, it was a win-win,” said Meghan Fellows, a county ecologist overseeing the project, which the government estimates will stop 718 pounds of nitrogen, 360 pounds of phosphorus and 20 pounds of sediment from entering the Chesapeake every year.

The scale of the plan alarmed some residents. Soon, the neighbors were shouting during heated meetings.

Proponents of the restoration plan warned that the streams’ erosion would cause trees to come tumbling down on people’s houses and argued that the county is sparing the civic association the cost of repairing the streams itself.

Opponents — many of whom live near the streams and would be most affected by the construction — countered that some of the trees to be cut down are more than a century old and cannot be adequately replaced.

The fight escalated after the civic association voted in November to allow construction workers access to the streams, a decision opponents argued was rigged.

“My crap detector has been going way off the whole time,” said Jeff Chown, who has led the fight for cheaper stream repairs done without the county’s help.

Chown, whose work as a residential architect includes saving trees from potential harm, drew fire from several neighbors when he suggested that Lardner was working to push through the county project because she would benefit professionally in some way.

Lardner called the suggestion “offensive.”

“I don’t understand what our other choices are,” she said.

The hostility is regrettable, said the civic association’s president, Patrick Kelly. But the stream deterioration, he added, is urgent.

“There’s definitely some pain as a result, especially for homeowners who live . . . where the construction vehicles will be entering and leaving their materials,” Kelly said. “It’s a challenge.”

Supervisor Daniel G. Storck (D-Mount Vernon), who represents the area, said he supports the project but is open to scaling it down. The overall goal, he said, is dealing with the hundreds of streams in older neighborhoods that have been overwhelmed by a tsunami of urban runoff.

On a recent afternoon, aluminum tags hung from dozens of trees surveyed by the county, in part to determine whether they will be cut down when the project launches later this year.

Marc Shapiro, 71, said the loss will not be worth whatever the restoration accomplishes in the neighborhood he’s known since childhood. He pointed to a broken storm water pipe and the puddle of water below it, which sometimes dribbles toward one of the streams.

“And they want to come the last 100 yards up here to fix that and cut down trees all along the way,” Shapiro scoffed. “This has been there forever. It hurts nothing.”