Roughly 700 feet above the Hudson River, 60-some miles north of New York City, Harrison Ford dips his Bell 407 helicopter toward the river's eastern bank and frowns. You know the frown, of course: It's the frown on Han Solo's face when he's ensnared in the Death Star's tractor beam, or of Jack Ryan untangling some terrorist plot. Twenty-five feet tall on the silver screen, that frown – with its crooked, clenched jaw – has become iconic. Up in Ford's helicopter, however, it merely means that something is wrong. "There's a brown pipe coming out from that quarry," Ford says to his passengers. "Comes out at the woods there. See it?"

Seated beside Ford is Basil Seggos, a 27-year-old legal investigator for Riverkeeper, an environmental watchdog group that tracks and prosecutes Hudson River polluters. Seggos cranes his neck for a better view. "Whoa," he says. "Look at this." Down below, a rust-colored pipe leading out from a gravel quarry is spewing a foamy jet of pump water into the wetlands that line the river. "We're flying directly over a pipeline that this quarry is using to pump out sediment," he explains to the three of us in the helicopter's rear: Alex Matthiessen, Riverkeeper's executive director; photographer Timothy White, an old pal of Ford's; and me. "Oh, man," says Seggos, his voice almost cracking. "That's gotta be illegal."

In April, after years of hunting down polluters with kayaks, powerboats, hiking boots, and subpoenas, Riverkeeper's investigators received an offer that they leapt at: A New York-based pilot was willing to take them up on aerial surveillance runs, and, to boot, he had a photographer friend willing to document the abuses. That this pilot, a longtime Riverkeeper patron, happened to be the biggest-grossing star in cinema history was a glitzy bonus, but it was the prospect of using Harrison Ford's helicopter that truly thrilled Seggos and company.

"See all that shit coming out of it?" says Ford, his eyes on the pipeline. "Tim, I'm going to give it to you on the right here." White lifts his camera up to the window glass, aims, and focuses. "Go ahead, Tim," says Ford. "Fire away." Click. Within each frame, gallon after gallon of discharge spills into the marsh and seeps toward the Hudson in a curved plume, marking its entrance into the river's wide stream. Click. Click. Click-click-click. Up front, Seggos is giddy; he and Matthiessen came with a list of surveillance targets – sprawling riverside junkyards, leaky Superfund sites, illegally filled-in-wet-lands – but this discovery is a surprise. "We never would have seen that without a flyover," Seggos says to Ford. The actor doesn't respond, which is not unusual for him, but the frown seems to soften.

This isn't the first time Harrison Ford has put his helicopter to good use. Two years ago, he piloted a rescue team to the top of Wyoming's Table Mountain, near the home in Jackson where he lives part time, to help a 20-year-old hiker who was too sick and dehydrated to descend. Ford ferried her to a local hospital, where she recovered. Last year, he located and rescued a Boy Scout who'd been missing for a night in Yellowstone National Park.

Not that Harrison Ford wants you to call him a hero. He doesn't want you to think, "Gee, what a guy, saving Boy Scouts and damsels, whapping polluters with a real-life bullwhip." And he certainly doesn't want you to say that the planet would be worse off if it weren't for him. In fact, Ford really doesn't want to talk about this extracurricular stuff at all. He'd almost prefer that questions be addressed to his helicopter, since it's really the helicopter doing the heavy lifting. That's why Ford has rarely taken the standard celebrity tack in do-gooding, that is, being a spokesman. "Emotion is the language of movies," he once said. "Not yammering on." It's a dictum he applies to the wider world as well.

As a 10-year board member of Conservation International, a nonprofit group that seeks to safeguard biodiversity trouble spots around the globe, Ford has traveled to endangered zones in Brazil, Venezuela, and other South American nations. "He's an engaged and immersed leader," says Peter Seligmann, Conservation International's chairman, "who's deeply involved in the substantive issues of the organization. Visibility is the most minor part of what he gives us. He's a strategic thinker, and as passionate an advocate as we have." In other words, Harrison Ford is a man of action, literally. "I am not," as Ford says, "a poster child."

Case in point: Riverkeeper. Ford had been sending checks to the New York-based organization for years when he encountered Riverkeeper's chief prosecuting attorney, Robert F. Kennedy Jr., at a fundraiser this spring. Although Ford spends a good deal of his time at his 800-plus-acre Wyoming ranch, he keeps his main residence close to the Hudson River, in New York City, where two of his children attend private school. An avid, even obsessive, pilot for the past six years, he stores some of his six aircraft – including a single-engine de Havilland Beaver biplane, a twin-engine Gulfstream IV jet, and his $2 million Bell helicopter – in a hangar in Teterboro, New Jersey, just across the river.

"I think I said to Bobby, 'If there's anything I can do to help . . .'" Ford recalls. "He said he might be looking for pilots to fly the Hudson watershed, and I said, 'Let me know when you're ready, and I'll do what I can.'" Kennedy says the surveillance flights were Ford's idea, not his, but we can probably chalk that disagreement up to dueling modesty. Either way, Riverkeeper gained its first aerial volunteer.

Back in Ford's helicopter, Matthiessen follows up. "We recognized that with a staff of 15 or 20, it would be impossible to crack down on all the polluters along the river. So we've embarked on a program to enlist average citizens, and" – with a nod toward Ford – "not-so-average citizens, to be our eyes and ears. The idea is to allow local people to do some of the work of protecting their river – to let them serve as mini-riverkeepers."

"So, something like a neighborhood watch for the environment?" I ask.

"Exactly," Seggos says. "By the program's first anniversary, we'd recorded 169 complaints, and the rate is skyrocketing. We accept help from anyone who wants to give it to us, from anonymous sources, at the low end of involvement, and, at the another end, a few full-time volunteers: fishermen, kayakers, people like that. Right now, though, Harrison is the only volunteer who can give us an aerial view."

"Red-tail right beneath us," Ford announces. All eyes focus on the hawk as it glides upstream, until a junkyard strewn with tires and rusted automotive husks appears on our left. White lifts his camera as Ford swings the helicopter toward their target.

Riverkeeper grew out of the dismay of fishermen, both commercial and recreational, back in the sixties, when the Hudson River was a national joke, a viscous, sludge-streaked, chemical-ridden brown sewer line running beside New York City. Under the provisions of an obscure 1888 law that awards citizens a bounty for turning in polluters, the fishermen began waging a legal war against companies that were dumping into the Hudson, shutting down the polluters and collecting half the fines. In 1983, the Hudson River Fishermen's Association, as the organization was then called, hired its first riverkeeper, a full-time investigatory with a commission to patrol for evidence of pollution. A year later, Robert F. Kennedy Jr., the son of the slain U.S. senator, joined the organization, eventually becoming chief prosecuting attorney in 1993. His goal was simple: expose and prosecute, expose and prosecute. In the past two decades, Riverkeeper has used that tactic to nail more than 300 polluters, including behemoths like Exxon, General Electric, and, most recently, the city of New York, which pleaded guilty last year to negligence that had led to mercury leaking into the drinking supply.

The resurrection of the Hudson River, partly creditable to Riverkeeper's dogged fieldwork and legal campaigns, has been dramatic. Some anecdotal evidence: My next-door neighbor is a commercial fisherman on the river, which is walking distance from my door. Just the other day, he delivered to me a weighty sack of soft-shell crabs he'd pulled that morning from the Hudson, a routine kindness that not so long ago would have been unthinkable. Two decades back, a sack of Hudson River crabs on the doorstep would have carried the same message as a black rose. But today? "Today, it's the only major river system that has strong spawning stocks of its historic migratory fish," says Kennedy. Though polluters still menace it (and fish-consumption warnings still stand), the single largest threat to the river these days is from development; after decades of strenuously avoiding the Hudson, says Kennedy, "people now want to live beside it."

Unintended consequences aside, the river's turnaround has proved a durable model for environmentalists nationwide; there are currently 90 licensed "keepers" on waterways across America, with an additional 300 applications awaiting approval, according to Kennedy. "We get up every morning and fight for rivers," he says. "We'd like to see ourselves put out of business, but I don't think that's going to happen."

A few hundred feet about the Hudson's eastern bank, Ford's helicopter hovers atop a square of land jutting into the river in a clearly artificial way – clearly artificial, at least, from this altitude. "This is a yacht club," explains Seggos. "A while back, they illegally filled the riverbank to make themselves some new space. We also have an informant telling us that oil drums and old appliances were being dumped here."

"This was a case referred by a citizen watchdog," adds Matthiessen.

"Looks like they filled in about an acre, huh?" says Ford, and the frown returns. The Hudson may be 306 miles long, and an acre just an acre, but it's this type of thing, this gnawing away at nature, that annoys him. "We keep pulling out pieces of the puzzle," he tells me later, "and expecting it to heal itself." The Hudson may be hemmed in and its banks built up, but its current still runs wild.

Wildness, on water and land and in the sky, has beckoned Ford since his childhood. As a boy growing up in Morton Grove, Illinois, Ford retreated from neighborhood bullies – who liked to roll him down a hill – into the wildish edges of his subdivision, which sparked his early ambition to be a forest ranger. But after flunking out of Wisconsin's Ripon College, Ford headed to California, where he flirted with acting and then went with carpentry, which provided him a living for nearly a decade. Then came American Graffiti (1973), then Star Wars (1977), and you know the rest. Never comfortable with fame, never at ease with Hollywood's tacky apparatus, Ford found refuge in his Wyoming ranch, and then, later, in the cockpits of his plane and helicopter, where, he says, he can keep "from thinking about anything but flying." That and the wide earth below him.

"There's still time left to hold the line," he tells me, "to save sufficient biodiversity, to preserve what's necessary for nature to function." Does it ever frustrate him, I ask – flying over some denuded stretch of clear-cut forest? "No," he says. "Never. People don't need to despair. They need to know that there are solutions – good, practical solutions – and they need to keep up the good fight."

"You getting your shot back there, Tim?" Ford says as he buzzes the yacht club, dipping toward it. "About three o'clock on the right, where it juts out into the river. I'll make another pass. The rule, by the way, in case anyone is interested, is that we're supposed to be 2,000 feet horizontal and 1,000 feet vertical from any person or object. Obviously we've dicked the shit out of that one today."

Call it vigilante environmentalism. If the government can't enforce the laws, the people will, by finding and exposing polluters, taking them to court, forcing them to shut off an ugly spigot. "We usually get calls before the government does," Kennedy tells me later. "If you call the Department of Environmental Conservation, or the EPA, whomever, the chances of something getting done are slim. And don't even think about calling after hours or on a Saturday. But if people call us, they know that something will get done." Riverkeeper's mission is not fluffy and utopian; it's a grouchy, populist, your-dog-can't-shit-on-my-sidewalk kind of environmentalism. As Ford puts it, "This is simple stuff; that's a violation of the law, and the law is there to be enforced."

Up above the Hudson, headed south, Seggos is explaining what will happen next with the quarry we spied earlier. "We'll talk to the state Department of Environmental Conservation, check on the permits using the Freedom of Information Act, investigate it further on the ground, and then, if necessary, take them to court." (As it turns out, the quarry does have a permit to discharge 8 million gallons per day of "pump-out water and storm water," and the state alleges that the drainage does "not have a significant impact on the environment." Which means it's legal, but, as Seggos later tells me, that doesn't always mean it's kosher. Riverkeeper has filed a complaint with the state about the quarry's permit, and investigators will collect water samples in the fall, from boats and kayaks, to monitor the discharge's actual contents.)

"See all those swans over there?" Ford cuts in. Near the Hudson's western edge, a bevy of swans sit bobbing in the river. "They're trumpeters," he says, and banks the helicopter eastward. "I don't want to disturb them." The swans continue their rest as if they never noticed us, high above them in this jet-powered contraption, and that, it seems, is exactly the point. "That's cool," Ford whispers, and, with the Bronx ahead of us in the hazy urban distance, he follows the river home.