Most of the time, a mere ribbon of water and algae collects in the center of this 40-foot deep, 100-foot-wide culvert, and in 1989, a local politician actually proposed turning the river into a dry-season freeway. So when we round a bend in the concrete-walled canyon and spot the maintenance truck rolling down the streambed, it's hard to say which is more out of place—the four men in orange shirts and rubber boots scooping debris into the back of the truck, or our fleet of 12 paddlers in yellow kayaks and one green canoe.

I try to float casually by—hard to do on three inches of water–but the crew boss walks straight toward me through the ankle-deep water with a cell phone to his ear and an officious look on his face. He doesn't say hello; just fires the question we've all been dreading: "Do you have a permit?"

THE CITY OF LOS ANGELES HAS BIG PLANS for the concrete-clad waterway that bears its name. After decades of citizen lobbying, in 2007 the city embraced the Los Angeles River Revitalization Master Plan, a $2 billion blueprint encompassing 239 potential projects to create greenways, reduce flooding, and restore wildlife habitat throughout the river's 51-mile course. In the words of Mayor Anthony Villaraigosa, it will create an "emerald necklace" through the industrial heart of Los Angeles, which has fewer parks per capita than any other American city.

The following summer, however, the L.A. River's prospects dimmed when the Army Corps of Engineers decreed that it is not a "traditionally navigable waterway." That decision exempted most of the watershed from Clean Water Act protection and put at risk the master plan's primary goal—citizen access to a clean, revitalized river. Suddenly our trip leader George Wolfe, who had been planning the expedition to see the river before the master plan is implemented, had a larger cause and a much bigger stage. Navigating the full length of the river, even in sit-on-top kayaks, would expose the folly of the Corps' ruling. But to make the trip—indeed to set foot legally anywhere within the river corridor—we would need a permit from the Corps. We were asking the notoriously pro-development agency to greenlight a publicity stunt designed to make it reverse course.

When the Corps refused our application, George, a wiry, almost concave man who runs a satire website called LaLaTimes, called a press conference to announce that we were going anyway. For the next two days, he hammered the agency on local radio, taunting the Corps spokesman who'd decreed that paddling on the L.A. River "is an unsafe and incompatible activity."

"Incompatible with what?" sneered George. "Water?" "Our permits were largely denied under the pretense of safety concerns," he said. "Some children drown in bathtubs each year; should we outlaw bathing?"

At stake was not just our planned three-day descent, he explained to reporters and anyone else who would listen, but the entire decades-long effort to revitalize the river corridor.

Our group consisted of a handful of Los Angeles River advocates and a film crew to document it. Also along, in secrecy, would be Heather Wylie, the petite, 29-year-old Corps biologist who first leaked the document classifying the river as un-navigable, and whose presence would eventually spark an uproar at the agency. Finally, on the afternoon we launched, and probably just to shut him up, the Corps faxed George a permit to film inside the basin. It didn't actually allow us to paddle the river, but we figured that in Los Angeles, a film permit probably excuses murder.

That's the document we flash at the foreman, who tells whoever is on the other end of the line, "Yes, they have a permit." Our passage is safe for now, but we expect stiffer challenges. Signs posted on the chain-link fences lining the banks promise a $500 fine for being in the river, and the police have kicked other paddlers off the water, including a group George led the previous year. But as we float past him, one of the maintenance crew smiles down at us and makes a crankshaft motion with his arms, the universal sign for kayaking. "I did that once," he says.

THE LOS ANGELES RIVER RUNS through the suburban sameness of the San Fernando Valley, past the back lots of Warner Brothers and Universal Studios, by the rail yards and chemical plants near downtown, divides gritty neighborhoods like Maywood and Compton and finally spills into the Pacific Ocean at Long Beach. Yet most Los Angelenos would have a hard time picking it out from the city's tangle of roads and freeways. Even in its namesake city, the L.A. River is best known for its film cameos: The climactic drag race in Grease was filmed there, as were countless chase scenes, from The Italian Job to Terminator 2, in which future California governor Arnold Schwarzenegger flees a liquid-metal robot driving an 18-wheeler. Because of the Army Corps' permit requirements, there have probably been more movies and television shows filmed in the Los Angeles River than completed paddling expeditions.

We float close together so there's less chance for passing motorists to report us. Wylie, the Corps biologist, quickly grows impatient with the group's slow pace and charges ahead. As a whistleblower, she's used to breaking ranks. I paddle quickly to catch her, and then we wait out of sight under one of the more than 100 bridges that cross the river, "Laurel Canyon Blvd." stenciled on the sheer walls beside it.

You couldn't pick a less-likely looking bureaucrat—Wiley is tiny, maybe 100 pounds and is as effusive as a kindergarten teacher. "I thought I'd landed my dream job," she says. "I thought I'd be helping to enforce the Clean Water Act (CWA)." In 2006, the U.S. Supreme Court ruled that the CWA is enforceable only when violations can be proven to affect a "traditionally navigable waterway," a test that is difficult to meet on seasonally flowing streams. Since the court's decision was handed down, violations on seasonal streams across the country have gone unpunished, clean water advocates say.

When Wylie read the Corps' internal memo branding the L.A. River as not traditionally navigable, she felt that the agency was using the Supreme Court ruling to avoid enforcing potential CWA violations—such as filling dry riverbeds—in the L.A. River basin. So she decided to leak the memo to L.A. Congressman Henry Waxman, who immediately petitioned the EPA to supersede the Corps and protect the L.A. River's CWA rights, an authority it had never exercised.

"Kayaking is something I do sometimes to survey rivers for work," says Heather, her straw hat hanging down her back by a lanyard around her neck. "But in this case, I'd get fired if they knew I was here. But you know, this is my day off and I want to support this."

Eventually, the group catches us and we proceed downstream through the concrete canyon, whose 20-foot walls are topped by chain-link fences. Beyond them rises another wall separating the river from the 12 lanes of Highway 101. The buzz of traffic is plainly audible. We occasionally pass round openings in the wall—storm-drain tributaries that add a trickle of lawn-sprinkler runoff to our flow. A great blue heron lurches incongruously from one of them and takes flight around the next bend. Though the water seems clean enough, I'm shocked that there's enough prey in this concrete trench to sustain the three-foot bird.

Then, a mile downstream, the river's entire flow slots into an eight-foot-wide, three-foot-deep trench running down the center of the concrete canyon. I drop the tiny falls at the head of the algae-lined trench, dodge a stray shopping cart, and am whisked downstream with surprising speed at the head of a bobbing line of boats. The trench is too narrow to maneuver in, so we sit back and let the current flush us past the graffitied walls of the industrial riverway, like passengers on some blighted Disney Small World ride.

SEVERAL HOURS LATER, I'm wondering whether the river is navigable after all. The center trench is long gone, and the river's anemic flow spreads over the entire 150-foot-wide riverbed. It's not deep enough to float our kayaks, so by the time we pass behind the blocky warehouses of Dreamworks Studio in Glendale, I've been dragging my kayak for hours. The ordeal has broken our previously cohesive group into a row of stragglers stretching more than a mile. What's more, I just slipped on the algae and whacked the back of my head against the concrete bottom. A red helicopter flies overhead, circles once above me, and then whirls downstream.

Joe Linton and Jeff Tipton have already turned the 90-degree corner where the river wraps around the brown hills of Griffith Park and a hulking industrial plant emits a noxious gas. Just ahead, flanked by a fenced-off recycling yard and partially hidden beneath the 14-lane Ventura Freeway Bridge, lies the confluence of Verdugo Wash, one of the Revitalization Master Plan's five so-called "opportunity sites"—it's biggest flagship projects. Right now, the spot is just an opening in the river walls and a thicket of weeds growing on the plume of sediment dropped there—so anonymous that it barely registers in the eye. But if the plan goes forward, the recycling yard will become a 15-acre wetlands park. Flood-resistant terraces traversed by boardwalks and viewpoints will replace the opposite shore. In the architectural drawings, people walk along an esplanade under blue skies, or sit on benches beneath shade trees. In one sketch, an attractive Caucasian couple points towards the water, presumably at some cute, cuddly baby animal. It's a seductive vision, and it suggests the river will become two things that it emphatically is not now: clean and safe.

Most of the river's flow is treated wastewater from three sewage-treatment plants along its course, but after a hard rain it can flash in minutes, washing every dog turd and brake fluid puddle in the Los Angeles basin straight to the river. In the 1920s, the city commissioned a plan to buffer the river with a series of parks that would absorb floodwaters. The plan was prescient, expensive and ill-timed: It was completed in 1930 as the country was careening toward the Great Depression, and the city parks department filed its copy without ever reading it. Eight years later a flood killed 85 people, caused millions of dollars in property damage, and sealed the river's fate, literally. Workers soon began turning the volatile river into a 51-mile concrete storm drain. When they finished, more than 650 million tons of cement and 7 million tons of rebar girded the banks. It was a sort of matricide of the river that had been the city's sole supply of water for its first 150 years.

The river remained all but forgotten until a citizen group called Friends of the L.A. River started advocating for its better treatment in the 1980s. But city government really only took notice when they learned of other cities creating real-estate bonanzas out of revitalizing tarnished rivers. Denver was first. Starting in 1975, the city's $75 million of improvements in the confluence of the South Platte River and Cherry Creek has attracted $5 billion in private investment, transforming a once gritty industrial area into the city's hippest new neighborhood. Other cities followed suit, including San Jose and Reno, which tore the concrete out of a half-mile stretch of the Truckee River and made it into a whitewater kayaking course and park. These projects brought prestige, money and a higher quality of life to their cities, and the Los Angeles planners took notice—if implemented, each of the five opportunity sites will be larger than anything undertaken in Denver, Reno or San Jose.

DESPITE THE THREAT OF FINES, we see a surprising number of people in the river. On our second day we catch a teen-aged graffiti artist at work on a bridge footing–his can runs out of paint and he tosses it over his shoulder into the meager current. We cross paths with dozens of Latino fishermen angling for carp, fat, oily bottom feeders native to China that can grow as long as three feet. Alarmingly, many fishermen say they eat the fish, and one member of our expedition tells me he once saw a man trap a 10-pounder in the concrete shallows with his T-shirt. In a stretch near Glendale where the riverbed isn't paved (though the sides are) wild tangles of 30-foot high giant reed trees, another invasive species, have coalesced the rocky streambed into islands. We spot a family harvesting leaves in a mid-stream island. The woman says she's going to make the leaves into a soup.

We are also on the lookout for Duckman, a notorious older fellow who comes to the river most days to feed the scores of ducks, many of which he allegedly bought in Chinatown food markets and released in the Glendale stretch. Anti-social and volatile, he accosts anyone he feels disturbs the birds—one such profanity-laden confrontation plays on Youtube.

"Hello," I say, brightly, hoping to smooth my sudden intrusion. Beside him is a small clearing in the dense vegetation. A tent is tucked into one corner and a mirror hangs on a tree at face level. A giant pink stuffed animal—a teddy bear–is tethered to shore where river foam collects around its wet fur. It seems like a camp where several people might live, though this man is the only person I can see. "We're paddling the river," I say, "all the way to Long Beach."

The people I'm most interested in are the homeless. No one knows the actual number, but the L.A. River is home to dozens, maybe hundreds. In the lower reaches, nearly every bridge has a camp crammed into the shady triangular space between the concrete bank and the roadway, and in the few earth-bottomed sections like in Glendale, people stake out space in the dense trees, where drivers flashing past on one of the interstate bridges never notice them.

Late in the afternoon on the second day, I'm bumping down the rocky streambed—paddling again now, but still trying to catch Joe and Jeff–when I pop into an eddy beside a mid-stream island and find a naked man squatting on a rock, a square washbasin in his hand and water dripping from his thick mop of hair and graying beard. He makes no move to cover himself and remains squatting on the rock, as though kayakers interrupt his bath every day.

He nods, and after a second says, "So, you have a permit then?" It strikes me as completely absurd that a man living in a streambed that is illegal to even visit is concerned whether or not I have a permit to travel the river. But I assure him that we do, even if it is only half true. He continues squatting on the rock, and tells me his name is Dimitri, that he's 51, and has been living at this camp for four months and on-and-off by the river for a decade. Despite his name, his accent is plainly American—he mentions a daughter in Sacramento and military service.

In the way of people who meet on water everywhere, he's especially happy to talk about the river. The authorities don't seem to mind people living in the river, he says. "It's safer than the streets. No one bothers us down here." He's heard of the city's plans to revitalize the river. "I think it's a good thing," he says, looking up and left, considering his opinion. "But I just hope no one gets pushed out. A lot of people have been living here a long time."

Dimitri's island might escape the bulldozers—it sits midway between the Verdugo Wash opportunity site upstream, and the huge Taylor Yard area two miles downstream—but no one from the city has gone on record about accommodating the homeless residents. After all, it's officially illegal to be in the river in any capacity, and proponents of the plan say that it isn't the responsibility of a river revitalization plan to solve the city's homeless problem. Not that there's full agreement on the plan, even amongst river activists: Melanie Winter, director of The River Project, a local L.A. River advocacy group, likens the city's plan to Disneyland. She doesn't like its emphasis on human use at the expense of a more naturally functioning ecosystem. "The only thing the city is interested in is drawing development to the river," she says flatly. Critics of the plan charge that the master plan will harm Latino communities—the river travels predominantly through low-income ethnic neighborhoods. Neighborhood representatives want soccer fields, not bird habitat, and fear the erosion of community through gentrification and even the loss of their homes through imminent domain.

The reality is that so far, there is little opposition to the plan. Few people, Dimitri included, think the river couldn't use some help. If real opposition materializes, it will likely happen after specific plans released for individual projects. In the meantime, Dimitri seems most concerned about finishing his bath, so I excuse myself and continue downriver. I've gone maybe 100 yards when I see the police helicopter hovering just downstream. A loudspeaker blares: "Get out of the river now!"

IT WOULD PROBABLY BE BETTER FOR THE RIVER if we were arrested. A confrontation with the police would keep the Army Corps's navigability ruling in the news, and increase public outcry against it. Heather, I know, is ready to argue our rights to be on the river based on California's Constitution—Article 10 guarantees access to waterways, though it has seldom been challenged in court. George, ever the satirist, has a pair of novelty handcuffs with him. "If the cops try to haul us off the river," he'd told me earlier, "I'm going to handcuff myself to my kayak. Hopefully, they won't notice the cuffs are made out of plastic."

However, I for one, would rather keep paddling. We're only halfway now, and I'm just into the rhythm of the river—imbued with the curiosity to know what lies downstream, the urge to see around the next bend.

When I arrive at the footbridge that crosses the river below Hyperion Ave, two policemen are studying the film permit. My tripmates are standing next to their kayaks on shore and our film crew's camera is rolling. After a minute the two officers, thick-chested with bulletproof vests, their radios squawking, walk a few yards away to confer.

"Duckman says he called the cops," says Melanie, floating just offshore in her kayak. "Then he called me every name he could think of—cow, bitch, whore…" On top of the concrete bank, a thick older man stands with his arms crossed, scowling. A small crowd has gathered on a footbridge that crosses the river above us, rubbernecking the proceedings. A few ducks lurk at the edges of the vegetation.

George is standing patiently on shore, perhaps mustering courage to make a scene. Heather, our expedition firecracker, is still behind us. No one in the group looks to be spoiling for a fight, and I'm hugely relieved when the police return and tell us they will allow us to continue. The permit seems to have done the trick. They give us a quick lecture about being careful, about how they don't want to have to come rescue us later. You can tell they have more important things to do than fool around with a bunch of nuts kayaking down the barely wade-able Los Angeles River, whatever Duckman may say about wildlife harassment.

In the end, our arrests weren't required. In July, three weeks after our trip, the EPA superseded the Army Corps ruling. David Beckman, National Resource Defense Council attorney, tells me that the EPA's action was almost certainly the result of Congressman Waxman's letter, combined with public outcry sparked by publicity, notably our kayak trip. "The Corps really kicked a hornet's nest," he says.

A few months later, Heather's supervisors would find out about her presence on the trip from a news photo on the Internet and use it as an excuse to try to terminate her. Because it's illegal to fire a whistleblower, her presence on the trip was the best reason they could find to fire her. Disillusioned with the agency, Heather ultimately negotiated a settlement with the Corps over her job, the terms of which neither side will divulge. She considers the L.A. River effort to be a success. "I think I'd outlived my usefulness as a watchdog at the Corps," says Heather. "I'm glad to leave with a victory."

Hopefully, it's the next step in the river's recovery. With nearly every rivulet in its watershed hemmed in by human settlement, the Los Angeles River will need all the help it can get. The adoption of the revitalization plan shows there's a public will, and from what we found, people might just need an example—something to make them start thinking of the river as more than just a flood control facility. As the police walk up the bank to talk Duckman down, and we climb into our boats and launch into the rocky stream, the gathered crowd lets out a cheer, doubtless glad for the little bit of dignity our expedition affords the downtrodden L.A. River.