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.