The mountains that Angelenos love—the sheer high-desert backdrop that defines the boundaries of their megalopolis and offers them a wild escape that's nearer and more varied than any other in the country—are trying to kill them. Or is it the other way around? It can be hard to tell in a relationship as complicated as that between the citizens of Los Angeles and the San Gabriel Mountains. Consider the most recent development in their 230-year-old union: Heavy rains in early February caused a catch basin to fill and mudslides to sweep down the mountains and into a neighborhood of La Canada Flintridge, damaging 43 homes and 25 cars.

The suburban culs-de-sac share space with the catch basin, a structure best thought of as an empty, perforated dam, built to capture the mud, rocks, and trees that people expect to come sliding down the mountainside. It was overwhelmed when a 10-ton boulder tumbled and blocked a key drain, which soon caused a 35-mile-per-hour tide of mud and bowling ball-size rocks to sweep into the streets, tossing and crumpling cars like tinfoil toys. The mud filled houses like they were cake molds. If you had been standing in the kitchen of one of those houses, you would have been chest-deep in what geologists call debris flow—a fast-moving mix of water, rocks, dirt, and detritus—except, of course, you would not have been standing. You would most likely have been killed. Fortunately, no one was.

San Gabriel Canyon and the east fork of the San Gabriel River.

The La Canada Flintridge slide was nature's payback for the largest wildfire in the modern history of Los Angeles County. The Station Fire rampaged through the San Gabriels from the end of summer until mid-fall 2009, burning 160,000 acres. Investigators believe the fire was intentionally set alongside Angeles Crest Highway. (A vast majority of California wildfires are started by humans, either by accident or as acts of arson. Of the 20 largest fires in the recorded history of the state, only 7 had natural causes.)

Nonlocals hear about the San Gabriels only when they're ablaze or falling on people. But when they're doing neither, they are much more interesting: an untamed wilderness coexisting with one of the world's largest metropolises, a safety valve for the psyches of 18 million jangled humans. The 1,000-plus-square-mile Angeles National Forest, which encompasses the mountain range, is where hikers and campers find solitude within 30 miles of the country's second-most-populated region. It's where children learn about nature, snowboarders carve, those without air conditioning seek relief, hunters and fishermen bag prey, off-roaders crack axles, motorcyclists experiment with asphalt skin grafts, gun lovers practice the rhythms of pop-pop-pop, and the religious test their faith by being baptized in the waters of a canyon sometimes called "Diaper Alley." And that's just what's legal; it doesn't include the potential of finding (or becoming) a bullet-punctured human skull.

The forest is heavily trafficked, underfunded in its upkeep, and remote in its steepness, a landscape that John Muir—who knew of such things—called "ruggedly, thornily savage." Muir continued: "Not even in the Sierra have I ever made the acquaintance of mountains more rigidly inaccessible," yet "down in the dells, you may find gardens filled with the fairest flowers, that any child would love, and unapproachable linns lined with lilies and ferns, where the ousel builds its mossy hut and sings in chorus with the white falling water."

The San Gabriels make up more than 70 percent of the open space in Los Angeles County, dwarfing urban parks and playgrounds. They are the park for the poorer and industrial parts of L.A. Three and a half million people visit the area each year, with a million and a half of them heading to the San Gabriel River above the foothill city of Azusa. Mostly they're day-picnickers, immigrant families escaping the blacktop heat of the sprawling San Gabriel Valley. On a weekend day in summer, some 8,000 visitors head into the canyon, along with 1,000 cars. There aren't enough parking spaces, trash cans, toilets, or trails to the riverbanks, but no matter. People scramble down anywhere they can. The trash piles up beside full bins or gets left in the sand. The U.S. Forest Service and its volunteers haul out 400 32-gallon trash bags a day, 150 tons of garbage a year. The agency figures it's scooping up less than two-thirds of the trash, maybe only half. This on a river that supplies a third of L.A.'s drinking water.

Graffiti mars the banks of the east fork of the San Gabriel River in Cattle Canyon.

What grew to become Angeles National Forest started as the San Gabriel Timberland Reserve in 1892. The designation ended the excesses of gold mining, cattle ranching, and logging scarce lumber-worthy trees. More than anything, the designation was meant to protect the San Gabriel watershed, then the water source for L.A.'s vast citrus groves. Today, the forest's 650,000 acres contain 650 miles of trails, 66 campgrounds, 36 picnic areas, five lakes, five protected wilderness areas, and the San Dimas Experimental Forest, a field laboratory for studying chaparral ecosystems. The land is spiderwebbed with 1,600 miles of roads, 950 miles of which are under Forest Service domain. There are 600 private cabins, a few hundred apiaries, 26 organizational camps (for Boy Scouts and such), five ski resorts, three managed off-road-vehicle areas, two educational centers, and two shooting areas.

There's no timber cutting or animal grazing, but there are electric lines, pipelines, and several dams. There are historical sites, like the remains of the old scenic railway, abandoned gold prospects, and the still-in-use Mt. Wilson Observatory, where astronomer Edwin Hubble realized that the universe is filled with galaxies accelerating away from our own, a discovery that led to the big bang theory and a grand expansion of the cosmos and human insignificance.

And for all this, the Forest Service has, at peak times, around 450 employees, 85 percent of whom work on fire prevention and suppression. Only 50 or so people manage the forest's swarm of visitors.

"We do the best we can," Forest Service supervisor Jody Noiron says. "There's no way we can do what we do without volunteers. They're a godsend." Many of the volunteers come from outdoors groups, including the Sierra Club. (See "Force of Nature," page 54.) They help pick up trash, erase graffiti, maintain trails, enforce rules, and inform the public about the land and fire safety. Noiron notes that the biggest challenge is illegal off-roading, which erodes fragile hillsides and adds the fire hazards of hot metal and sparks to dry brush. With communities right up against the forest, people can fire up their dirt bikes and ATVs, roll down their driveways, and be in the wilderness in minutes. "There's not a fence around it," Noiron says. "It's not like we can close the gate and lock it at night." She says this makes members of off-roader groups among the most valuable volunteers in the park. They police their less-informed brethren and help haul out cars and large appliances dumped in canyons.

That yes-in-my-backyard volunteer spirit inspired an innovative Sierra Club effort to expand wilderness areas and preserve a vital watershed in the San Gabriels while improving outdoor access for working-class and Hispanic Angelenos. As part of the San Gabriel Mountains Forever campaign, the Club has joined other outdoors organizations, churches, and community groups to push the federal government to create a San Gabriel Mountains National Recreation Area, an inland equivalent to the Santa Monica Mountains National Recreation Area, created on L.A.'s prosperous west side more than 30 years ago. The Club is also pushing to enlarge the existing Sheep Mountain, Cucamonga, and San Gabriel Wilderness Areas and make 46 miles of rivers and creeks part of the National Wild and Scenic Rivers System. 

Juana Torres, a Sierra Club regional organizer, says the 150,000-acre Santa Monica Mountains National Recreation Area outspends the much larger Angeles National Forest on visitor education and recreation by up to 15 to 1. "In our lifetime, there's no likelihood of any additional funding unless we change the equation," says John Monsen, a Club representative campaigning with Torres.

The process has already started, and the National Park Service is preparing a draft recommendation for Congress. The proposed recreation area would extend north almost to Palmdale, be bounded on either side by Highway 14 and Interstate 15, and reach from Monrovia to Claremont and down to South Whittier, an area that would include most of the San Gabriel River's watershed. If the land was designated a national recreation area, the recreation-focused National Park Service wouldn't replace the resource-minded U.S. Forest Service now running the show, but the area would gain new rangers and interpreters. And the hope of everyone involved—the activists, the Forest Service, everyone—is that the designation would free up additional federal money to assist the San Gabriel Mountains' used and abused recreational areas.

The San Gabriels' ruggedness is a product of their youth. A mere 1.65 billion years old in parts, they are a plutonic igneous intrusion of granite—molten rock pushed to the surface by a confluence of tectonic faults such as the San Andreas, the Sierra Madre, the Raymond Hill, and the Cucamonga complex. The 10,000-foot-plus Mt. San Antonio (better known as Mt. Baldy) is rising at a rate of two to five millimeters a year. During the 1971 Sylmar earthquake, part of the San Gabriels jumped six feet. The area around Cajon Pass, where Interstate 15 heads north into the Mojave Desert, has been surveyed since the 19th century and is climbing at a rate of 17 inches per century.

"That means within my lifetime it's probably risen about 14 inches, which in geological terms is really scary," says D. D. Trent, a professor emeritus of geology at Citrus College who's spent a lifetime studying the 63-mile mountain range at his school's back door. "Those people right up at the base of the mountains get to know exactly what an alluvial fan is because they've got debris flows coming down their streets and carrying away their cars."

There are 400 catch basins throughout Los Angeles County like the one in La Canada Flintridge that was at the center of February's spat between the mountains and the city. They're part of Los Angeles' attempt to stave off alluvial runoff, the very process that built the land the city sits on. The project started after a 1914 flood that inundated nearly 12,000 acres of land, blocked harbors, twisted railways, uprooted telegraph poles, and cut Los Angeles off from the world for almost a week. The flood caused more than $10 million in damages (about $215 million in today's dollars) but, to general amazement, no one was killed.

That was not the case on New Year's Day in 1934, when the mountainsides tumbled down Pickens Canyon to land on Montrose, killing at least 40 people, destroying more than 100 houses, and planting a 70-ton boulder on Foothill Boulevard. Four years later, another flood killed more than 100 people and left thousands homeless. Major floods followed in 1969 and 1978 but were restrained by the vast network of catches.

"You don't have to have an advanced degree to say, 'Well, this happens about three times a century.' It will happen again, and everyone will be amazed by boulders the size of Volkswagens and mud pies that can bury a house," says Ronald Quinn, a professor emeritus of biological science at Cal Poly Pomona, a chaparral expert, and a fifth-generation Californian. "The heroic geologic processes that created this area are still going on, and there's nothing we can do to stop them."

Humans, though, have turned up the volume on these catastrophes. The last 10 years in Southern California have been the driest decade in 150 years, says Jon Keeley, a wildfire expert and research ecologist with the U.S. Geological Survey. He says that while chaparral is adapted to fire and burns hot and heavy, we are in the midst of—and largely causing—an unprecedented period of burns. With nothing but lightning strikes, chaparral might burn every 30 or 130 years. Of what he considers the nine "mega-fires" in the past 50 years, five have occurred in the last decade. They are destroying the native vegetation, which needs 15 to 30 years to recover from a blaze. Does Keeley think we can stave off fire for that long? "Oh no. It's not possible," he says.

Fire and flood are exacerbated by and in turn exacerbate the problem of invasive species. If an area has been repeatedly burned or suffers from a drought after a burn, invasive species like brome grasses can move in. These seasonal grasses are more flammable, which harms the native vegetation that has hung on. And grass fires don't burn hot enough to trigger the seeds of native plants like mountain lilac, which require the scorching heat of a chaparral fire to generate a key protein for germination. (These species of plants also suffer more than any other during droughts.) The increasing number of fires and the shallow root systems of the invading species lead to an increasing number of landslides, opening up fresh earth for still more invaders.

The repeated lesson of the San Gabriels is that the boundary between the wild and the urban is fuzzy and unpredictable. And while the occasional mountain lion attacks, and bears sometimes wander out of the mountains to forage for suburban trash, humans are by far the most dangerous creatures in the forest. Drug dealers protect mobile meth labs and backcountry marijuana plantations—water-sucking pesticide- and fertilizer-intensive operations often protected by booby traps and armed guards. (Police estimate that at least three plantations were consumed in the 2009 Station Fire.)

Gangs brawl. The Angeles National Forest has long been a dumping ground for people looking to dispose of human bodies. In 1978 the Hillside Strangler left a victim there, decomposing in the trunk of her car. Police searching the forest recovered the remains of ex-Raiderette Linda Sobek and the model Kimberly Pandelios, the latter after she had been drowned in a mountain stream. The corpse of con man Ron Levin, killed in the '80s by members of the Billionaire Boys Club Ponzi scheme, is believed to be somewhere in the mountains.

Poachers kill black bears and sell their gallbladders at $265 per gram to purveyors of Asian folk remedies. Diverse religious rituals abound. One Halloween, a state game warden came across three young people in white robes and black masks sitting around chalked pentagrams with a sword, a jar of goat's blood, and a severed owl's wing as they chanted at the moon. The warden confiscated the owl's wing. (The worshippers of unspecified occult forces claimed the wing came from roadkill.) Sometimes ceremonies go awry. The Forest Service's report on 2002's 21,000-acre Curve Fire dryly notes that the cause "was determined to be a ritual involving the use of fire (candles) and animal sacrifices." In other words, a Santeria shrine. Even an off-season tour of the San Gabriel River can reveal a neon heap of fruit and flowers, murk-filled jars, and sometimes chicken feathers.

"We have to share it with everyone. That guy who killed the Raiderette, gold miners, nudists, and pot growers," says Darrell Kunitomi, an avid catch-and-release fly fisherman who heads into the San Gabriels most weekends year-round. Kunitomi comes across trash all over the place. Sofas, soiled diapers, old barbecues—depressing stuff. "Several guys I know refuse to fish locally because they get too angry," he says. He picks up what he can, stuffing the litter into an old army pack. The last time he went fishing, Kunitomi carried out 20 pounds of trash.

Why does he put up with it? In part because Kunitomi sees the San Gabriels as his birthright. His parents were Japanese American residents of Los Angeles interned at the Manzanar relocation camp in the eastern Sierra during World War II, after which they returned and raised him as a native Angeleno. He learned to fish on Echo Park Lake. The San Gabriels are his landscape, rugged and spectacular, full of wildflowers that attract insects that get fish biting. He's less than an hour away from streams swimming with rainbow and brown trout, wriggling little fingerlings he admires for their fight and perseverance in a land of drought and fire. He reels them in, takes a quick picture, and sends them on their way.

Kunitomi describes his perfect day: There's sparse traffic along the 210 freeway and he can get from Echo Park to a stream (he won't say which one or which part) in 55 minutes flat. It's a clear winter day, bright and 70 degrees Fahrenheit. Early wildflowers are blooming and the insects are out. There are blue-winged olive mayflies and swarms of pale morning duns, "one of the most poetic names in all of fly-fishing."

The fish are already jumping when Kunitomi steps in. He's casting a seven-foot, four-weight fly rod of split cane—the details are important—and they're willing, feisty, and fighting.

"My tippet is perfect, and my knots are holding. I catch fish, take a lot of pictures, and hike back home happy," he says with a sigh.

And on the way home, he stops for a bite at Manuel's, which has been part of East Los Angeles all his life. The sign out front says el tepayec. Ignore this. It's Manuel's. Kunitomi has the beef taquitos and guacamole. And thinks about the mountains, a matchup that matches L.A.