Nature reclaims Yosemite route

Winding lazily into the Sierra Nevada, California 140’s two asphalt lanes for generations served as the busiest road to Yosemite, with more than 1 million visitors each year rolling up the route to the magnificent granite valley.

But of late, the natural world has gotten in the way.

A dozen miles from the park, the old road has disappeared, its once-bustling blacktop buried under a rubble pile broad as two football fields.

With a geologic shrug of the shoulders, Mother Nature last spring sent roughly 90,000 cubic yards of boulders and debris sliding off Ferguson Ridge, blocking the highway and causing consternation from Yosemite’s west gate to the downtown streets of Merced.

Many visitors to Yosemite would catch California 140 in Merced and head northeast, passing through Mariposa on their way to the park 76 miles away.

For residents of Mariposa and even smaller communities that hug California 140 -- Midpines has a population of less than 1,500 -- short trips turned into two-hour detours. Local merchants groaned. It took months to fashion a one-lane bypass, which loops over a temporary bridge to the other side of the Merced River and then crosses back a few miles upstream to reconnect with the highway.

The slide, meanwhile, has been wired up like a cardiac patient, its belly flecked with high-tech monitoring devices borrowed from Mt. St. Helens’ lava dome. Radar scans it 24/7.

A long-term solution remains years away, a subject of scrutiny by top state transportation officials and a source of concern all the way up the statehouse food chain to the governor’s office.

For now, the big rocks have mostly stopped raining down, and the slide appears to have stabilized in a precarious angle of repose, its foot buried deep in the swirling waters of the Merced River.

Experts admit they can’t predict exactly how the rocks of Ferguson Ridge will behave.

A few more rainy winters could trigger a restart of the slide, with the catastrophic consequence of a natural dam blockading the Merced River, flooding its upstream reaches with a new lake, miles long.

The more likely scenario, officials say, is that it will mostly sit as it is for decades to come.

“What the slide will do, it’ll do in its own good time,” said Jerome DeGraff, a U.S. Forest Service geologist. “That’s a pain when you have a road running up a canyon.”

DeGraff saw it start, even if he didn’t realize it at first.

Last May, DeGraff and a colleague were traveling down the highway when they discovered several basketball-sized rocks on the roadbed. He rolled the worst of them off the road and went on his way.

In the following days, dirt and shale began cascading down the hillside. Road crews responded, figuring they could erect a concrete and steel barrier to hold back the worst of the rubble.

It was buried within days.

One boulder slammed into the side of a parked road crew pickup truck, caving in the door. Traffic was suspended. Yosemite employees and schoolkids who had to get to the other side were left adrift. Power line towers teetered at the edge of the slide, imperiling electricity to the park until the high-voltage lines were rerouted to temporary poles across the river.

By midsummer, road crews had installed a pair of one-lane steel bridges and blacktopped the old gravel road on the far bank of the Merced River. Signal lights at each end let traffic through at 15-minute intervals. Vehicles longer than 28 feet are prohibited, banning tour buses and big construction trucks.

Since it opened in the late 1920s, California 140 has remained the preferred route for such big rigs. It is far less curvy than other major routes -- California 120 to the north and California 41 to the south. It also doesn’t climb as high, making it far less prone to ice and snow.

Park officials worry about the increased number of tour buses on the other highways during the icy winter months, said Scott Gediman, a Yosemite National Park spokesman. “A permanent fix is incredibly important.”

Once used by 800,000 vehicles a year, this portion of California 140 has seen traffic dwindle with the restrictions caused by the slide.

Restoring the highway to full operation has become the highest priority for the California Department of Transportation district office in Stockton.

Among the alternatives being eyed are tunneling through Ferguson Ridge, erecting permanent bridges across the Merced River or building a rock shed, a long concrete structure, to shield motorists from further slides.

Caltrans is also looking at trying to remove the debris, a monumental task given that it could take more than 9,000 dump truck loads just to cart off what has fallen so far. An unlucky bulldozer also could unleash a far more devastating slide. Geologists estimate that about 1 million cubic yards of rock and soil -- 10 times what has already fallen -- remain poised on the slope.

No cost estimates have been hammered out for any fix, but local Caltrans officials have privately told Mariposa County supervisors it could run up to $100 million. A timeline for the project could be forthcoming in about a month.

Locals, however, aren’t optimistic. Leroy Radanovich, the county’s tourism director, predicted that it would take half a dozen years for any final solution to be built.

Though merchants in Mariposa have survived the road closure, he said, “we have to have full use of that highway.”

Forest Service geologist DeGraff, 61, figures he’ll be studying this slide until his retirement in five years.

A self-proclaimed “landslide nut,” DeGraff said he has seen worse. He traveled to Thailand in the late 1990s to see the devastating effects of typhoon-spawned debris flows that wiped out entire villages. A decade before, he witnessed the aftermath of a devastating slide that destroyed a highway, dammed a river and put the railroad town of Thistle, Utah, under water.

Hiking up the far ridge to get a good view one recent day, DeGraff gazed out at the vast debris field, 1,200 feet from top to bottom.

He pointed out three leggy steel contraptions called “spiders” sitting at various spots on the slide. First used by the U.S. Geological Survey at Mt. St. Helens, the 6-foot tripod contraptions were ferried into place a couple months back by helicopter. Each holds a steel box of global positioning telemetry gear, allowing experts to monitor movement of the slope hourly.

Two of the units -- given the fanciful names Chaos and Shangri-La -- haven’t budged, DeGraff said. But the third, dubbed Precipice, has inched a bit less than a foot in recent months.

Nearby, a mobile radar unit sits on the hillside, slowly scanning each foot of the slide zone every 10 minutes. If a single boulder were to let loose, the radar unit would let the experts know.

With such real-time monitoring providing early warning, a relative degree of safety can be assured the trickling traffic, particularly in a natural realm where rocks can fall off canyon walls just about anywhere they choose.

Mammoth slides in the Sierra have been relatively rare since the arrival of the first pioneers to California, he said.

Today’s shifting landscape dates to a time 150 million years ago, when the Sierra lay beneath the ocean. As tectonic plates collided and the mountains pushed skyward, rivers cleaved the slopes of hardened granite and fine sediments left by the ancient sea.

Ferguson Ridge’s phyllite rock first broke loose in prehistoric times but then settled into a long slumber until two wet winters kicked off the current events. Flowing rainwater lubricated fissures beneath the slope, boosted groundwater buoyancy and adding weight to the surface mass.

Last spring, DeGraff said, “gravity won.”

*

eric.bailey@latimes.com