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Alaska highway officials say they've never seen an avalanche this large touch a road

Michelle Theriault Boots,Zaz Hollander,Tegan Hanlon
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The volume of water flowing through and old railroad tunnel in Keystone Canyon is slowing down as the flow waters covering the Richardson Highway recede on Wednesday, Jan. 29, 2014.
Bill Roth
A front end loader is parked on the Richardson Highway near Mile 16 after having cleared some of the flooding debris on Wednesday, Jan. 29, 2014.
Bill Roth
Flooding debris and ice surround a Richardson Highway mile post sign on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Alpine subdivision of Valdez is still on the lookout for possible flooding as a result of the avalanches at Mile 16 of the Richardson Highway on Wednesday, Jan. 29, 2014.
Bill Roth
The 4,000 residents of Valdez are cut off from the road system as a result of a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River. Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway, near Mile 16, is still buried under debris from a massive avalanche in Snowslide Gulch, right, near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The 4,000 residents of Valdez are cut off from the road system as a result of a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River. Wednesday, Jan. 29, 2014.
Bill Roth
Photo by Jason Sakalaskas / ADOT&PF Robert Dunning, Valdez District Superintendent of the Department of Transportation and Public Facilities inspects avalanche debris in Keystone Canyon Tuesday, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Keystone Canyon avalanche, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Robert Dunning, Valdez District Superintendent of the Department of Transportation and Public Facilities inspects the Lowe River bridge in Keystone Canyon Tuesday, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Lowe River water rushes out of the tunnel near the Keystone Canyon avalanche January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Keystone Canyon avalanche, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Lowe River water rushes out of the tunnel near the Keystone Canyon avalanche January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF DOTPF personnel inspect the Lowe River bridges near the Keystone Canyon avalanche, January 28, 2014.
Jason Sakalaskas
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
A helicopter surveys the lake that has submerged the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the lake that has submerged the Richardson Highway Monday, Jan. 27, 2014 outside Valdez. Snow slides covering the highway have also damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the scene where a natural avalanche and another avalanche triggered by blasting have covered the Richardson Highway and damned the Lowe River to create a lake on Monday morning, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Photo courtesy Alaska Department of Transportation and Public Facilities
A helicopter surveys the scene where an avalanche has damned the Lowe River and submerged the Richardson Highway in snow and water Monday, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Alaska Department of Transportation and Public Facilities
A snow slide at Mile 39 is one of three smaller slides that have covered parts of the Richardson Highway in addition to two major avalanches that have buried the road in Keystone Canyon.
Alaska Department of Transportation and Public Facilities
An aerial survey shows how avalanches have damned the Lowe River and submerged the Richardson Highway in snow and water Monday, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the lake that has submerged the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the lake that has submerged the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the avalanche covering the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public FacilitiesAlaska Department of Transportation and Public Facilities
A helicopter surveys the scene where a natural avalanche and another avalanche triggered by blasting have covered the Richardson Highway and damned the Lowe River to create a lake on Monday morning, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Photo courtesy Alaska Department of Transportation and Public Facilities
This photo provided by the Alaska Department of Transportation & Public Facilities shows multiple avalanches that crossed the Richardson Highway in the Thompson Pass region of Valdez, Alaska, causing flooding. Alaska highway officials say the only highway into the city of 4,100 people will be closed until further notice, for at least a week, if not much longer.
Alaska Department of Transportation and Public Facilities via AP
A helicopter surveys the scene where an avalanche has damned the Lowe River and submerged the Richardson Highway in snow and water Monday, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Alaska Department of Transportation and Public Facilities
Skies finally cleared in Valdez, AK on Sunday, January 26, 2014, giving residents a full view of the major avalanches that occurred the previous day on Town Mountain.
Matt Kinney
Alyeska Pipeline Company photo Slides spill off the mountain sides near the Richardson Highway at MP 42. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo Slides spill off the mountain sides near the Richardson Highway at MP 42. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo These slides covered the road and dammed the Lowe River in Keystone Canyon at MP 15.5. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo Slides cover a length of the Richardson Highway at MP 45 just south of Stuart Creek. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The volume of water flowing through and old railroad tunnel in Keystone Canyon is slowing down as the flow waters covering the Richardson Highway recede on Wednesday, Jan. 29, 2014.
Bill Roth
A front end loader is parked on the Richardson Highway near Mile 16 after having cleared some of the flooding debris on Wednesday, Jan. 29, 2014.
Bill Roth
Flooding debris and ice surround a Richardson Highway mile post sign on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Alpine subdivision of Valdez is still on the lookout for possible flooding as a result of the avalanches at Mile 16 of the Richardson Highway on Wednesday, Jan. 29, 2014.
Bill Roth
The 4,000 residents of Valdez are cut off from the road system as a result of a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River. Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway near Mile 16 is still buried under debris from a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The Richardson Highway, near Mile 16, is still buried under debris from a massive avalanche in Snowslide Gulch, right, near Keystone Canyon and the Lowe River as road crews wait for the flooding to recede on Wednesday, Jan. 29, 2014.
Bill Roth
The 4,000 residents of Valdez are cut off from the road system as a result of a massive avalanche in Snowslide Gulch near Keystone Canyon and the Lowe River. Wednesday, Jan. 29, 2014.
Bill Roth
Photo by Jason Sakalaskas / ADOT&PF Robert Dunning, Valdez District Superintendent of the Department of Transportation and Public Facilities inspects avalanche debris in Keystone Canyon Tuesday, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Keystone Canyon avalanche, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Robert Dunning, Valdez District Superintendent of the Department of Transportation and Public Facilities inspects the Lowe River bridge in Keystone Canyon Tuesday, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Lowe River water rushes out of the tunnel near the Keystone Canyon avalanche January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Keystone Canyon avalanche, January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF Lowe River water rushes out of the tunnel near the Keystone Canyon avalanche January 28, 2014.
Jason Sakalaskas
Jason Sakalaskas / ADOT&PF DOTPF personnel inspect the Lowe River bridges near the Keystone Canyon avalanche, January 28, 2014.
Jason Sakalaskas
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
Alaska Department of Transportation and Public Facilities did an overflight of the avalanches in Keystone Canyon to monitor the situation Monday January 27, 2014.
ADOT&PF
A helicopter surveys the lake that has submerged the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the lake that has submerged the Richardson Highway Monday, Jan. 27, 2014 outside Valdez. Snow slides covering the highway have also damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the scene where a natural avalanche and another avalanche triggered by blasting have covered the Richardson Highway and damned the Lowe River to create a lake on Monday morning, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Photo courtesy Alaska Department of Transportation and Public Facilities
A helicopter surveys the scene where an avalanche has damned the Lowe River and submerged the Richardson Highway in snow and water Monday, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Alaska Department of Transportation and Public Facilities
A snow slide at Mile 39 is one of three smaller slides that have covered parts of the Richardson Highway in addition to two major avalanches that have buried the road in Keystone Canyon.
Alaska Department of Transportation and Public Facilities
An aerial survey shows how avalanches have damned the Lowe River and submerged the Richardson Highway in snow and water Monday, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the lake that has submerged the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the lake that has submerged the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public Facilities
A helicopter surveys the avalanche covering the Richardson Highway Saturday, Jan. 25, 2014 outside Valdez. Snow slides covering the highway have damned the Lowe River, complicating efforts to clear the road.
Alaska Department of Transportation and Public FacilitiesAlaska Department of Transportation and Public Facilities
A helicopter surveys the scene where a natural avalanche and another avalanche triggered by blasting have covered the Richardson Highway and damned the Lowe River to create a lake on Monday morning, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Photo courtesy Alaska Department of Transportation and Public Facilities
This photo provided by the Alaska Department of Transportation & Public Facilities shows multiple avalanches that crossed the Richardson Highway in the Thompson Pass region of Valdez, Alaska, causing flooding. Alaska highway officials say the only highway into the city of 4,100 people will be closed until further notice, for at least a week, if not much longer.
Alaska Department of Transportation and Public Facilities via AP
A helicopter surveys the scene where an avalanche has damned the Lowe River and submerged the Richardson Highway in snow and water Monday, Jan. 27, 2014, in Keystone Canyon outside Valdez. Some water is escaping through a railroad tunnel.
Alaska Department of Transportation and Public Facilities
Skies finally cleared in Valdez, AK on Sunday, January 26, 2014, giving residents a full view of the major avalanches that occurred the previous day on Town Mountain.
Matt Kinney
Alyeska Pipeline Company photo Slides spill off the mountain sides near the Richardson Highway at MP 42. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo Slides spill off the mountain sides near the Richardson Highway at MP 42. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo These slides covered the road and dammed the Lowe River in Keystone Canyon at MP 15.5. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo Slides cover a length of the Richardson Highway at MP 45 just south of Stuart Creek. DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company
Alyeska Pipeline Company photo DOT has closed the road from Mile 12 (about 12 miles north of Valdez) to Mile 64. Avalanches in Keystone Canyon near Valdez have buried parts of the Richardson Highway several hundred feet long and between 30 and 40 feet deep.
Alyeska Pipeline Company

A massive pile of avalanche debris kept the Richardson Highway closed on Monday, and officials were reluctant to offer any forecast on when the only road into Valdez would reopen.

The threat posed by a half-mile-long lake pooled behind a snow dam in Keystone Canyon and continuing avalanche danger from the slopes above make it too dangerous for crews to move in and begin cleaning up, officials said.

"There are just too many unknowns at this point," statewide maintenance engineer Mike Coffey said.

Valdez city officials have consistently said they expect the highway to be closed "for at least one week, but very possibly longer," according to city spokeswoman Sheri Pierce. The Alaska Department of Transportation had been more optimistic, saying on Sunday it could be re-opened as early as Tuesday. But a flyover Monday showed the scale of the dammed water, Coffey said, and the state said simply the road was closed "until further notice."

Valdez is prepared to ride out being cut off. Groceries have been replenished at the town supermarket. Extra flights to and from Anchorage were added. The Alaska Marine Highway System will make additional runs to Valdez, ferrying passengers to the road system via Whittier.

The situation on the Richardson Highway, involving multiple avalanches on both sides of Thompson Pass, is "extraordinary," Coffey said.

"We haven't had to deal with anything quite like this before."

BIG SLIDES

At least two giant avalanches -- one natural and one triggered by blasting -- and a half-dozen smaller ones covered parts of the Richardson Highway on Monday, according to DOT officials.

The largest slide is also the closest to Valdez, at Mile 16 of the highway in Keystone Canyon.

The avalanche debris field in the canyon is estimated to be 100 feet tall and between 1,000 feet and 1,500 feet long.

Highway officials say they've never seen an avalanche this large touch a roadway.

The other major slide is at Mile 39, toward the north end of the closure. It is estimated to be between 30-40 feet deep.

Between a half-dozen and a dozen smaller, isolated slides dot the highway closure area, between Mile 12 and Mile 64.

Most only cover a single lane of the roadway, Coffey said.

SNOW DAM

The real problem is the snow dam created by the enormous Keystone Canyon slide, and the water it is holding.

That avalanche choked off the Lowe River, which normally snakes through the narrow canyon alongside the highway.

The river is typically frozen at this time of year, officials said.

But with heavy rains and unseasonably warm weather for January, it has been moving at about a third of its normal summer flow -- when it becomes a destination for white water rafters, said Valdez DOT superintendent Robert Dunning.

Over the course of Saturday and Sunday, the snow dam flooded the valley upstream, pooling a half-mile-long lake dotted with ice and snow clumps. At its peak, that lake was rising by an inch or more an hour, Coffey said.

The backed-up water would pose a grave risk to workers downstream if there were to be a catastrophic break of the snow dam, he said.

That's unlikely, but it poses enough of a threat that workers can't be allowed in until the water drains, said Jeremy Woodrow, a state Department of Transportation spokesman.

So far, there are encouraging signs: The water appears to be receding and flowing through an old railroad tunnel and the snow pack.

A voluntary evacuation advisory for the Nordic and Alpine Wood subdivisions, about two miles from the Keystone Canyon, is still in effect Monday, said Holly Wolgamott, deputy city clerk in Valdez.

Wolgamott said she's heard that some residents have left their homes in the subdivisions to stay with friends and family or at a hotel, but no one has stayed at the shelter.

The National Weather Service issued a flash flood advisory until noon Tuesday for the area along the Richardson Highway from Keystone Canyon to Mile 5.

If sirens sound in the area under voluntary evacuation, all residents must immediately evacuate, said the Valdez statement.

When there's no risk from a surge of water, crews can get into the area to begin carting away snow, ice, and rock brought down by the avalanche, Coffey said. That work should take "days, not weeks," he said.

PIPELINE OK

The avalanches and dammed water don't appear to pose a risk to the Trans-Alaska Pipeline System, according to Michelle Egan, a spokeswoman for Alyeska Pipeline Service Co.

The 800-mile pipeline, which starts at Prudhoe Bay and ends at the tanker terminal in Valdez, goes underground at Mile 63 of the Richardson Highway -- more than 20 miles from the closest avalanche, Egan said.

During construction in the 1970s, engineers decided to bury the pipeline to safeguard it from the risk of avalanche or rockslide in the rugged, steep terrain, she said.

The pipeline crosses the Lowe River just under a mile and more than 30 feet higher in elevation from the lake formed by a slide that dammed the river.

Alyeska officials are keeping an eye on that section and a check valve -- used to prevent the reverse flow of oil - encased in pipe about nine feet down. Egan said even if water reached the area of the valve, it wouldn't be expected to cause any hazards.

Alyeska is conducting daily helicopter surveillance flights to monitor the area, she said. "There's a significant weather event in that area so we're watching it really closely ...it's changing conditions all the time. We're glad to see the water level going down."

LOTS OF SNOW, LOTS OF RAIN

The conditions that led up to the flurry of avalanches can be summed up easily, said Valdez Avalanche Center forecaster Pete Carter: lots of snow followed by lots of rain -- nearly a foot in the hours before the road-blocking slides began on Friday.

With temperatures rising, the warming snowpack became "a quivering bowl of jelly," Carter wrote in an email Monday.

Thompson Pass has been in a state of heightened avalanche activity since Jan. 14, Carter said.

"This was a long-foreseeable event," Carter said.

On Friday, Valdez residents woke up to huge slabs of snow missing from the mountains that jut up behind town.

"Everybody woke up Friday morning and the mountains had fallen down around town," Carter said. "There was nobody who didn't realize something was going on."

The first big slides in Thompson Pass were natural, officials said.

But on Saturday DOT workers using explosives triggered a massive slide bigger than the natural one.

Triggered slides are a way of releasing snow in a controlled way, Woodrow said.

Unstable snowpack is still clinging to mountainsides in Keystone Canyon, Carter said.

KNOWN FOR SNOW

Meanwhile, the airport and port at Valdez remain open and DOT is working to reroute two additional ferries to Valdez this week to and from Whittier while the road is closed, Woodrow said.

Valdez is known for snow. Thompson Pass gets an average of 600 inches per year. During the winter of 1952, 974.1 inches of snow fell in Thompson Pass, the most ever recorded in the U.S., according to the city.

That snow can halt a major artery in the state's highway system is not hard to believe if you've ever seen an avalanche, Carter said.

From a helicopter, a huge snow slide starts with a fissure in the mountainside and transforms into something that Carter describes as looking like a "dragon" of snow: "They have those great snaking tails and are just always moving and pushing and building."

 

Road closures and warnings
Alaska weather warnings and advisories
By MICHELLE THERIAULT BOOTS, ZAZ HOLLANDER and TEGAN HANLON
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