ANCHORAGE, Alaska (AP) — Highway access to the city at the end of the trans-Alaska pipeline has been cut off indefinitely by avalanches, including one that dammed a river and created a lake up to a half-mile long across the roadway in a 300-foot wide mountain canyon.

State Department of Transportation officials said Monday that the snow slide about 12 miles outside Valdez on the Richardson Highway cannot be cleared until water behind the snow drains. The water in Keystone Canyon is too deep on the upstream side for crews to approach the snow dam. Officials fear that digging from the downstream side could trigger a dangerous surge of water and harm work crews.

"At this time there is no safe way to approach relieving that water," said Jason Sakalaskas, northern region maintenance engineer, at a news teleconference.

The avalanche was in a location prone to avalanches and known locally as Snow Slide Path, said Mike Coffey, the DOT's statewide maintenance engineer.

"This is of a magnitude we probably have never seen before," Coffey said.

The Lowe River is a whitewater stream with Class 4 and 5 rapids. Rafting companies use it in summer for trips, but usually in winter it's just a trickle, Valdez DOT Superintendent Robert Dunning said. The rain and melting snowpack have filled the river to about one-third its summer capacity, he said.

Coffey estimated snow is piled 100 feet high on the Lowe River and up to 50 feet high on the highway.

A second avalanche is blocking the highway at Mile 39. Up to 10 more avalanches are blocking parts of the highway that stretches north 360 miles to Fairbanks.

The avalanches have not affected the trans-Alaska pipeline, which is buried in the area. Air and water access remains open to Valdez.

The city's 4,100 residents, especially old-timers, are taking the inconvenience in stride, city Clerk Sheri Pierce said.

"We're doing our best to let people know we have food supplies at the grocery stores," Pierce said.

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Fuel companies expect no shortages. Mail is being flown in, Pierce said. Additional state ferries have been scheduled for people who want to leave with cars.

Valdez is on Prince William Sound 105 miles east of Anchorage. The Richardson Highway goes through the Chugach Mountains at the 2,800-foot-high Thompson Pass, one of the snowiest areas of Alaska.

Snow fell by the foot in early winter, Coffey said. Late last week, however, Alaska's unseasonably warm temperatures were accompanied by inches of rain. The precipitation trigged avalanches Friday morning that nearly trapped a trucker on the highway.

Transportation Department officials had hoped to reopen the highway early this week, but they remain stymied by unstable snow pack conditions, including the threat of additional avalanches.

They used a 105-mm howitzer and 50-pound bags of explosives dropped from helicopters to trigger additional snow in avalanche zones to make cleanup conditions safer for crews. Freezing temperatures also could stabilize snow on mountainsides.

Efforts during the weekend to clear avalanche zones were been delayed when two people walking toward Valdez on Saturday refused orders to stop. The two were eventually flown to the city by helicopter, arrested and charged with disorderly conduct and obstruction of a highway.

The highway is officially closed between Miles 12 and 42.