Detailed Accident Report

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Date: 2002-04-15
Submitted By: Doug Fesler AMSC
Place: Twin Lakes, Kenai Peninsula
State: AK
Country: USA
Summary: At least 143 Caribou killed in slide that occured 12/23/2002

Report by Doug Fesler Alaska Mountain Safety Center

Killey River Caribou Herd, Alpine Lake Avalanche Accident, Headwaters of the East Fork of Benjamin Creek, Kenai Mountains, Kenai National Wildlife Refuge, T2NR5W, Field Notes

Date of Avalanche: Unknown, Estimated at late December

Starting Zone Elev.: Approximately 4,600?

Runout Zone Elev.: Approximately 2,700?

Vertical Drop: ? 1,900?

Runout Angle: 26? (measured)

Starting Zone Angles: (mostly high 30?s (35? - 45?)

Slope Aspect: Northeast

Slope Surface: Alpine Tundra, scree, and seasonal snow

Slope Configuration: Track consistently smooth and steep with a narrow sloping upper bench (400-500 vertical feet above the runout zone) and a wider sloping lower bench (only slightly higher than the valley floor). In profile: U shaped glacial valley with lateral moraines.

Type of Avalanche: Unknown, but very likely soft slab avalanche involving dry cold snow

Failure layer: Unknown, but most likely a weak layer of faceted snow formed during early season cold temperatures and shallow snowpack.

Triggering Mechanism: Unknown, but most likely triggered by the weight of the caribou herd moving onto or across the slab in the upper third of the path.

Weather Background: The 2001-2 season was characterized by early season shallow snowpack and cold temperatures which resulted in the widespread development of very weak faceted snow on the ground surface. As winter progressed, several warm periods of precipitation were interspersed by cold events forming layers of weaker snow capped over by denser layers. The first significant warm event with precipitation occurred around the December 23 and resulted in widespread avalanche activity throughout the Kenai and Chugach Mountains. It is likely that the caribou heard inadvertently crossed this slope during this time, either seeking forage along the windblown ridges or moving to a better location. It is also possible that they were caught by a sudden storm and were heading for a more sheltered location. Prior to this storm, the snowpack would have been generally shallow and faceted with some windslab at higher elevations.

Accident Observations: As of May 28, 2002, approximately 49 caribou skulls or carcasses had been confirmed as dead on site. Most likely, others are still buried, while injured animals likely succumbed to predation later. The distribution of the victims? bodies (spread across an area measuring roughly 1000? X 1200?) is consistent with victims who have been caught by a slab avalanche high on a mountain and carried at great speed (estimated in this case at ?80 mph) downslope.

A high degree of trauma (evident from broken antlers and bones) indicates that the herd was swept down the mountain in a turbulent manner and impacted against rocks and each other. It is likely that they were hit by a natural release while grazing at the bottom. The fact that the animals were spread out across more than 1/8 of a mile wide, strongly supports the theory they were caught by an extensive slab avalanche, not killed by a fall while ascending.

My guess is that they were traversing the slope in parallel lines (or spread out with some traveling above others) heading in a northwesterly direction (entering the path from the south east) and that when they were approximately half way into the starting zone (upper 1/3 of the mountain), the slope ripped out. More than likely the area had recently been loaded with new snow (either from cross loading winds or straight precipitation). The slope was likely triggered by a combination of this recent loading and the added weight/ vibration of the herd. I suspect that there had to have been some wind action or a warming event prior to the accident to create a more consolidated slab layer over the weaker faceted snow. Without a stiffer slab to support their weight, their feet would have punched right through the faceted snow and they would have been walking essentially on the ground, unable to trigger a slide.

Two factors support the idea that the herd was high on the mountain when caught: 1) The extent of traumatic injures observed in the main body of victims found on the lower bench could only have occurred if they had fallen (and been tumbled) a long distance, and 2) remains of 3-4 victims were found on the upper bench, well above the main deposition area.

The avalanche, a moderate-sized, soft slab avalanche with a powderblast component, probably extended at least to the center of the valley bottom, with the majority of the victims terminating on the lower bench where the slope angles decrease significantly. From an avalanche perspective, a decrease in slope angle means an increase in friction or resistance, thus a decrease in forward momentum and velocity, and a higher probability that debris (and victims) will come to rest. It is interesting that most of the victims came to rest on the lower bench, but that 3-4 stopped on the upper bench, and 2-3 were carried to the valley floor. All indications are that the avalanche was very energetic and fast moving. More than likely the uppermost victims in the debris were uppermost when caught and/or along the edge of the path.

Certainly the terrain where the accident happened is a classic textbook avalanche path: steep, smooth, leeward, and cross-loaded top to bottom. Caribou, like humans, probably develop a sense of complacency about the risks they are exposed to, based upon their previous successes of having survived similar exposure (though not identical conditions) many times in their lives.

Updated Information: A skull count taken in August 2002 revealed a minimum of 143 fatalities confirmed. With snow debris still on site, the final count is likely to be larger.

Grim revelations

Biologists find skulls of at least 143 caribou killed in Kenai avalanche

By Jon Little

Anchorage Daily News

(Published: October 16, 2002)

The death toll of caribou killed in a single devastating avalanche last winter high in the Kenai Mountains has reached a dizzying 143, according to a biologist who helped count the skulls.

"It's the only incident I've ever heard of where so many animals from such a small herd were taken out," said Rick Ernst, a biologist and pilot for the U.S. Fish and Wildlife Service. Before the snow slide, the Killey River herd numbered about 700.

Several trips to the site, near Skilak Glacier 30 miles southwest of Sterling, turned up bone after bone as summer's brief heat melted most of the snow from the rocky tundra.

Biologists made new piles of skulls on each visit to make sure they weren't recounting the same carcasses.

The Killey River herd grazes on lichens growing on the flanks of mountains between Tustumena and Skilak lakes.

Investigators believe a string of caribou began traversing the steep, snowy ridge near Alpine Lake in late December 2001, and their hooves triggered an avalanche that swept scores of the animals downhill at speeds approaching 90 mph. Warm, wet storms dumping snow over a hard-frozen crust were triggering other slides at the time, according to experts.

This avalanche began at an elevation of 4,600 feet and petered out at 2,700 feet. Some of the caribou rolled and bounced the entire way to the bottom of the treeless slope.

The force coughed snow and animals over a bench, depositing the remains in this U-shaped valley. They lay there undetected, by humans at least, for about two months.

Ernst was the first to suspect something was wrong.

He was flying along the steep ridges bordering Alpine Lake in early March listening for radio signals from 21 collared caribou. The transmitters issue a distinct mortality beacon when an animal hasn't stirred for several hours, and Ernst picked up five of the tones while observing the snow slide from the cockpit of his plane. He heard four more of the mortality signals on a later flight.

If nine of 21 collared caribou were caught in the slide, he reasoned, there probably were many more.

Investigators helicoptered into the area in May for a first look from the ground and were stunned by the carnage. They saw broken bones poking out of a debris field blanketed in gray fur. Bears, eagles and wolves had feasted on the remains.

Biologists went back in July, and found yet more carcasses revealed by melting snow. Ernst returned one last time Sept. 9 and found more dead caribou.

Even at that time, snow still covered some of the area. So more animals may lie underfoot, and some wounded may have wandered off to die, he said. He described the 143 figure as "a minimum." By now, the valley is under this season's blanket of fresh snow.

Before the avalanche, state and federal biologists were worried about the Killey River herd growing too large and overgrazing its winter forage. They were encouraging hunters to thin the herd's ranks by about 200.

So will this single hit have any bearing on next fall's Killey River caribou hunt? Ernst said it was too early to say.

A lot of the animals killed by the avalanche were cows and calves, so the herd will take a while to recover, he said.

A new Killey River caribou head count this fall may help biologists from the state Department of Fish and Game, U.S. Fish and Wildlife Service and U.S. Forest Service gauge the situation, he said. The Board of Game meets in March to decide.

Reporter Jon Little can be reached at or at 907-260-5248.

Melting snow reveals mass caribou carnage

KILLEY RIVER HERD: At least 50 animals died in a late December avalanche.

By Jon Little

Anchorage Daily News

(Published: June 3, 2002)

Shattered antlers, broken bodies and hoofs poke out of the rotten snow at the base of a steep, treeless

slope near Alpine Lake in the western Kenai Mountains.

Some of the carnage has been exposed by warming temperatures. But much of it has been pawed out of

the snow and gnawed on or devoured by bears, wolves, wolverines and bald eagles. Bones and strips of

hide lie scattered.

"When the helicopter came in to land, caribou fur rose like dust in the air," said Jim Hall, deputy manager of

the Kenai National Wildlife Refuge.

Hall and two other investigators, state wildlife biologist Ted Spraker and avalanche expert Doug Fesler,

choppered into "the valley of death," as Hall called it, last week for some sleuthing.

They concluded that a string of caribou began traversing the steep, snowy ridge late in December, and

their hooves triggered an avalanche that swept scores of the animals downhill at speeds approaching 90


How many caribou died still isn't known. But the count has exceeded 50.

"I think all the wrong things happened at the right time," Spraker said. "It turned out to be kind of a

catastrophic event."

Judging by the mutilation, the caribou must have been near the top of the snow slide. Some rolled and

bounced about 1,900 feet to the bottom of the treeless slope, Fesler said. The avalanche began at an

elevation of 4,600 feet and ran out at 2,700 feet.

The force blew snow and caribou over a bench to be deposited in this U-shaped valley near Skilak Glacier.

The animals' remains were spread in an area 1,000 feet wide and 1,200 feet long.

Fesler, an avalanche hazard consultant with the Alaska Mountain Safety Center, said the slide probably

happened around Dec. 23, when numerous slides were happening in the area. Warm, wet storms had dumped snow atop a thin layer of

powder laid during a cold snap, he said.

He drew a mental picture of the final moments.

"The animals were traveling from left to right in a northwesterly direction and they were probably either in a line or a couple of lines,"

Fesler said. About halfway across this slope, one which they probably crossed safely many times before, the snow gave way.

"They were tumbling, flying through the air and landing hard," he said. "We saw one bull, its back was almost like a Z'."

After a laborious count of skulls, they know that at least 53 of the 700-strong Killey River caribou herd perished in this single slide. More

bodies lie beneath the snow.

"I'd be surprised if there isn't at least another dozen or more," Spraker said.

It is common to find the body of a hapless moose, mountain goat or Dall sheep at the base of an old snow slide. But Hall, Fesler and

Spraker said they were unaware of any case on this scale.

"It's a unique phenomenon," Hall said. "This is the largest mortality from an avalanche that I'm aware of, as far as the number of animals


Still, it easily could have gone unnoticed if not for a joint state-federal project to radio-collar young caribou. Researchers dart and weigh

the animals in the fall and spring to see how they've fared over the winter. It helps managers gauge the herd's health.

In March, biologists flying in the area picked up mortality signals from six of the 12 radio transmitters attached to collared caribou. They

all came from the base of the snow slide. The beacons are set off when a collar doesn't move for hours.

Spraker said he has since recovered three of the transmitters, all chewed up by predators.

Still, this death toll probably won't change next fall's scheduled caribou hunt, he said. Even if 100 had died, the Killey River herd's

population would still be 100 higher than the state's goal of 500.

The smaller herd would better match the amount of winter food, such as lichens, available in the range between Killey River and Twin

Lakes, Spraker said.

While hunters may frown at the reduced chance of bagging a bull, the calamity has been a boon to predators. Several wolves, eagles

and one brown bear were feeding on the carcasses when the helicopter set down.

"It looked like a smorgasbord," Spraker said.

One of Hall's fears is that the scene will become a magnet to hunters, antler collectors or curiosity seekers. He pointed out that

gathering racks on the wildlife refuge for sale is illegal, and these antlers are mostly reduced to shards anyway.

A visitor would surely run into one or more of the predators, and wouldn't be well received by a brown bear protecting its food supply,

he said. The bear could wind up being shot.

"My fear is we're going get people up there blasting wolves and bears," Hall said.

"That's useless, just for curiosity's sake," he said.

He said he would not accept a claim of self-defense from anyone who kills a bear at the slide site.

Reporter Jon Little can be reached at or at 907-260-5248.

Caribou casualties counted

AVALANCHE Initial check shows at least 12 died in Kenai slide.

By Jon Little

Anchorage Daily News

(Published: April 15, 2002, Page B-1)

State and federal game biologists inspected a remote Kenai Mountain valley near Twin Lakes last week to assess how many Killey River caribou died in a March 5 avalanche.

Caribou antlers poked from the snow as a helicopter hovered over the avalanche chute, churning up loose snow in its rotor wash.

An initial count indicates at least 12 were killed.

There were three medium-sized bulls near the surface. "We could just see the antlers and heads sticking out," said Ted Spraker, area management biologist on the Peninsula. Spraker also surveyed the scene from the ground as U.S. Fish and Wildlife Service officials made flybys aboard a Piper Super Cub.

They saw strips of hides and bone pieces strewn about, left behind by a brown bear that had pawed holes into the snow to reach the fresh meat, Spraker said. Wolverines also had prowled around.

"There's also a pack of 12 wolves there, in the same area. I'm sure they've been making use of the caribou," he said.

Beyond natural curiosity, biologists need to keep tabs on the herd's size. The largest of four herds on the Kenai Peninsula, the Killey River group has grown to a population of 700 since caribou were introduced to the area in 1986. It has been targeted for aggressive hunting next fall to reduce it to 500. Biologists say there is only enough winter habitat to properly support the smaller number.

If the slide turns out to have claimed a significant percentage of the herd, state biologists have plenty of leeway to alter the fall hunt, Spraker said.

He said he has never seen anything quite like it.

The occasional Dall sheep or mountain goat will take a misstep or succumb to a snowslide, he said. He has never heard of a single avalanche wiping out so many caribou. But given their habit of foraging in winter for lichen along wind-swept ridges, he said, he wasn't too surprised.

"After following these caribou in the Kenai Mountains and Killey River, it is absolutely amazing how they will go up these big slides, and they will graze across the front of a big, open face, with big cornices of snow hanging above them," Spraker said. "It seems like they really utilize the habitat that's available and sometimes it puts them in places they really shouldn't be. I'm sure they trigger avalanches from crossing some of these open ridges."

Biologists found out about this avalanche because it claimed half a group of 12 calves they had collared last fall. On March 5, six of the dozen radio collars began emitting mortality signals, a faster-paced beep set off when the collar is stationary for hours.

The six are still buried deep beneath the snow, Spraker said.

Because caribou calves often group with cows, with bulls forming their own subunits, Spraker guessed that the majority of those animals killed in the avalanche were calves and cows.

The caribou were swept to their death, possibly when a cornice gave way on the 40-degree slope, triggering a 1,300-foot slide from the top of an unnamed 4,000-foot peak to the valley below.

Spraker said he expects the count to rise as higher temperatures melt the snow.

"There's no speculation on how many have died," he said. There are 10 or 12 animals we have evidence of. It'll be August before we sort it all out for sure."

State and federal agencies will keep tabs on the snowmelt. They can count skulls, pelvic bones and spines left behind by scavengers to reach a final death toll.

Reporter Jon Little can be reached at or at 907-260-5248.