Hydrologist Carolyn Dreidger hovered in a helicopter yesterday over Mount Rainier, looked down and saw the surge.
Tons of volcanic rock and debris pouring into the mountain's Van Trump basin. It looked like concrete going through a chute.
But this stuff was moving faster than concrete, and the chute, she remembered after the awe of the moment, went through a popular trail and under a main bridge along the road to Paradise at the foot of the volcano.
"Heads up high," she shouted as the helicopter buzzed over hikers, who she feared might be threatened by the oncoming muck.
Dreidger had witnessed a debris flow triggered by the rapid melt of a glacier along the mountain's south flank. The melt continues to course from the Kautz Glacier, flushing waves of water, mud and rock down the adjacent Van Trump Creek drainage.
The first and most serious surge, on Tuesday evening, took out a trail bridge and briefly prompted evacuations of some homes downstream.
Geologists initially thought the flows were caused by a "glacial outburst flood," which occur as snowmelt pools within glaciers and then suddenly releases. Glacial outbursts can sometimes touch off large, destructive mudslides known as lahars.
But a helicopter inspection by Dreidger and other U.S. Geological Survey (USGS) officials yesterday showed no such sudden release.
Instead, the warm, dry summer is turning ice to water along a finger of the Kautz Glacier that peeks over a ridge known as Wapowety Cleaver. The melting outcropping of ice is shooting a fire-hose-like stream into the adjacent drainage basin.
The water is soaking and eroding loose volcanic soil and rocks as it moves downhill.
At least two additional surges flushed down the Van Trump Creek drainage yesterday.
No hikers were injured, but National Park Service officials closed a short trail within the debris flow zone.
Downstream communities do not appear to be in harm's way, Pierce County officials said.
"There's tremendous melting along the Kautz Glacier and that could continue for a while," said Cynthia Gardiner of the USGS.
The melt from these glaciers ultimately is carried away by the Nisqually River, which winds west through Pierce County.
Late Tuesday, the Nisqually appeared muddier and higher than normal for August, but a gauge near the town of Ashford measured far below flood stage.
By yesterday afternoon, river flows had receded from Tuesday night's levels.
"It's well within with seasonal ranges," said Jim Gregory, an Ashford-based fire chief for Pierce County.
Still, the flow was an impressive, fearsome sight as it coursed through the Van Trump Creek drainage. And Tuesday night, with darkness preventing aerial reconnaissance, authorities hoped for the best but believed they had to prepare for the worst.
"Why did we ramp up so significantly?" said Steven Bailey, director of Pierce County Emergency Management as he watched footage from the helicopter of yesterday's surge.
"The answer is right before your eyes. The unknown did not allow us to do nothing."
While no formal evacuations were ordered late Tuesday, emergency officials in low-lying areas were told to prepare for a possible lahar.
Officials soon realized a lahar wasn't coming, but not before some residents had been put on alert.
As long as sunny, warm weather continues, the risk of debris flows will remain high.
More than a dozen glaciers wrap in and around the ridges on Mount Rainier. The glaciers constantly move, melt and rearrange the mountain's face.
From 1967 to the present, at least 26 debris flows have been caused by Rainier's South Tahoma Glacier, Dreidger said. She guessed the Kautz has caused about a half dozen in the same time period.
The debris flows tend to happen late in the afternoon and at night, when the melt water is at maximum discharge, she said.
Hikers should be vigilant.
"If they hear what sounds like a freight train moving down a valley — where there's no track — they need to get up-valley," Dreidger said.
Don Willard of Puyallup heard the sound of a debris flow coming toward him yesterday morning as he hiked the Comet Falls trail.
"I got behind a tree because the river was cracking," he said, referring to the sound geologists say is being made by car-sized boulders washing down the Van Trump Creek.
He made it through the surge fine.
So did all the other hikers on the two-mile trail who were evacuated yesterday around noon by park rangers.
The Comet Falls trail will be closed indefinitely because of flows, said Jill Hawk, a Mount Rainier ranger. A footbridge across the Van Trump River has been washed out and the trail switchbacks near Comet Falls are coated with mud.
"If (the weather) cools off, this could taper off, " said Tom Sisson, a geologist with the USGS who has been studying Rainier for years.
But when fall comes, he said, rains could trigger more debris flows.
Still, Sisson said there's not enough loose material to push the Nisqually out of its banks and harm downstream communities.
The debris is dispersing by the time it enters the Nisqually River.
But Tuesday evening, when the first flows were noticed, no one could be certain just what was happening.
Park Service officials began getting reports of the flows around 9 p.m., said Gregory, the Ashford fire chief, who monitored their radio communications.
At 9:43 p.m., Gregory said, he got a formal call from Park Service officials to inform him of the mud flows.
"The Park Service, you got to realize, they didn't have a lot of good information. It was pitch black and night," Gregory said.
Gregory and volunteer firefighters notified riverside residents they should prepare for a possible evacuation order.
Evacuation orders are serious business in Ashford, which is near Rainier and could be greatly affected by a volcanic eruption. Coincidentally, Ashford-area homes and some businesses are scheduled by December to be equipped with radio devices that broadcast emergency evacuations.
On Tuesday night, some people along the river didn't wait for a formal order to get out.
Sandra Mecarini, visiting from New York, was staying at the Stone Creek Lodge along the Nisqually.
"About 10 (p.m.) the manager very violently knocked on the door and opened it and said `There's an emergency. You have to vacate immediately. Go toward Elbe,' " she said.
Mecarini and her husband jumped in their car and fled to the fire station in Ashford about four miles away. Others were there.
When they were told evacuation was unnecessary, Mecarini and her husband returned to the lodge, but slept with their clothes on just in case.