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The day the Grasshopper burped
A glacier melts in the Wind Rivers, unleashing a 600-million-gallon flood.
By Angus M. Thuermer Jr.
An unusual geological event illustrated the power of nature in September when a 30-acre lake contained by a Wind River Range glacier broke loose and flooded a mountain canyon.
There might have been two or three witnesses to the flood a NOLS instructor and some wranglers when a lake 12,000 feet above sea level eroded through the Grasshopper Glacier and roared down Grasshopper Creek to the Downs Fork and into Dinwoody Creek.
In the course of about four days beginning Sept. 6, a gauge 17 miles downstream on Dinwoody Creek shot up from 200 to 900 cubic feet per second, a peak exceeding this spring's runoff. The deluge an estimated 650 million gallons gouged a 30-foot deep trench a half-mile down the glacier. It carried tons of glacial silt into Grasshopper Creek and deposited much of it across 70 acres of the Downs Fork Meadows in some places piling up sand bars several feet high. The flood clouded Mud Lake, Upper Dinwoody and Dinwoody lakes, and filled normally dry irrigation ditches more than 20 miles away in Fremont County with a silty brew.
The outburst overwhelmed outfitter Clayton Voss' camp in Downs Fork Meadow of the Fitzpatrick Wilderness of the Shoshone National Forest and sent his wrangler scrambling through a foot of water to move tents to higher ground. Nobody was harmed in the incident that changed the face of at least eight miles of three mountain drainages spanning 2,800 vertical feet.
Liz Oswald, a hydrologist with the Forest Service, flew over the area Sept. 22 and photographed the trail of sediment, silt and sand.
"I was just so excited because it was such an event huge," Oswald said. "The scale is so enormous. It sort of makes you feel a little small."
Hank Williams, a Forest Service employee in Pinedale who treked into the Grasshopper Glacier in October to document the drained lake, said nature was still in action more than a month after the flood.
"It was fascinating," he said. "It was very warm for October. The trench [in the glacier] was very dynamic. There were things falling in rock, mud water all kinds of condensation, water dripping. A rock came down and pegged me on the shoulder."
The now-drained and unnamed lake has been contained at the top of Grasshopper Glacier since before 1966, the date of aerial photographs on which current U.S. Geological Survey maps are based. It lay east of the Continental Divide below 13,340-foot Pedestal Peak and 2.5 miles north of Gannett Peak, Wyoming's highest mountain.
The lake was perched at the upper end of Grasshopper Glacier, an expanse of ice up to a mile wide that flows north more than two miles. The Grasshopper, named for locusts found frozen in its core, prevented the lake from flowing directly downhill; it's outlet was to the east, over a rocky gap and into Klondike Lake.
In the usual ebb and flow of mountain seasons, the lake would accept spring runoff, summer glacial melt and rainfall, disgorging the excess through the rocky gap rather than down the slope underlying the glacier.
But Grasshopper Glacier, like all other glaciers in the range, is shrinking. The integrity of the glacier as a dam was being compromised by summer temperatures. Ultimately, the ice dam sank to the elevation of the rocky gap and the lake carved a new outlet. In what was probably a fast-moving chain of events, water carved a channel in the ice culminating in the flood of Sept. 6-10. Glacier experts in Iceland call such an outburst a jokulhlaup (pronounced yodel-alp).
Outfitter Voss said wranglers noticed the Downs Fork leaving its banks and started moving his camp to higher ground. "They were able to walk around in a foot of water," he said. "Next day it came with a larger amount of water. It had a two-day rise."
Voss said now he knows how a sandy meadow near his camp was formed.
"It filled most of the [Downs Fork] canyon with sand," he said of the recent flood. "It's quite different looking."
Hydrologist Oswald flew over the lake and its outwash Sept 22. She estimated the lake's area and that 60 feet of water drained during the jokulhlaup. The flood may not have been of biblical proportions, but Oswald calculated that 1,800 acre feet or about 600 million gallons of water were set free in the deluge.
The wave that came through Grasshopper Creek could have been 10 feet high, she said. "It's hard to estimate without actually going in there and actually measuring it."
Outwash deposits filled Grasshopper Creek, she said. Lower, at the confluence of the Downs Fork and Dinwoody Creek, the water ponded temporarily. "We saw brightly colored white deposits surrounding the Downs Fork Meadow," she said of her flight.
Oswald guessed that water up to five-feet deep covered the meadow and that sediments may have been deposited four feet deep. A bridge in Downs Fork Meadow survived, but Forest Service officials are uncertain about its integrity. Mud Lake, 17 miles from the glacier lake, was full of sediment as was Upper Dinwoody Lake and Dinwoody Lake, she said.
Voss and Oswald said the fishery in the Downs Fork was probably wiped out.
"I think the fish were overwhelmed with sediment," Oswald said.
Water quality consultant Art Shoutis, who works for the Wind River Indian Reservation, said fish in the drainage have adapted to periodic waves of silt.
In the lowland lakes, "I don't think it's going to be real significant in terms of fisheries and biology," he said. "These are normally glacier creeks anyway. There's traditionally a pulse of sediment in warmer months.
The fish, "I think they are pre-adapted to a lot of that," he said. "There may be short-term effects, for sure."
In the high country the impacts may be more significant, he said. "Certain species that spawn in the fall could be impacted. Some of the trout food may be impacted."
Williams' visit to the glacier revealed a 60-foot high ice wall on one shore of the disappeared lake. Icebergs that had been afloat were stranded on talus slopes that had been the lake bottom. He estimated 90 percent of the lake drained.
In the glacier where the lake drained, Williams found a trench 40 feet wide, 30 to 40 feet deep and half a mile long.
"It drained underneath the glacier for some time," Williams said of the lake. "A cavern formed under the glacier. Due to warming climate, water flowed across the top. When it cut down to where the cavern existed, that's when the water [burst] out."
Western Wyoming Community College professor Charlie Love, based in Rock Springs, has been studying Wind River Range glaciers for two decades. He said such outbursts are normal.
"They're quite common around the world," he said. In the mid 1990s there was a similar event on the Mammoth Glacier on Gannett Peak. Often the outbursts happen at a glacier's moraine when it impounds water, then fails.
Wind River Range, glaciers are shrinking, Love said, and floods are becoming more common. "They're all melting dramatically," he said. His surveys of the Knifepoint Glacier near Fremont Peak show it receding 42 feet a year between 1985 and 1995. "Knifepoint Glacier will disappear within this century," he said.
There is a clue that the September 2003 jokulhlaup was not Grasshopper's first. USGS records from the Dinwoody gauge show a smaller spike in September 2002. Did the trench begin to form then? Will the Grasshopper shift and close the trench, setting the stage for another jokulhlaup?
Outfitter Voss, who has 18 years' experience in the Dinwoody, knows that when it comes to wilderness, nature is often in control and unpredictable.
"We had two years a row of fires," he said, "then we got about a billion gallons of water."
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