Posted: Tuesday, November 11, 2003 12:00 am
PINEDALE (AP) - A 30-acre mountain lake contained by a glacier broke through and flooded a remote mountain canyon in September, changing the face of at least eight miles of three mountain drainages.
The unusual geological event occurred in the Wind River Range in western Wyoming. Only a few witnessed the resulting flood, and nobody was harmed.
The unnamed lake was located 12,000 feet above sea level, just east of the Continental Divide and about 2.5 miles north of Gannett Peak, Wyoming's highest mountain at 13,804 feet.
The lake apparently 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, more than during spring runoff.
The 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 several downstream lakes and filled normally dry irrigation ditches more than 20 miles away in Fremont County with a silty brew.
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."
The 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.
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 glacier, named for locusts found frozen in its core, prevented the lake from flowing directly downhill; its outlet was to the east, over a rocky gap.
But Grasshopper Glacier, like all other glaciers in the range, is shrinking. Ultimately, the ice dam sank to the elevation of the rocky gap and the lake carved a new outlet.
Hank Williams, a U.S. Forest Service employee in Pinedale who hiked into the Grasshopper Glacier in October to document the drained lake, said nature was still in action more than a month after the flood.
Williams said he saw a 60-foot high ice wall on one shore of the drained lake. Icebergs that had been afloat were stranded on slopes that had been the lake bottom. He estimated 90 percent of the lake drained.