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"SUDDEN LAKE" AND THE GROS VENTRE FLOOD

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The Gros Ventre landslide in June of 1925 "couldn't have happened at a better time for water users", according to the Snake River watermaster report. The slide occurred on the Gros Ventre River east of Jackson Hole, about 35 miles from Yellowstone Park. In just three minutes, fifty million cubic yards of rock and debris slipped from the north slope of Sheep Mountain to the canyon bottom and up the other side, forming a dam 225 feet high and half a mile long, blocking the river and creating a lake--called "Upper Slide Lake" by some and "Sudden Lake" by others--four to five miles long and up to three-quarters of a mile long. Three ranches and a ranger station, about two thousand acres, were submerged under about 150,000 acre feet of water. Bill Bierer, a rancher, had long predicted the slide and hence sold his land to Guil Huff in 1920, but he died in 1923 so never saw his prediction come true.


As a result of the slide, about two thousand second-feet of water were withheld from the Snake River system, but it came at a time when users had yet to start pulling irrigation water for their crops. And as the natural reservoir filled, the dam began seeping, eventually returning a flow of about 500 cfs to the system--almost the same as the flow on normal years.


In March of 1927, a paper was given on the Gros Ventre slide at the Geological Society Convention. W. G. Alden expressing concern as to what would happen if a snowpack similar to 1918's, when the river flow at Kelly (four miles downstream from the dam) reached six thousand second feet and averaged four thousand for two weeks. At 10 AM on May 18, 1927, the question was answered.


The winter of 1927 had seen heavy snowpack, and subsequent rain on the snow in May made the dam overflow. As the Gros Ventre flooded, Ranger Dibble saw what was coming and so drove to town to warn residents. They had just fifteen minutes to get out of the way of the waters. By four PM, just six hours after the damburst when the waters receded, a half million dollars in damage had occurred, leaving six dead. Of lesser importance, the now ruined town of Kelly was not awarded the county seat--it was given to Jackson.


The dam break had released sixty thousand acre feet of water, about a quarter of that of the Teton Dam failure nearly fifty years later. Bridges were destroyed and irrigation structures ruined. The water flooded farms and damaged highways, reached Heise the following morning, the river reaching an estimated 60,000 cfs before dissipated throughout the valley above and around Rexburg. Rigby and Lorenzo saw little flooding, and the river raised just three feet at Idaho Falls. Further downstream, the Woodville water gage barely registered a difference, the flow less than that which would occur later in the year at high water. It was believed that debris from the flood created smalller dams along the water route which obstructed the flow enough to ease the destruction.


The following is a long personal account by a survivor of the flood:


Ralph Thurston is the author of The Shanghi Plain: Bingham County's Early History, available at Kesler's Market in Blackfoot and online at https://www.amazon.com/dp/B0CCCS7XLR?ref_=pe_3052080_397514860




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