Among the cataclysms of nature, none is more terrifying than an
earthquake, and huge slides, like the one triggered by the Madison
Canyon earthquake, are perhaps the most dramatic type of geologic
change. In one sudden, spectacular moment, changes take place that make
us think of the tremendous energy released by atomic fission, the
earth's mass moves in a volume that rocks the imagination, and its
effect on the people who are near, or in the path of nature's huge,
impulsive-seeming change helps us to realize how infinitesimal we are
before the forces and laws of nature.
In 1903, a 40 million cubic yard rock slide crashed down from the crest
of Turtle Mountain onto the coal-mining town of Frank, Alberta, killing
70 people.
But the consequences of such huge slides aren't completed when the cliff
toppling ceases. Take the case of the famous June 23, 1925, Gros Ventre
slide in northwestern Wyoming, 40 miles south of Yellowstone Park. An
estimated 50 million cubic yards of rock and debris plunged down the
steep canyon wall, shot across the valley floor, and rushed some 350
feet up the opposite wall of the canyon before it settled back, like
water sloshing in a huge bowl.
Nobody was killed when this slide choked the Gros Ventre River. It
covered parts of two ranches and buried six head of cattle. But two
years later, in May, 1927, the water dammed by the slide pushed out a
big section of the slide and the sudden wave of water and debris washed
away the town of Kelly, Wyoming, killing seven people.
This kind of possibility was in the mind of Army Corps of Engineers
Missouri River Chief Keith R. Barney as he and Lt. Col. Walter W.
Holgrefe of the Corps district offices at Garrison Dam in North Dakota,
discussed the Montana earthquake's action. The slide represented a
double threat to people in the Madison Valley below.
As an immediate effect of the slide, the water flowing over Hebgen Dam
was stopped by the slide. The formation of a lake behind the slide began
the moment of the slide. When it filled, this 240-foot deep impoundment,
called Earthquake or "Quake" Lake, would exert an enormous pressure on
the slide. If the slide was composed of unstable material, its collapse
could, in a repeat of the Gros Ventre tragedy, bring death and
destruction to the valley towns of Ennis, Three Forks, and Trident
below.
[Illustration: The Hebgen slide on September 10, when Quake Lake
first began to flow through the 14-ft. deep channel cut by the Army
Corps of Engineers. The immense erosion that followed prompted
another 50-ft. channel cut on the slide. The lower picture shows the
completion of this project on October 17, 1959.
(U. S. Army Corps of Engineers)]
[Illustration: The completion of the channel.]
The second and greater threat was the discovery that when Quake Lake
filled, its impounded water would lap at the foundations of Hebgen Dam,
and quite possibly undermine it, releasing a volume of water seven times
that of the earthquake-caused reservoirs, which could also sweep part of
the slide along in its mad rush.
Like the threat of a time bomb, the rising level of Quake Lake, and the
increasing pressure of the water against the slide, augmented by rumor,
kept the downstream towns in constant anxiety.
The Army Corps of Engineers rushed into emergency action. They flew in a
50-man-staff, and set up headquarters in the Stagecoach Inn at West
Yellowstone.
[Illustration: Bridge repair.]
The mines in Butte were on strike, and huge earth moving equipment from
the open pit operations, along with rigs from other contractors, worked
around the clock to cut a 250-foot wide and 14-foot deep channel across
the mile and a half long slide and armor it with rock so the water
couldn't cause sudden erosion of the rest of the slide before the water
topped the huge natural dam.
On September 10, water licked over the new spillway, running into the
river bed just below, which had been dry since the quake. To the Corps's
great surprise, severe erosion tore the downstream face of the slide. To
remedy this, they launched another crash program to cut a 50-foot deep
channel across the top of the slide. It was completed October 29.
The Corps's operations on the slide took a total of close to $1,700,000
of funds set up for such emergencies. As a result, the towns below the
slide are safe from the flood threat the slide might represent.