Wyoming And The Largest Landside In American History

On June 23, 1925, an entire side of Sheep Mountain in northwest Wyoming's Gros Ventre Range collapsed, creating one of the largest landslides in North American history.

Around 4 p.m. roughly 50 million cubic yards of rock, dirt, and trees roared down the mountainside in less than two minutes.

It happened because heavy spring snowmelt, intense rain, and a small local earthquake saturated weak clay and shale layers beneath slipping sandstone. That is a lot of bad things coming together at once.

The result was debris that piled up 200 feet deep across the valley, blocking the Gros Ventre River and forming Lower Slide Lake. Miraculously, no one died during the initial slide. The slide from the side of the mountain can still be seen to this day.

Rancher Guil Huff, gave a firsthand account to geologists who were studying the event. Mr. Huff narrowly escaped the surge.

But the event was not over. On May 18, 1927, snowmelt from the swollen Gros Ventre River breached the landslide dam. Everything came rushing down.

The flood destroyed the town of Kelly, 4 miles (6 kilometers) downstream from the dam, and resulted in six fatalities.

Many lessons were learned. The Kelly Flood provided a crucial lesson in the aftermath of the 1959 Madison Slide, a consequence of the M7.3 Hebgen Lake earthquake, when engineers averted a similar disaster by constructing a spillway to lower the water level in the lake that formed on the Madison River upstream of the slide.

The danger is not over in the area, and it never will be. These mountains are young and so are prone to constant, sudden, and devastating change. The Gros Ventre River valley is a high-risk area for landslides as it features steep slopes and weak rocks, which is common for many of the mountainous areas surrounding Yellowstone National Park.

Learn more at this link.