The Gros Ventre Slide, one of North America’s largest landslides, occurred on June 23, 1925, in the Gros Ventre River Valley of Wyoming. It took place on the north slope of Sheep Mountain, near the community of Kelly and the popular Jackson Hole area. The sudden movement of earth dramatically altered the local landscape, triggering a chain of events that would later result in a devastating flood. The slide remains a notable feature of the Bridger-Teton National Forest, offering a stark reminder of the power of natural forces.
The Geological Conditions That Led to Failure
The failure of the mountainside resulted from unstable geology and environmental triggers. Sheep Mountain’s structure was predisposed to failure, with layers of sedimentary rock tilted at an angle of roughly 20 degrees down toward the Gros Ventre River. The underlying layer was the Amsden Formation, which contains weak, impermeable shales. Above this lay the Tensleep Sandstone, a porous rock unit that allowed water to easily penetrate and collect.
Water saturation acted as a lubricant between the rock layers. Heavy spring rains and rapid snowmelt in 1925 saturated the Tensleep Sandstone. This water collected on the Amsden shale, lowering the frictional strength of the weak siltstone layers. Furthermore, the Gros Ventre River had eroded the base of the slope, undercutting the Tensleep Sandstone and removing the necessary support for the rock mass. Local reports of earthquake tremors in the weeks leading up to the slide suggest a final seismic event may have initiated the collapse.
The Immediate Event of 1925 and River Blockage
The mountainside gave way in the late afternoon of June 23, 1925, lasting only a few minutes. An estimated 50 million cubic yards of rock and debris hurtled down the slope of Sheep Mountain. This massive volume of material traveled at speeds around 50 miles per hour, creating a tremendous roar and a cloud of dust visible for miles.
The mile-wide landslide swept across the valley floor and rode 300 feet up the opposite mountainside. The debris completely blocked the flow of the Gros Ventre River. This created a natural earthen dam over 200 feet high and 400 yards wide. Water immediately began to back up behind the unstable structure, forming the five-mile-long body of water known as Lower Slide Lake. The presence of this massive, naturally formed dam instantly created a recognized hazard for the communities located downstream.
The Secondary Catastrophe: The 1927 Kelly Flood
Nearly two years later, following a severe winter with heavy snowpack, a rapid thaw and intense spring rainfall caused the water level in Lower Slide Lake to rise dramatically. On May 18, 1927, the rising water began to spill over and erode the crest of the dam.
The erosion quickly cut a channel through the loose debris. A wall of water, estimated to be at least six feet deep, rushed down the valley for over 25 miles. This wiped out the small town of Kelly, located only six miles downstream from the dam. The flood destroyed approximately 75 buildings and resulted in the loss of six lives.
The Enduring Physical Evidence
The landscape today bears the scars of the 1925 slide and the subsequent flood. The sheer face of Sheep Mountain exhibits a massive, treeless scar where the mountainside collapsed, a prominent feature in the Gros Ventre Wilderness. This exposed rock face shows the volume of material that was displaced.
The natural dam remnant, though significantly eroded by the 1927 flood, remains across the valley floor. The river now flows through a notch cut into the debris field. Lower Slide Lake still exists behind the remnant of the dam, though it is much smaller than its maximum size achieved before the breach. The area is preserved as the Gros Ventre Slide Geological Site, offering a unique opportunity to observe the long-term impacts of the event.