Geologic hazards of the Bighorn River Floodplan

In my initial article on geologic hazards of the Bighorn River floodplain, I described how towns along the Bighorn River are at risk due to possible seismic activity causing the Boysen Dam to become damaged or to fail. The following scenario is a theoretical combination of events. Thanks to multiple monitoring government agencies, we would have adequate warning and preparedness such that loss of property and life would be minimized.

Now consider a combination of circumstances that could come together to create the “Perfect Storm.”

A winter season providing well-over normal amounts of snow in the Wind River Range is poised to deliver huge amounts of water to Boysen Reservoir. An early period of warm weather and rain causes the snow to melt rapidly. The reservoir fills quickly to capacity even though the Bureau of Reclamation is releasing water at the highest rate possible without flooding towns downstream. Folks who live along the river are scrambling to sand bag their homes against the anticipated high water coming their way.

These circumstances place the Wind River Canyon hydraulic system at maximum vulnerability. The dam can no longer provide flood control since the reservoir is at maximum capacity. The dam itself is not in jeopardy because of the immense spillway overflow capacity that engineers built into its design.

While our river system is in this vulnerable state, a large low-pressure weather system moves into the area and provides a hard rain the likes of which we haven’t seen in many years.

After two days of continuous downpour, the walls of the Wind River Canyon have mobilized. Huge boulders and massive amounts of rock debris traverse the highway and railroad and cascade into the river. The large dormant landslide, visible just east of the Thermopolis water plant, has been reactivated.

The massive landslides have dammed up the Wind River. The landslide dam creates a reservoir of water behind it geologically-termed as a “landslide lake.” Since Boysen Dam is already at capacity, it can do little to hold back the water entering the canyon. As a landslide lake accumulates behind the dam, the potential energy can build to a high level. Imagine a Wind River Canyon filled with water to a depth of 25 feet. A landslide dam will fail suddenly once the water breeches the top and rapidly erodes downward. All of that water, and the destructive energy it carries, would flood onto the Bighorn River floodplain.

The next part of this series will address seldom-considered geologic hazard threats to the Bighorn River floodplain.