Geologic hazards of the Bighorn River floodplan

Our town and county leaders wisely built the first county hospital on “hospital hill” overlooking Hot Springs State Park. They realized from experience that the floodplain of the Bighorn River was subject to flooding on a semi-annual basis. Locating a hospital on the floodplain would subject it to periods when it could not be used due to flooding and potential ruin.

The building of the Boysen Dam during the 1950s has provided many great benefits including limited flood control. Consequently, the current hospital was built directly on the floodplain. From a geologic hazard aspect, this was a huge mistake. A result of building the Boysen Dam was the control of frequent flooding which had caused primarily low-level property damage and inconvenience. Unfortunately, the Dam also created a huge reservoir of potential energy with the capacity to destroy everything on the floodplain and cause serious loss of life.

The Independent Record article of May 5, 2016 “Preparing for if the dam breaks” is an interview with Bill Gordon, our County Emergency Management Coordinator. The article points out the facts of a failure of the Boysen Dam. The town would have 90 minutes before an on rush of water reaching 50 feet deep would destroy the buildings, infrastructure and people on the floodplain and the adjoining valleys and lower-lying topography. How likely is this catastrophic event to happen? All dams represent man’s attempt to control nature and all dams will fail over geologic time. However, dams do fail prior to their estimated useful lifetimes and Boysen Dam has issues that are of concern.

Several years ago, I was asked by the Boysen State Park Superintendent to do a field study and prepare a summary of the Park’s geology for distribution to tourists. I concentrated a part of this study on Boysen Dam and the immediately-surrounding geology. The Dam is of earthen construction and is sited in an area of several small-displacement faults. Even if no fault plane actually passes under the dam, the shaking caused by a nearby seismic event on one of these faults can have a liquefying effect on the water-saturated dam material. Of even more concern is the very-large-displacement fault that is located near the entrance to the first tunnel at the beginning of the Wind River Canyon. This mega-fault has brought Precambrian crystalline rocks, which are billions of years old, up against sedimentary rocks which are only hundreds of millions of years old. This fault is the geologic reason that the three tunnels are required. A substantial displacement along this fault would result in an earthquake rating high on the Richter scale. It is doubtful that Boysen Dam could sustain such an earthquake without significant damage or failure.

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