Geologic hazards of the Bighorn River Floodplain

There is a lesson to be learned from the recent near-catastrophic failure of the Oroville Dam in California. That dam is of similar construction as our Boysen Dam. Extended heavy rains filled the reservoir behind the dam to the extent that the spillway was activated to prevent overflow and breaching of the dam. The spillway itself began to erode and fail. An emergency evacuation of 180,000 people below the dam was instituted because the dam was in danger of failing.

I had an enlightening experience about rain potential in Colorado. A good friend and I had each purchased high-performance motorcycles. We decided to ride them on a camping trip over the weekend and enjoy the sinuous canyon roads above Denver. That Saturday night we had no specific plan on where to camp so we just stopped when it looked like rain. After pitching our tents there was some light rain and we got a good night’s sleep. Sunday morning we headed for US Highway 34 through the Big Thompson Canyon for our trip back down from the mountains and return to Denver. When we got there the road was blocked by the Sheriff and Highway Patrol. The Canyon had been devastated by a huge flash flood. The road was washed away along with 361 homes, 52 businesses and 139 lives. Some areas of this river’s drainage basin had received 12.5” of rain in a 4-hour period on Saturday night. It was estimated that the flow through the Canyon was equal to 3.8 times the typical 100-year flood. It was the weekend of July 31-August 1, 1976. It could have been us camping in that Canyon.

Although the Bureau of Reclamation takes appropriate precautions, we are at risk of a similar event occurring in the Wind River drainage basin above the Boysen Dam. Right now there is 2.37 times the normal amount of snow in the Wind River Range. Couple that with a torrential rain and/or a sudden thaw and the Dam’s capacity could be exceeded. Once the spillway is activated, flood control is no longer possible. At best, the floodplain that Thermopolis sits on will be flooded. At worst the Dam could fail and the town would be devastated by the power of rushing water and end up under an estimated 72 feet of water.

Currently, we are relying on the Federal Government to detect that the dam has failed and to provide us with an adequate warning for evacuation. We need our own, independent means of detecting a dam failure and a fail-safe means of getting that information to dispatch. We can rely on dispatch to give us the warning and to issue an order to evacuate the town to high ground. If the system works perfectly, we would have about an hour to react.

What should we do if we get the evacuation order? The folks being detained in the jail need to be released immediately and our elders and disabled folks will need our help. Recently, I comfortably walked a mile in less than 20 minutes. So walking (fast!) east or west will get you out of town and to high ground before the water arrives. Don’t take your car. The roads will be blocked by panicked motorists. The main thing is your survival. Don’t worry about logistics, government agencies and your neighbors on high ground will take care of them in due time.

The provided map of Thermopolis, which can be seen on page 12 of this paper, indicates the area of the current Bighorn River floodplain* inside the red line (4340’ elevation) and the potential 80-foot fill line in green (4400’ elevation) above which is safe ground. The floodplain area is subject to flooding up to several feet deep. The 80-foot fill line encompasses the maximum potential area affected by a Boysen Dam failure if the narrows at the Swinging Bridge becomes dammed up by a few dislodged railroad cars and mobile homes such that flood water is backed up behind it. Hint: the old airport is more than 200 feet above the Bighorn River and is a great safe place to hike to.

*Due to grade adjustment for uplift, the current Bighorn River floodplain is less extensive than the geomorphic floodplain created over geologic time.