June 5, 1976 was not a red-letter day in the history of the Bureau of Reclamation (Bureau). The Bureau was in the process of filling their latest construction, Teton Dam, on the Teton River in the northeast corner of Idaho when that morning the dam failed catastrophically.
The Teton River is a tributary to the Snake River. The dam failure and subsequent devastating flood cost the lives of 11 people. It also killed some 13,000 cattle. Both property and ecological damage were major with the flood damaging or destroying structures, eroding the Teton River canyon, removing topsoil in the floodplains and damaging habitat in both the Teton and Snake River valleys.
Dam construction cost in the neighborhood of $100 million, a significant sum in the 1970’s. In the years following the disaster, the Federal government paid over $300 million in claims. The dam was not rebuilt.
The Bureau proposed the dam in 1963 citing power generation and flood control as the purposes of the project. Congress authorized it in 1964. An environmental impact statement issued in 1971 did not consider dam failure.
Construction of the dam began in 1972 and was completed in November 1974. The dam was an earth-fill dam, over 300 feet high in Teton Canyon. After completion, filling of the reservoir began. By the time the dam failed, the reservoir was at near capacity with a water depth of 240 feet behind the dam.
Failure of the dam happened rapidly. Early in the morning of June 5, a leak appeared and by mid-morning about 25 cubic feet per second of water was being discharged from a muddy spot on the face of the dam. A crew sent to try to plug the leak was unsuccessful. Shortly after 11:00 am the order went out to evacuate downstream residents which likely saved many lives. Failure began just before noon when the dam crest collapsed followed by the collapse of the entire right side of the dam. By 8:00 pm, the reservoir had completely emptied.
Why did Teton Dam collapse? One could claim that it was the result of the “Law of Unintended Consequences”. Yes, it was, but actually it was much more than that. At least some of the soil used to fill the dam was permeable loess. The canyon walls of the dam site consisted of volcanic material, basalt and rhyolite, both of which are highly permeable. Cores drilled prior to construction, particularly on the right side of the dam site, showed that the rock was fissured and unstable. However, Bureau engineers felt that the problem would be solved by sealing the fissures with grout under high pressure. That was what they did.
After the failure, studies blamed the collapse on both the permeable dam fill and the permeable rhyolite on the dam abutments. Water seeped through both the fill and through unsealed rock. A panel of experts that studied the failure summed it up: “The fundamental cause of failure may be regarded as a combination of ecological factors and design decisions that, taken together, permitted the failure to develop.”
It was a disaster waiting to happen. As I stated in my previous column, the Bureau no longer builds dams but rather is dedicated to the operation and maintenance of existing facilities. I can’t help but speculate that the failure of Teton Dam hurried this change along.
Some of the material in this column is adopted from the article “Teton Dam” in Wikipedia.