The Teton Dam was an earthen dam in the western United States, on the Teton River in eastern Idaho. It was built by the U.S. Bureau of Reclamation, one of eight federal agencies authorized to construct dams. Located between Fremont and Madison counties, it suffered a catastrophic failure on June 5, 1976, as it was filling for the first time.
The collapse of Teton Dam killed 11 people and 16,000 livestock. The dam cost about $100 million to build and the federal government paid over $300 million in claims arising from its failure. Total damage estimates reached $2 billion. The dam has not been rebuilt.
Interest in building a dam in the eastern Snake River Plain had arisen for many years to control spring runoff and provide a more constant water supply in the summer. The area had suffered a severe drought in 1961, followed by severe flooding in 1962. The Bureau of Reclamation proposed the Teton Dam in 1963 and Congress passed, without opposition, an authorizing bill the following year. The planned dam was to be an earthen structure 310 feet (94 m) high and 0.6 miles (1.0 km) long, creating a reservoir 17 miles (27 km) in length. The impounded water would be used to generate hydroelectric power. An environmental impact statement was issued for the dam in 1971, but it did not raise the possibility of a collapse. Lack of funding and site preparation work and questions surrounding the required environmental impact statement stalled the project. Barely 14 pages long, the statement quickly drew the ire of opponents of the project.
On September 27, 1971, several environmental and conservation groups filed a lawsuit in Idaho District Court to stop the construction. Opponents of the dam questioned the project's justifications. They argued that damming a wild and scenic river would result in the destruction of its trout fishery and other wildlife habitat and "replace a unique resource with a vulgar one." The suit questioned the economic return on the investment, the bureau's compliance with the National Environmental Policy Act, and the geologic soundness of the dam's location. Concerns over the seismic conditions of the dam site delayed the bid process pending further review by the Department of the Interior. Pressure from Idaho's congressional delegation stopped the review, and in spite of the lawsuit, bids were taken in Idaho Falls on October 29, 1971. The $39 million contract was awarded in December 1971 to Morrison-Knudsen Co. of Boise, assisted by Peter Kiewit Sons Co. of Omaha, Nebraska. In spite of the lawsuit, work began in February 1972. After various motions, amended complaints, attempted injunctions, and appeals, the suit was dismissed on December 23, 1974.
The eastern Snake River Plain is underlain almost entirely by basalt erupted from large shield volcanos on top of rhyolitic ash-flow tuff and ignimbrites. The Huckleberry Ridge Tuff, a late-Cenozoic volcanic rock, is 1.9 million years old. The dam site is composed of basalt and rhyolite, both of which are considered unsuitable for dam construction because of their high permeability. This was confirmed by long-term pump-in tests at rates of 165 to 460 US gal (620 to 1,740 L) per minute. Test cores, drilled by engineers and geologists employed by the Bureau of Reclamation, showed that the canyon rock at the dam site is highly fissured and unstable, particularly on the right side (as one faces the direction of flow). The widest fissures were determined to be 1.7 inches (4.3 cm) wide. The bureau planned to seal these fissures by injecting grout into the rock under high pressure to create a grout curtain in the rock.
In addition, an investigation of the area by geologists of the U.S. Geologic Survey indicated that it was seismically active; five earthquakes had occurred within 30 miles (50 km) of the dam site in the previous five years, two of which had been of significant magnitude. This information was provided to the Bureau of Reclamation in a memorandum, but the geologists' concerns were considerably watered down in the 6-month redrafting process before the USGS sent the final version of the memo to the USBR in July 1973.
In 1973, when the dam was only half built, but almost $5 million had already been spent on the project, large, open fissures were encountered during excavation of the keyway trench near the right end of the dam, about 700 ft (210 m) from the canyon wall. The two largest, near-vertical fissures trended generally east–west and extended more than 100 ft (30 m) below the bottom of the key trench. Some of the fissures were lined with calcite, and rubble filled others. Several voids, as much as 6 in (15 cm) wide, were encountered 60 to 85 ft (18 to 26 m) below the ground surface beyond the right end of the dam and grout curtain. The largest fissures were actually caves. One of them was 11 feet (3.4 m) wide and 100 feet (30 m) long. Another one was 9 feet (2.7 m) wide in places and 190 ft (60 m) long. These were not grouted because they were beyond the keyway trench and beyond the area where the bureau had decided grouting was required. This necessitated using twice as much grouting as had been originally anticipated; the total injected grout included 496,515 ft3 (14,060 m3) of Portland cement, 82,364 ft3 (2,332 m3) of sand, 132,000 lb (60 t) of bentonite, and 418,000 lb (190 t) of calcium chloride, injected into 118,179 ft (36.021 km) of drilled holes.
Later, the report of a committee of the House of Representatives, which investigated the dam's collapse, felt that the discovery of the caves should have been sufficient for the Bureau of Reclamation to doubt its ability to fill them in with grout, but this did not happen. Even after the dam had failed, the bureau continued to insist that the grouting was appropriate.
In December 1972, concerned about the geological conditions of the Teton River Canyon, USGS geologist David Schleicher wrote about the Teton Dam while it was still under construction, "A final point is that flooding in response to seismic or other failure of the dam—probably most likely at the time of highest water—would make the flood of February 1962 look like small potatoes. Since such a flood could be anticipated, we might consider a series of strategically placed motion-picture cameras to document the process."
The dam was completed in November 1975 and filling the reservoir began at the standard rate of one foot (30 cm) a day. However, snows were heavy that winter, and five months later, the project's construction engineer requested permission to double the filling rate to deal with the additional spring run-off, while continuing to inspect for leaks and monitor the groundwater. A month later, though monitoring showed that groundwater was flowing a thousand times faster than had been originally anticipated, the filling rate was doubled again, to 4 feet (1.2 m) a day.
On June 3 and 4, 1976, three small springs were discovered downstream of the dam, although the water running through the leaks was clear and such leaks are not unexpected for an earthen dam. At the time, the reservoir was almost at capacity, with a maximum depth of 240 feet (73 m). The only structure that had been initially prepared for releasing water was the emergency outlet works, which could carry just 850 cubic feet per second (24 m3/s). The main outlet works and spillway gates were not yet in service; the gates were cordoned off by steel walls while they were being painted.
On Saturday, June 5, 1976, at 7:30 am MDT, a muddy leak appeared, suggesting sediment was in the water, but engineers did not believe there was a problem. By 9:30 am, the downstream face of the dam had developed a wet spot, which began to discharge water at 20 to 30 cubic feet per second (570 to 850 L/s), and the embankment material began to wash out. Crews with bulldozers were sent to plug the leak, but were unsuccessful. Local media appeared at the site, and at 11:15 am officials told the county sheriff's office to evacuate downstream residents. Work crews were forced to flee on foot as the widening gap, now larger than a swimming pool, swallowed their equipment. The operators of two bulldozers caught in the eroding embankment were pulled to safety with ropes.