Teton Dam
The Teton Dam was a federally built earthen dam on the Teton River in southeastern Idaho, set between Fremont and Madison Counties, USA which when filling for the first time suffered a catastrophic failure on June 5, 1976. The collapse of the dam resulted in the deaths of 11 people and 13,000 head of cattle. The dam cost about USD $100 million to build, and the federal government paid over $300 million in claims related to the dam failure. Total damage estimates have ranged up to $2 billion. The dam was never rebuilt.

The dam site is located in the eastern Snake River Plain, which is a broad tectonic depression on top of rhyolitic ash-flow tuff. The tuff, a late- Cenozoic volcanic rock dating to about 1.9 million years, sits on top of sedimentary rock. The area is very permeable, highly fissured and unstable. Test boreholes, drilled by engineers and geologists employed by the Bureau of Reclamation, U.S. Department of the Interior, showed that one side of the canyon was highly fissured, a condition unlikely to be remediated by the Bureau's favoured method of "grouting" (injecting concrete into the substrates under high pressure). The dam was completed in November of 1975 and no seepage was noted on the dam itself before the date of the collapse. However, on June 3, 1976 workers found two small springs had opened up downstream.

The collapse and flood
At the time of the collapse, spring runoff had almost filled the new reservoir to capacity, with a maximum depth of 240 feet (73 m). Water began seeping from the dam on the Thursday before the collapse, an event not unexpected for an earthen dam. The only structure that had been initially prepared for releasing water were the emergency outlet works, which could carry just 850 cubic feet per second (24 m 3/s). Although the reservoir was still rising over 4 feet (1.2 m) per day, the main outlet works and spillway gates were not yet in service. The spillway gates were cordoned off by steel walls while they were being painted. On Saturday, June 5, 1976, at 7:30 a.m., a muddy leak appeared, suggesting sediment was in the water, but engineers did not believe there was a problem. By 9:30 a.m. the downstream face of the dam had developed a wet spot erupting water at 20 to 30 cubic feet per second (0.57 to 0.85 m 3/s) and 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 officials told the county sheriff's office to evacuate downstream residents. Work crews were forced to flee on foot as the widening gap, now over the size of a swimming pool, swallowed their equipment. The operators of two bulldozers caught in the eroding embankment were pulled to safety with ropes. At 11:55 a.m. Mountain Daylight Time (UTC-6:00), the crest of the dam sagged and collapsed into the reservoir; two minutes later the remainder of the right-bank third of the main dam wall disintegrated. Over 2,000,000 cubic feet per second (57,000 m 3/s) of sediment filled water emptied through the breach into the remaining 6 miles (9.7 km) of the Teton River canyon, after which the flood spread out and shallowed on the Snake River Plain. By 8:00 p.m. that evening, the reservoir had completely emptied, although over two-thirds of the dam wall remained standing.

Cause of the collapse
Study of the dam's environment and structure placed blame on the collapse on the permeable loess soil used in the core and on fissured (cracked) rhyolite in the foundations of the dam that allowed water to seep under the dam. The permeable loess was found to be cracked. It is postulated that the combination of these flaws allowed water to seep through the dam and led to internal erosion, called piping, that eventually caused the dam's collapse. The Panel had quickly identified piping as the most probable cause of the failure, then focused its efforts on determining how the piping started. Two mechanisms were possible. The first was the flow of water under highly erodible and unprotected fill, through joints in unsealed rock beneath the grout cap, and development of an erosion tunnel. The second was “cracking caused by differential strains or hydraulic fracturing of the core material.” The Panel was unable to determine whether one or the other mechanism occurred, or a combination. “The fundamental cause of failure may be regarded as a combination of geological factors and design decisions that, taken together, permitted the failure to develop.” A wide-ranging controversy erupted from the dam's collapse. According to the Bureau of Reclamation,

Deaths, damage and property claims
Teton Canyon comes to an end approximately six miles below the dam site, where the river flows into the Snake River Plain. When the dam failed, the freed waters struck several communities immediately downstream, particularly Wilford at the terminus of the canyon, Sugar City, Salem, Hibbard and Rexburg. Thousands of homes and businesses were destroyed. The small agricultural communities of Wilford and Sugar City were wiped from the river bank. Five of the fourteen deaths attributed to the flood occurred in Wilford. The similar community of Teton City, on the south bank of the river, is sited on a modest elevation and was largely spared. One Teton resident was fishing on the river at the time of the dam failure and was drowned. An elderly woman living in Teton City died as a result of the evacuation. One estimate placed damage to Hibbard and Rexburg area, with a population of about 10,000, at 80 percent of existing structures. The Snake River flows through the industrial, commercial and residential districts of north Rexburg. A significant reason for the massive damage in the community was the location of a large commercial lumber yard directly upstream. When the flood waters hit, thousands of board feet of timber snapped from their moorings, caught fire from leaking gas, and were swept downstream. The force of the logs and cut lumber, and the subsequent fires, practically destroyed the city. The flood waters traveled west along the route of the south fork of the Snake, around the Menan Buttes, significantly damaging the community of Roberts. The city of Idaho Falls, even further down on the flood plain, had time to prepare. At the older American Falls Dam downstream, engineers increased discharge by less than 5% before the flood arrived. That dam held, and the flood was effectively over, but tens of thousands of acres of land near the river were stripped of fertile topsoil. The force of the Teton dam failure destroyed the lower part of the Teton River, washing away riparian zones and reducing the canyon walls. This seriously damaged the stream ecology, and the native cutthroat trout population has been endangered. The force of the water and excessive sediment also damaged stream habitat in the Snake River and some tributaries, at least as far downstream as Fort Hall, Idaho. After the dam's collapse, debris clean up began immediately and took the rest of the summer. Rebuilding of damaged property continued for several years. Within a week after the disaster, President Gerald Ford requested a $200 million appropriation for initial payments for damages, without assigning responsibility for Teton Dam’s failure. The Bureau of Reclamation set up claims offices in Rexburg, Idaho Falls, and Blackfoot. By January 4, 1977, disaster victims filed over 4,800 claims totalling $194 million. By that date, the Federal government paid 3,813 of those claims, $93.5 million. Originally scheduled to end in July 1978, the Claims Program continued into the 1980s. At the end of the Claims Program in January 1987, the Federal government had paid 7,563 claims for a total amount of $322 million.

Possible reconstruction
No plans were made for rebuilding the Teton Dam immediately after the disaster.