Teton Dam — Piping Through the Core on First Fill, Gone in Hours

On the morning of 5 June 1976, the Teton Dam — a brand-new 305-foot (93-metre) earthfill embankment built by the U.S. Bureau of Reclamation on the Teton River in eastern Idaho — eroded itself open from the inside and released roughly 250,000 acre-feet of water onto the towns of Sugar City, Rexburg and Wilford below. Eleven people died, some 25,000 were left homeless, around 16,000 head of livestock drowned, and property losses ran from hundreds of millions of dollars into the low billions. The reservoir had never been full before; the dam was failing the first time it was asked to hold its design load. The cause was not overtopping and not an earthquake. It was internal erosion — piping — driven by water that fractured the dam’s silt core and tunnelled through open joints in the rock the core was keyed into.

The Teton was a conventional zoned embankment with an impervious central core of wind-blown silt — loess-derived material that compacts well and seals beautifully, but that erodes readily once water moves through it and holds an open pipe without collapsing. That core was trenched down into the canyon’s foundation rock: a fractured, highly permeable volcanic rhyolite riddled with open joints. The job of sealing those joints fell to a grout curtain and to slush grouting along the key trench. That sealing was incomplete. The most heavily loaded interface on the site — the contact between an erodible silt core and a jointed rock that could carry concentrated seepage — was left as a path waiting for water.

The reservoir filled fast. Impounding effectively began in October 1975 behind the still-finishing dam, and through the spring of 1976 the level rose at roughly a foot a day, accelerating to about four feet per day by June as snowmelt poured in. Springs and small seeps appeared in the right abutment in the first days of June. On the morning of 5 June a clear leak turned muddy, then grew. By mid-morning a wet spot on the downstream face was discharging twenty to thirty cubic feet per second and a whirlpool was visible upstream. Crews drove bulldozers into the widening hole in a last attempt to plug it; the machines were swallowed. At about 11:57 the crest gave way and the reservoir emptied through the breach in a matter of hours.

Two federal inquiries — the Independent Panel impanelled by the Secretary of the Interior and the Governor of Idaho, and the Interior Department’s own Teton Dam Failure Review Group — reached the same family of conclusions. The dam failed by internal erosion of the silt core, most probably initiated by hydraulic fracturing of the key-trench fill and by seepage through unsealed joints in the rhyolite beneath the grout cap. The Teton became the United States’ canonical first-filling failure, the case that forced filters, controlled filling and independent review into the heart of American dam safety, and the disaster that ended Reclamation’s era of unquestioned authority over its own designs.

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