Case Study · Dam Failure · 1976
June 5, 1976, 11:57 AM. The Teton Dam in Idaho — built by the Bureau of Reclamation, essentially complete 7 months earlier — failed catastrophically during its first reservoir filling. The highest dam in US history to ever fail. The failure mode was piping through the porous volcanic rock foundation — a vulnerability that had been identified and not adequately addressed. The dam went from first visible seeps on June 3 to complete catastrophic failure on June 5 in approximately four hours of final warning. Eleven to fourteen deaths. Three hundred square miles flooded. Two billion dollars in damage. Built right, it would never have failed at all.
Teton River Valley, Idaho · June 5, 1976
The Teton Dam was a major Bureau of Reclamation project — the premier US government dam-building agency, with decades of experience constructing some of the most sophisticated dams in the world. Construction began in 1972. By November 1975, the earthfill embankment was essentially complete: 305 feet tall and 3,100 feet across, making it at that time the highest dam ever to fail in the United States. First reservoir filling began after construction, with water rising at a rate of approximately 3 feet per day in June 1976. On June 3, two small seeps appeared at the downstream toe of the dam — a sign that water was moving through the dam or its foundation. The ASDSO dam failure case study describes those initial observations: the seeps "released clear seepage and measured less than ¼ cfs." Clear seepage is less alarming than turbid (muddy) seepage; it suggests water is flowing through the foundation without yet carrying soil particles.
On June 5, that changed. The UW engineering case study of the Teton failure documents the progression: "By approximately 7:30-9 AM, a large seep with brown muddy water was observed." Brown muddy seepage means the water is now carrying soil particles — internal erosion (piping) is actively underway. The piping process, once started, is typically progressive and accelerating: each grain of soil eroded enlarges the flow path, which erodes more soil, which enlarges the path further. By approximately 10:00 AM, a crack appeared in the embankment. At 10:30 AM, dispatchers at Fremont and Madison County sheriffs' offices were notified that the dam was failing. The flood warning had been issued. But by that point, the time available to act was measured in minutes to hours. At 11:57 AM, complete catastrophic failure. Approximately 80 billion gallons of water — 300 million cubic meters — was released into the Upper Snake River Valley.
The Idaho Office of Emergency Management account of the disaster documents the flood: "Water touched nearly 300 square miles of land, impacting up to 100,000 acres of agricultural land and 32 miles of rail lines. Numbers of livestock lost range from 13,000 to 20,000." The NWS Heritage account documents the downstream communities: "the towns of Wilford, Sugar City, Rexburg, and Roberts" were in the flood path. "Homes were swept from their foundations, and the towns were struck by cars, trees, and even animals being carried by the floodwaters." Despite the catastrophic flood, the death toll was remarkably low: 11-14 people died. The downstream warning — issued when dam failure became evident — and the roughly 45-minute to 1-hour lead time before the flood reached major communities allowed most people to evacuate. The $2 billion in damage (1976 dollars; approximately $9.4 billion today) included 250 businesses, 733 homes destroyed, and 3,000 additional homes damaged.
June 5, 1976
Date
First filling
7 Months Old
11-14
Deaths
$2 billion
Damage (1976 dollars)
4 hours
Seep to Failure
The Science
Think of piping failure as a progressive underground erosion process. An earthen dam retains water through an impermeable core — typically clay — that prevents water from flowing through the embankment. The core must seal tightly against the foundation rock at the base of the dam (the "key trench"). If water finds a pathway through a gap between the core and the foundation — through cracks in porous rock, through inadequately filled voids, or through improperly compacted fill — the flowing water begins eroding soil particles from the surrounding material and carrying them downstream. This is piping: a pipe-like channel that grows as each eroded particle makes the channel slightly larger, which carries slightly more water, which erodes slightly more material. Once started, piping is typically self-reinforcing and accelerating. At Teton, the foundation was highly jointed rhyolite (volcanic rock) with open cracks that were inadequately grouted during construction. The ASDSO image caption of the Teton failure site shows "the jointed and permeable nature of foundation" — visible in the exposed rock after failure. This is the pathway through which piping occurred.
The Teton failure is one of a documented category of dam failures that occur during the first filling of the reservoir. The reason is specific: an earthen dam's interaction with its foundation is tested for the first time at full hydraulic head (water pressure) during first fill. Any seepage pathways that were not detected during construction — cracks in foundation rock, voids in the compacted fill, or imperfect core-to-foundation contact — are subjected to the full reservoir pressure for the first time. If those pathways initiate piping, the dam may fail before operators have the opportunity to lower the reservoir and investigate. At Teton, the dam was filling at approximately 3 feet per day. The UW engineering account documents that the reservoir was at elevation 5,301.7 feet — only 3.3 feet below the spillway sill — when failure occurred. The dam was nearly full for the first time in its history when it failed. The ARGOSHE account notes: "Within hours of the first visible leak, the entire structure collapsed, unleashing an estimated 80 billion gallons of water downstream." First-fill monitoring is the critical safety protocol for any new earthen dam: independent observation of downstream conditions, instrumentation of seepage and pore pressure, and clear protocols for stopping first fill and draining the reservoir if anomalies are detected.
The Teton failure, occurring in the same year as other US dam safety incidents (1976), directly motivated major reforms in federal dam safety programs. The NWS Heritage account documents one specific reform: "The NWS began a program to develop a flood-forecasting procedure that is specifically designed to cope with the characteristics of dam-break floods, as opposed to the more common floods that occur as a result of rainfall." Dam-break floods differ from rainfall-driven floods in their speed of onset, their extreme peak flows, and their shorter warning times. The Teton failure demonstrated that a dam can go from first visible distress to complete failure in approximately four hours — a window far shorter than the days of warning that riverine floods typically provide. Post-Teton, the Bureau of Reclamation significantly revised its design standards for earthfill dams on porous foundations. The Intermountain Histories account notes: "The fallout of this disaster led to the creation of many oversight groups and committees, dedicated to maintaining the integrity of dams all across the country to ensure that tragedies of this magnitude would never happen again."
Timeline
01
1972: Construction begins. Teton Dam is a Bureau of Reclamation zoned earthfill embankment on the Teton River, southeast Idaho, designed to provide irrigation water, flood control, and hydroelectric power. The foundation is highly jointed and permeable rhyolite volcanic rock. Some engineers raise concerns about the porous foundation and the adequacy of the grouting program to seal it. The design proceeds. November 1975: dam construction essentially complete. 305 feet tall, 3,100 feet crest length. The tallest dam ever to fail in the United States. Reservoir begins first filling.
02
June 3, 1976: two small seeps observed at the downstream toe of the dam. Seeps are clear (not turbid/muddy) and small (less than ¼ cfs). Clear seepage is less immediately alarming but must be monitored. Bureau of Reclamation personnel observe and document the seeps. Reservoir continues to fill at approximately 3 feet per day. Reservoir level: approximately 3.3 feet below the spillway sill — the dam is nearly full for the first time. June 4: continued monitoring. No action to halt filling.
03
~7:30 AM: Large seep with brown (turbid/muddy) water — piping actively underway. ~10:00 AM: crack appears in the embankment. ~10:30 AM: dispatchers at Fremont and Madison County sheriffs' offices notified dam is failing; evacuation warnings issued. 11:57 AM: catastrophic failure. 80 billion gallons (300 million m³) released into Upper Snake River Valley at 10 mph. Flood overtops Teton River canyon and spreads across Snake River Valley. Towns of Wilford, Sugar City, Rexburg, and Roberts struck. Homes swept from foundations; cars, trees, and animals carried by floodwaters. 80-mile flood extent; 300 square miles inundated. Flood eventually contained by the empty American Falls reservoir downstream.
04
Deaths: 11-14 (sources vary). Injuries: 2,000+. Damage: $400M-$2B (1976 dollars; ~$9.4B today). 250 businesses, 733 homes destroyed; 3,000+ homes damaged. 13,000-20,000 livestock lost. 200+ landslides triggered in inundated river canyon. Independent Panel convened: failure attributed to piping through/adjacent to key trench in right abutment; design and construction inadequacy in porous rhyolite foundation. Bureau of Reclamation revises earthfill dam design standards. NWS develops dam-break flood forecasting procedures. Teton Dam is never rebuilt. Multiple oversight groups and committees created to improve US dam safety. A sign posted during cleanup read: "Was it worth it?"
Human Decisions
The engineering failure
The ARGOSHE account of the Teton failure documents the engineering concerns that existed before construction: "The Teton Dam was designed to be an earth-filled dam... unfortunately, the materials used in construction were not adequate to withstand the water pressure, especially in combination with the flawed foundation. There was also insufficient attention given to potential failure modes, such as erosion through the foundation, which proved to be fatal in this case." The ASDSO photograph of the Teton failure site specifically shows "the jointed and permeable nature of foundation" — the rhyolite rock with its open joints and cracks that water found its way through. The grouting program that was supposed to seal these joints was inadequate for the scale of the problem. The Independent Panel that reviewed the failure confirmed: it was caused by "piping through or adjacent to the key trench in the right abutment" — the exact vulnerability that the porous foundation created. The concerns that were raised during design and construction about this vulnerability were not resolved. The gap between raised concerns and built reality is the specific Teton lesson.
The ASDSO account documents that the first signs of potential problem (clear seeps on June 3) were observed and documented without halting the fill. In modern dam safety practice, the appearance of seeps during first fill is a signal to conduct engineering assessment before continuing to fill. Clear seeps may be less alarming than turbid seeps, but seepage from the downstream face during first fill at near-full reservoir elevation is a known piping precursor. Stopping the fill on June 3, draining the reservoir, and conducting a geotechnical investigation of the seep locations would have been the risk-averse response. The reservoir filled 3 more feet — to within 3.3 feet of the spillway — over the two days between the first seep observation and catastrophic failure. Had filling stopped on June 3, the failure might have been detected at a lower reservoir level, with less water to release and more time to respond. This is the specific first-fill monitoring lesson that Teton contributed to modern dam safety practice.
What saved lives — and the US aging dam problem
The remarkably low death toll from the Teton Dam failure — given the scale of the flood and the $2 billion in damage — was the result of the evacuation warning that was issued when dam failure became apparent at approximately 10:30 AM. The warning was "believed to have been timely" according to the Independent Panel. Downstream communities had approximately 45 minutes to 1+ hour before the flood reached populated areas. The flood came on a Saturday morning with clear weather and good visibility — conditions favorable to quick evacuation by car. The contrast with the Banqiao disaster (where communities received essentially no warning) and Vajont (where the wave arrived within one minute of the slide) is direct: the Teton warning, though short, was enough time for most people to move to safety because the warning was issued, received, and acted on.
The Teton Dam failed in 1976 — 50 years ago. The vast majority of the approximately 90,000 dams in the FEMA National Inventory were built between the 1930s and 1970s. Most are approaching or have exceeded their 50-year design life. The Association of State Dam Safety Officials (ASDSO) regularly reports on the state of US dam infrastructure: in the 2023 Infrastructure Report Card, ASDSO gave US dams a D+ rating. The number of high-hazard-potential dams (dams where failure would likely cause loss of life) that are in "poor" or "unsatisfactory" condition runs into the thousands. The Teton lesson is not historical — it is directly applicable to the aging dam infrastructure question the US faces today: engineering concerns about dam conditions need to be acted on before failure, not after.
The cascade lesson
The Teton failure is the new-dam construction defect case study — the demonstration that a dam built by the most experienced dam-construction agency in the world can fail in its first filling if engineering concerns about its foundation are not adequately addressed. Its lessons are directly applicable to the current US dam safety situation: most US dams are now approaching or exceeding their design lives; many have known deficiencies that are underfunded for repair; the ASDSO gives US dams a D+ rating. The Teton failure's four-hour warning window — from first muddy seep to complete collapse — is the realistic timeframe for dam failure events, and it defines why pre-positioned evacuation planning and alert system registration are the preparedness actions that save lives.
What You Can Do Now
Teton's lesson is about speed and warning: four hours from first sign to collapse, and relatively few deaths because most people evacuated on the warning. These five actions address what you do before a dam emergency and in the critical hours when one begins.
The Teton Dam evacuation warning that saved most lives was issued through county sheriff dispatchers and the emergency alert system of 1976. Today, dam Emergency Action Plans specify exact alert chains, thresholds, and communication channels. Your county emergency management office maintains these plans for regulated dams in your area. Search "[county name] dam Emergency Action Plan" or contact your county emergency manager. Additionally, register for your county's emergency notification system so that you receive alerts automatically when a dam emergency is declared — the same way residents in Fremont and Madison counties received their notification on June 5, 1976.
Dam emergency alert registration guideThe Teton Dam collapse released 80 billion gallons that flooded 300 square miles. Eleven to fourteen people died because most had time to evacuate — but only because they knew where to go and acted immediately. Identify your evacuation route from your home and workplace to high ground above the dam inundation zone. Note any roads that cross the river valley and would be cut off by a flood. Practice the route. A go-bag with essential documents, medications, and supplies — accessible in under five minutes — is specific dam failure preparedness that paid off in the Teton communities where residents had minutes to leave.
Dam failure go-bag and evacuation guideTeton Dam went from first muddy seep (visible sign of active piping) to complete failure in approximately 4 hours. The Johnstown Flood reached Johnstown 35 minutes after dam failure. The downstream warning at Teton reached communities approximately 45 minutes to 1 hour before the flood. In a dam failure event, there is no "wait and see" option. Unlike a hurricane that can be tracked for days, or a flood that rises slowly from rainfall, a dam failure can arrive within minutes to an hour of the alert. When a dam emergency is declared for a dam upstream of you, leave your home immediately. Do not stop for unnecessary belongings. Do not try to watch the event. Do not return until cleared by officials.
Dam failure emergency response guideThe Association of State Dam Safety Officials (ASDSO) reports that there are approximately 90,000 dams in the US, most built in the mid-20th century, many approaching or exceeding their designed lifespans. In ASDSO's most recent infrastructure report, US dams received a D+ grade. Thousands of high-hazard-potential dams — dams where failure could cause loss of life — are in poor or unsatisfactory condition. Many state dam safety programs are chronically underfunded. Asking your state legislators about dam safety inspection funding, supporting dam safety ballot measures and infrastructure bills, and being aware of the condition of high-hazard dams in your area are community-level actions with direct safety implications. The Teton lesson is that the gap between engineering concerns and adequate funding can be fatal.
Dam safety advocacy and infrastructure guideThe Teton Dam failure destroyed 250 businesses and 733 homes and damaged 3,000 more — totaling $2 billion in property losses. Standard homeowners insurance policies in the US generally do not cover flood damage, including flood damage from dam failures. FEMA's National Flood Insurance Program (NFIP) provides flood insurance for properties in NFIP-participating communities. Properties in dam inundation zones may also be eligible for additional flood protection. If you live in a dam inundation zone — or downstream of a dam system in a river valley — understanding your flood insurance coverage and specifically whether it covers dam failure scenarios is directly applicable Teton-motivated preparedness.
Flood insurance for dam failure scenarios guideDam Failure case study series
Johnstown 1889 covers the defining US dam failure. Vajont 1963 covers dam overtopping from a landslide without structural failure. Banqiao 1975 covers the largest death toll in dam failure history and cascade failure. Oroville 2017 covers deferred maintenance and the US aging dam infrastructure crisis.
Full dam failure case study seriesSources