According to the Federal Emergency Management Agency (FEMA) and the National Inventory of Dams, there are more than 80,000 dams in the United States--and about ⅓ of those pose a significant threat to life and property if a failure were to occur. Disastrous failure, whether from natural causes like rainfall or human causes like improper maintenance, can occur with very little warning. Such was the case in southeastern Idaho in the summer of 1976, when the Teton Dam catastrophically failed. 

In February of 1972, construction on the planned Teton Dam began in earnest, taking just over four years to fully complete. With a width of about 3,200 feet, the dam contained about 260,000 acre feet of water, covering more than 17 miles of Idaho land. By June of 1976, the dam was being filled for the very first time -- and, unfortunately, the last. On June 3, water began seeping through the canyon wall just downstream of the dam. By June 5, a leak had appeared, and the hole quickly grew to reach 25 feet in diameter. Just after 11 am on that day, a flood of water burst through the wall of the dam, releasing more than one million cubic feet of water per second. Within six hours of the initial failure, the entire reservoir had drained and had sent floodwaters cascading towards the neighboring towns of Sugar City, Rexburg, and Wilford. 

As water from the failed dam rushed through the valley below, roads quickly became impassable. Homes were swept from their foundations, and the towns were struck by cars, trees, and even animals being carried by the floodwaters. In total, there were 11 reported deaths, and the flood caused more than $2 billion in damages to property and infrastructure. 

In the aftermath of the Teton Dam failure, as well as others that occurred around that time, 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. Several forecasting models have been developed, and NWS River Forecast Centers continue to explore how and when to use modeling tools when facing a dam failure.

Flooding is a threat that affects the US and its territories -- from coast to coast -- almost every day. Since its founding, the National Weather Service has been dedicated to providing information on staying safe and protecting property before, during, and after a flood. 

Additional Reading: 

• https://www.usbr.gov/pn/snakeriver/dams/uppersnake/teton/index.html

• https://www.weather.gov/safety/flood-hazards

• https://www.nws.noaa.gov/oh/hrl/hsmb/hydraulics/dam_break.html