Taum Sauk was/is a hydroelectric pumped storage facility located in Missouri. Pumped storage facilities are used by electrical system operators to store power (in the form of water at increased elevation) when demand is low, and to generate power when it is needed most.
Pumped storage units will use their generators as motors, and their turbines as pumps, to pump water to an upper reservoir. This is typically done at night or during weekends, which are periods of low electrical demand. When peak electrical load is needed, the water is released from the upper reservoir to a lower one, through the turbine, which spins the generator for additional power. The process repeats as needed.
Taum Sauk began operation in 1963, with an upper reservoir built of earth atop a hill in Missouri. Initially the generating capacity was 2 x 175 MW turbine-generators, but this was upgraded in 1999 to 2 x 225 MW.
On December 14, 2005, at 5:12 AM, the dam failed. The only good thing about this event is that nobody was killed.
Because this was a pumped storage reservoir, someone had to decide when the upper reservoir was full enough to stop pumping water into it at night. Unfortunately, the device they used to tell them how high the water level was, came loose from its mount. More unfortunately, nothing was done about it. The loose level indicator gave so many spurious alarms that the "High Level" alarm setpoints were changed so that they would never alarm.
Given this scenario, it was inevitable that at some point, the water would overrun the top of the reservoir. When the water topped the reservoir, it quickly eroded the earth fill dam. The entire upper reservoir burst out, sending a 20ft. high wall of water down the side of the mountain. The lower reservoir, having been emptied by pumping to the upper reservoir, was designed to hold that quantity of water. It withstood the flood and contained it.
Courtesy Wikipedia contributer Kbh3rd
Only one family was swept away, the Superintendent of Taum Sauk State park and his family. Fortunately, they all survived. Not everyone downstream from a failed dam is so lucky. This wasn't a particularly large or lethal dam failure - about 1 billion gallons of water were released and then recaptured downstream. As is customary these days, nobody within the company went to jail. Instead the company was fined $15 million - mainly hurting a host of hapless stock owners.
The upper reservoir was rebuilt with concrete, and the power plant returned to operation in 2010.
Taum Sauk upper reservoir during the reconstruction process. Photo courtesy KTrimble at English Wikipedia