 It is a summer evening in Rapid City, and many in this medium sized town settle down to watch TV and eat dinner. The area has seen several days of heavy rain culminating in an intense storm. This puts a damper on the Friday night, but the subdued and miserable start to the weekend just a few hours later would feel like a high point. As the final few hours of the 9th of June 1972 played out, disaster would strike. The Canyon Lake Dam, groaning under the strain of the heavy shower failed, sending millions of gallons of water hurtling towards the some 43,000 residents in Rapid City, South Dakota USA below. The four flood sirens intended to give residents some warning fail to activate, meaning the occupants of the city would be at the mercy of the torrent of water heading for them. The causing flood of damage would be one of the nation's worst dam failures, killing an estimated 238 people. The event would not leave as much a remark on the national psyche as the Johnstown flood or the tombstone of St Francis Dam, but a financial and human cost would be just as horrific. Today we're looking at the Rapid City Dam failure and I'm going to rate this disaster here 7 on my scale and here 5 on my legacy scale. Our story starts over 30 years before that fateful summer evening. It is 1935 and the USA is languishing in the Great Depression and millions are unemployed. Politicians at the time across the world are faced with an economic nightmare. Many homeless, jobless and helpless are created after the Wall Street crash of 1929. One such solution to the employment crisis, at least in the United States, is to make lemonade with lemons so to speak. With so much available manual labour, an idea to develop the country's infrastructure is posed by the government under FDR. It seems like a golden bullet for the countries that economic woes, providing jobs and in return improving the nation's infrastructure. On the 6th of May 1935, FDR issued Executive Order 7034, establishing the Works Progress Administration. This would allow funds to hire manual labour to develop new parks, buildings, roads, railways, bridges, amusement parks and even dams. One such project was just 7 miles outside of Rapid City, South Dakota. The idea was to build a reservoir for recreational activities for the nearby residents. This wouldn't be the first project in the area, as in 1936 a dinosaur park was opened by the Works Progress Administration. To create a reservoir, the Rapid Creek needs to be blocked off upstream of Rapid City. To create the block, the Canyon Lake Dam was built, a 20 foot high, 500 foot long, simple earth-filled structure with a spillway. The new dam would hold back the 192 acre feet recreational reservoir. Like all dams similar to this, the spillway is vital in keeping the weight of the water from overcoming the structure. The spillway employed gates which allowed control of the reservoir's level, for example when heavy rain is anticipated. Space can be made, so the dam won't be overcome. You see with an earthen dam, an overtopping can be even more catastrophic than a stone or concrete structure. This is due to a thing called erosion. As water flows over the top of the dam and down the downstream face, the soil that makes up the structure is moved away with it. After a certain point of erosion, the strength of the dam is severely compromised and as such can lead to a rapid and total failure. Rapid City had become a victim of its own success in some regards, as many developments for residential and commercial buildings were constructed in the flood plain of Rapid Creek and this brings us to early June 1972. The week preceding the 9th of June can only be described as wet. Multiple showers had left the ground along the Rapid Creek sodden. To add further weather issues to the mix, a low level air mass flowing from the east forced moist air up to 7,000 feet high mountain slopes of the Black Hills. In the morning of the 9th, the weather forecast went from bad to worse. Both national and local weather predictions were calling isolated thunderstorms approaching severe limits, with hail larger than three quarters of an inch and wind gusts stronger than 60 miles an hour. The afternoon was battered by two individual thunderstorms. These increased the levels of water in the Rapid Creek reservoir. As the afternoon gave way to the evening, at 7.15pm a flood warning was issued for the northern Black Hills, but shortly afterwards this was extended to Rapid Creek. In little over 6 hours, a total of 10 inches of rain had fallen over a 60 square mile area. The flash flood carried debris laden water from upstream of the dam into Canyon Lake Reservoir, where it accumulated. To try and counteract this, the spillway gates were opened, but debris blocked the flow of water. To add insult to injury, many recreational boats were released from their moorings and began to make their way towards the spillway. Several workers tasked with trying to clear the debris from the spillway were soon overcome. With little else that could be done, the level of the reservoir started to rise. Eventually, the inevitable would happen, and at around 10pm the dam was overtopped. As the water worked its way down the downstream face, it started to erode the soil on the top. The main structure would withstand 45 minutes of punishing flash flooding, but eventually disaster would strike, and a total failure of the central part of the dam happened. This released millions of gallons of water down the Rapid Creek towards Rapid City. Meanwhile, many in the city downstream settled down for an evening inside their homes due to the poor weather. But as the evening gave way to the early hours of the 10th, the flowing water, which was estimated to be around 50,000 sq. ft. per second, herded its way into the city. The water smashed its way through bridges across the creek, destroying any buildings in its path, and pushed any vehicles along with the flow of the flood. Many unsuspecting residents were drowned instantly as the water engulfed their homes as they slept in the summer evening. 1,335 homes were destroyed by the flood with an additional 2,500 significantly damaged. 238 people were estimated to have been killed, but this number would never be 100% accurate. An estimated $160 million worth of damage was inflicted on the city and surrounding areas. The nearby town of Keystone was completely erased and hundreds of businesses just completely disappeared. The early warning system designed to avoid such an incident failed to activate, resulting in such a high death toll. The dam was a victim of a freak deadly cocktail of weather, all of which contributed to an increase of 12 feet of the reservoir level. The event was theorised to be a once-in-a-500-year scenario in 1996. The dam didn't stand a chance, but a loss of life and property could have been lessened by a proper flood warning system and better city area zoning. As part of the reconstruction of Rapid City, the federal government put money into a project called Greenway, where some of the worst areas susceptible to flooding were converted to parkland. Properties still in the flood plain were rebuilt above ground level to mitigate the risks of flood, and some 754 acres were subsequently designated as floodway. More reliable flood warning systems were installed in the 90s as the risk of flooding will never go away in the area due to the steep watersheds. The dam was rebuilt in 1976, and it is hopeful that the mitigation strategies implemented will reduce the chance of another tragedy. Visit the Plain Difficult Production, all videos on the channel are creative commons attribution share alike licensed. Plain Difficult Videos are produced by me, John, in a sunny south-eastern corner of London UK. Help the channel grow by liking, commenting and subscribing, check out my Twitter for all sorts of photos, nods and sods, as well as hints on future videos. I've got Patreon and YouTube membership as well, so if you fancy, check them out. And all that's left to say, is thank you for watching.