The Black Hills Flood of 1972, also known as the Rapid City Flood, was the most detrimental flood in South Dakota history, and one of the deadliest floods in U.S. history.
The flood took place on June 9–10, 1972[1] in the Black Hills of Western South Dakota. 15 inches (380 mm) of rain in a small area over the Black Hills caused Rapid Creek and other waterways to overflow. Severe flooding of residential and commercial properties in Rapid City occurred when Canyon Lake Dam became clogged with debris and failed in the late evening hours of June 9th resulting in 238 deaths and 3,057 injuries.[2] Over 1,335 homes and 5,000 automobiles were destroyed. The value of property damage was estimated to be over US $160 million in 1972 dollars ($936 million in 2017 dollars[3]).
Source:
https://en.wikipedia.org/wiki/1972_Black_Hills_flood