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 in the Black Hills of Western South Dakota. 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 9 resulting in 238 deaths and 3,057 injuries. Over 1,335 homes and 5,000 automobiles were destroyed. The value of property damage was estimated to be over US$160 million in 1972 dollars ($ in dollars). Flooding also occurred in Battle, Spring, Bear Butte, and Boxelder creeks.
Causes
A few days before the Rapid City Flood, "earlier rains had left the soil saturated, increasing the amount of runoff of the flood to come." On the afternoon of June 9, substantial rains fell on the area, caused by "an almost stationary group of thunderstorms."
Additionally, "a strong low-level easterly flow which forced the moist unstable air up-slope on the hills. This sustained orographic effect helped the air to rise, cool, and release its moisture in repeating thunderstorms. Another contributing factor was the unusually light winds at a higher atmospheric levels which did not disperse the moist air nor move the thunderstorms along to prevent an extreme concentration of rainfall."
According to Herbert Thompson the air pattern causing this storm was noticed in big measure over the Great Plains, with a minor scale to the east of the Rockies. There were only what appeared to be light winds, so nothing indicated that there was a huge storm underway. Thompson further indicates that a cold high pressure region was pushed from Canada into the Great Lakes region leading into the western part of South Dakota. A "mesoscale cloud mass" from Colorado also moved towards Rapid City. One of the cloud masses amplified the "mid-level moisture over Rapid City," while the other mass caused the pressure to stay, as the high pressure pushed downward, thus creating the right conditions to produce rainfall. The rainfall from the upper "cloud mass" above the Black Hills formed into new smaller masses downwind which reprocessed the rain allowing for the constant rainfall. The storm was described by Nair, Hjelmfelt, and Pielke as "convective cells of high precipitation efficiency a characteristic of tropical precipitation systems. Cloud seeding experiments being conducted by the Institute of Atmospheric Sciences on clouds west of Rapid City were speculated to have contributed to the unusual amount of rain. However, there is no evidence that the two phenomena were related.
The immense precipitation was based over the "Rapid, Boxelder, Spring, and Battle Creeks," creating run-off, along these creeks which led to flooding throughout the surrounding areas.
The intense rain began on the afternoon of June 9 and continued until after midnight on June 10. The immense amount of rain the Black Hills received during the thunderstorms ranged from 4 to 15 inches, causing Rapid Creek and surrounding creeks to overflow and creating massive amounts of runoff that resulted in flood waters. The run-off carried rubble to the Canyon Lake Dam creating a barrier in front of its spillway. This resulted in the "increase in depth of water behind the dam of 11 to 12 feet," which caused the release of more water adding to the already moving floodwaters. The flash flood hit Rapid City the hardest around midnight on June 9; the flood waters also hit the small town of Keystone nearby.
Damage
The flood caused a tremendous amount of damage. Flood waters displaced large rocks, trees, trailers, and vehicles, and carried homes away. In Rapid City the flood resulted in the deaths of "238 including 5 missing," 14 of the deaths were trained professionals. The flood resulted in over 3,000 people being injured. A total of 1,335 homes were ruined, and 2,820 homes were damaged. More than 200 businesses were ruined, and around 5,000 cars were demolished. The damage in Rapid City totaled $66 million in 1972 dollar value. As for Keystone, "eight people were killed and much of the town was washed away." The damage in Keystone totaled around $1.5 million. The total cost of the 1972 Black Hills flood totaled $165 million, including infrastructure and utilities. The total amount of rain the Black Hills received was "800,000 acre feet" equal to "1 billion metric tons of water."