Timeline of Events June 8-9, 1972
The National Severe Local Storm Forecast Center (now known as the Storm Prediction Center) advised that isolated thunderstorms approaching severe limits (hail larger than ¾” and wind gusts stronger than 60 mph) were expected in western South Dakota during the late afternoon and evening.
The Rapid City National Weather Service (NWS) office predicted isolated thunderstorms approaching severe limits (hail larger than ¾” and wind gusts stronger than 60 mph) in the afternoon and evening in their release to the media.
Thunderstorms over the Black Hills were detected on Ellsworth Air Force Base (AFB) weather radar.
A line of thunderstorms developed southeast of Rapid City and moved west-northwest.
The Ellsworth AFB weather radar observer notified the NWS of precipitation over the Black Hills.
Rain began at Pactola Reservoir.
State Radio Dispatch requested commercial radio and television stations to announce that motorists should avoid Boulder Canyon because of high water between Deadwood and Sturgis.
State Radio advised the NWS of 12 inches of water over the highway west of Sturgis and heavy rain southwest of Rapid City.
The South Dakota School of Mines and Technology (SDSMT) radar operator told the NWS about strong thunderstorms in the Hermosa area.
The Pennington County Civil Defense Director called the Rapid City police to report 3 inches of rain at Pactola Dam, and that Rapid Creek was rising rapidly.
The National Guard Commander was informed of water over the road in Boulder Canyon by the Lawrence County Sheriff.
The SDSMT radar operator advised the NWS of heavy rain from Piedmont to the northern Hills, with radar indications of rain falling at the rate of 2 inches per hour.
7:00 pm – The NWS received a report of 4 inches of rain since 5:00 pm at Galena (southeast of Deadwood).
The NWS issued a flash flood warning for the northern Black Hills.
Rapid City Mayor Don Barnett was notified of the flash flood warning; the National Guard and local law enforcement were called to duty.
KOTA received a call from the dam tender at Pactola of rapid rises along several creeks.
A person in Nemo informed the NWS that Box Elder Creek was flooding, two dams upstream had broken, and low-lying areas of Nemo had been evacuated.
The NWS expanded the flash flood warning to include Rapid Creek and Box Elder Creek.
The Rapid City–Pennington County emergency operations center opened.
The Ellsworth AFB radar observer reported heavy precipitation over the central Black Hills.
Commercial phone service at the NWS office at the Rapid City Airport became intermittent and unreliable.
NWS informed radio and television stations that the heavy rain will continue until about midnight.
The Brookdale residential area (near the Central States Fairgrounds) was evacuated.
In a television and radio broadcast, Mayor Don Barnett urged immediate evacuation of low-lying areas.
Canyon Lake Dam failed, adding to the flood waters rushing through Rapid City.
June 10, 1972
A flood crest estimated at 50,000 cubic feet per second reached downtown Rapid City.
Rapid Creek was once again within its banks.