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The 1972 Black Hills-Rapid City Flood RevisitedRapid Creek Hydrograph - June 9, 1972 through June 10, 1972
Although Pactola Reservoir was effective in storing runoff that originated upstream of Pactola Dam, the heaviest rainfall occurred downstream of the dam. Most of the flow that passed through Rapid City via Rapid Creek originated in the 51 square mile drainage between Pactola Dam and Canyon Lake. Flood waters that reached Rapid City above Canyon Lake Dam between 9:30 and 10:00 p.m. on June 9 carried large amounts of debris that clogged the spillway of Canyon Lake Dam. The dam at Canyon Lake failed at about 10:45 p.m. However, the effect of this dam failure on the subsequent flood wave into urban Rapid City (figure 1) has been difficult to assess because the amount of water coming down Rapid Creek and several tributaries (accounting for 86 percent of the peak flow) far over-shadowed the amount of water in the small lake (Swartz and others, 1975). The peak discharge of 50,000 cubic feet per second (ft3/s) was carried through Rapid City via Rapid Creek at about midnight on June 9, while many people were asleep and unaware of the impending flood (figure 1, Rapid Creek at Rapid City).
The floods struck quickly and forcefully, but did not last long nor did they make much impact further downstream in the basins. Water in Rapid Creek was back within the banks of the stream by 5:00 a.m. on June 10. As the peak flows moved into less-steep terrain, the flows spread out over a wide area and much of the water was stored in the flood plains (figure 1, Rapid Creek near Farmingdale).
Figure 1. Hydrograph of flows at three Rapid Creek USGS gaging stations.
Schwarz, F.K., Hughes, L.A., Hansen, E.M., Petersen, M.S., and Kelly, D.B., 1975, The Black Hills-Rapid City Flood of June 9-10, 1972—A description of the storm and flood: U.S. Geological Survey Professional Paper 877, 47 p.
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