50th Anniversary of the 1972 Black Hills Flood and Subsequent Canyon Lake Dam Failure – A Disaster of PMP-level Proportions

Friday afternoon of June 9th, 1972 was the beginning of a tragedy for Rapid City, South Dakota along the eastern slopes of the Black Hills mountain range. Scattered showers from the previous days had left the ground saturated while additional storms passed slowly over the area. The thunderstorms that struck produced an average of 10 inches of rainfall over a 60-square-mile area in just 6 hours (which has been estimated to be the equivalent of 90 percent of the Probable Maximum Precipitation).
Canyon Lake Dam was a 20-foot-high, 500-foot-long, earthfill dam located 7 miles up the meandering Rapid Creek that flows through Rapid City. The flash flood within the watershed upstream of the dam carried debris-laden water down into Canyon Lake Reservoir where it accumulated, clogged the spillway, and quickly caused the dam to overtop and fail. The storm also caused overtopping and near failure of Fort Meade Dam in an adjacent watershed. Fort Meade Dam was drained and breached by the U.S. Army Corps of Engineers shortly after the flood to avert an uncontrolled failure.
Rapid City (1972 population of approximately 43,000) did not anticipate either the magnitude of the storm or the breach of Canyon Lake Dam. On June 10th at approximately 12:15 AM, the flood wave reached downtown Rapid City with little to no warning. The overall flood event resulted in the loss of 238 lives and total damages of $160 million (or over $1 billion in 2022 dollars).
This paper discusses the historic storm event that led to the overtopping and failure of Canyon Lake Dam, attempts to differentiate life loss consequences of the dam failure as compared to general flooding, and presents lessons learned on emergency warning, response, and evacuation. The significance of this case study as it relates to major changes to dam safety regulation in the 1970s is also discussed.