EVIDENCE OF EXTREME FLOOD EVENTS IN THE BLACK HILLS REGION OF SOUTH DAKOTA

Rare but exceptionally large floods affect the Black Hills region of South Dakota, including the 1972 flash flood that killed 237 people and caused $160 million in damage in the Rapid City and Keystone areas. The peak flows from this intense thunderstorm, technically a mesoscale convective complex, greatly exceeded all other gaged flood peaks for several drainages in the central Black Hills, including some drainages with records extending back to the early 1900s. This situation leads to highly uncertain frequency assessments—a matter of practical concern for floodplain management and infrastructure design in this rapidly developing region. To improve estimates of peak-flow frequencies, a paleoflood assessment was conducted for four major drainages in the central Black Hills: Elk Creek (46 sq mi), Boxelder Creek (120 sq mi), Rapid Creek (371 sq mi), and Spring Creek (170 sq mi). Fine-grained flood sediment, deposited in rock shelters and caves, provides detailed stratigraphic records of past large floods. These records, in conjunction with radiocarbon and optically stimulated luminescence (OSL) dating and hydraulic analyses, allow for development of detailed chronologies of large floods for the last 2-3 thousand years in each of these drainages. Preliminary results show that (1) multiple floods in all four drainages have occurred with flows equaling or exceeding the 1972 flood, which was thought to be an extremely rare event; (2) the largest of these floods had flows roughly 2-3 times larger than those of the 1972 flood and occurred 800-900 years before present (BP); and (3) although records date back to ~3800 years BP, the frequency of large floods increases significantly about ~1150 years BP, coinciding with the onset of the Medieval Climate Anomaly (A.D. 850-1300).