PALEOFLOOD HISTORY OF RAPID CREEK IN THE FOOTHILLS OF THE BLACK HILLS, SOUTH DAKOTA

Drainages in the Black Hills area of South Dakota can experience extreme flash flood events similar to those that occurred in 1972, which claimed 238 lives. Peaks of record for many U.S. Geological Survey (USGS) gaging stations are about an order of magnitude larger than the next largest peak flows, and existing gage records have insufficient length to adequately estimate the frequency of occurrence of extreme events.

Seven sites on a 3-mile stretch of Rapid Creek in the east-central Black Hills of South Dakota were examined for paleoflood deposits. Sites were located in limestone caves and alcoves from a few feet to tens of feet above the low-flow water surface in the active channel. These sites contain deposits from the 1972 flood as well as evidence of large, unrecorded floods. Statigraphic analyses and geochronologic techniques were used to date the flood events. Geographic information systems and the US Army Corp of Engineer’s (USACE) Hydrologic Engineering Center – River Analysis System (HEC-RAS) one-dimensional hydraulic model were used to estimate the magnitude of the flood peaks. Results indicate that floods similar to or greater than the 1972 flood event, that caused extensive damage to the Rapid City area, have recurred multiple times over the last 2000 years. All sites on Rapid Creek contain evidence of floods greater than either the current peak of record (~31,000 cubic feet per second (ft³/s)) or the pre-1972 peak of record (2,600 ft³/s). The largest flooding event on Rapid Creek in the last few thousand years may have been as large as 120,000 ft³/s, which is 4-5 times the peak flow of the 1972 flood. Carbon-14 data from one site indicate that at least 10 floods equaled or exceeded 65-70,000 ft³/s between ~2000 and ~1400 years before present.