Applicability of Paleoflood Surveys to the Black Hills of Western South Dakota: Executive Summary

Flood-frequency analyses for the Black Hills area have large uncertainties because of several complicating factors, including: effects of the massive 1972 storm near Rapid City; geologic influences; and potential influences of topography on precipitation patterns. The objective of this study was to assess the applicability of paleoflood hydrology techniques to generate better historical records on the magnitude and frequency of peak-flood events in the Black Hills area. As the study evolved, it became apparent that the paleohydrologic approach of greatest potential utility is detailed stratigraphic analysis of sequential deposits of slackwater flood sediments in caves, alcoves, and rock shelters, along with radiocarbon dating of entrained organic materials. This approach has been used worldwide in suitable environments to assess the frequency of large and infrequent floods and was applied at French Creek and Spring Creek, where detailed analyses demonstrated the regional applicability of this approach. Stratigraphic records for multiple sites in each stream reach provided excellent chronologies of previous large floods, indicating various flood events within the previous several millennia approaching or exceeding the approximate magnitude of 1972 flooding. Ensuing regional reconnaissance efforts indicated that local conditions are ideal for formation and preservation of flood slackwater deposits, especially within Paleozoic carbonate rocks ringing the periphery of the Black Hills, where abundant caves and alcoves protect deposits from erosion. Generally arid conditions aid in preservation of stratigraphic boundaries and detrital organic materials necessary for reconstructing flood chronologies. The overall conclusion of this reconnaissance-level study is that improved understanding of flood frequencies for the Black Hills region would result from implementation of future studies using established paleoflood techniques. Flood slackwater deposits have been identified in canyon sections of most major drainages in the Black Hills. These deposits show stratigraphic records of large floods that can be effectively used with radiocarbon dating and hydraulic analysis to determine the approximate timing and discharge of previous large floods. Specific flood issues that could be addressed by Black Hills paleoflood studies include: (1) determination of the frequency, magnitude, and spatial characteristics of 1972-scale floods for several or all of the major Black Hills drainages; and (2) evaluation of spatial patterns of large-flood generation that owe to topography, geology, and climatology.