IMPROVING FLOOD-FREQUENCY ESTIMATES FOR THE BLACK HILLS, SOUTH DAKOTA USING SLACKWATER FLOOD-DEPOSITS IN CAVES AND ALCOVES

An efficient means of improving flood-frequency analysis is by augmenting streamgage records using paleohydrologic techniques, typically from geologic and geochronologic analysis of stratigraphy, ages and magnitudes of floods that occurred prior to gaging records (paleofloods). The Black Hills region is nearly an ideal environment for such an analysis, largely due to its geologic and hydrologic environment. Erosion in the central Black Hills has exposed a core of Precambrian-age igneous and metamorphic rock units that are flanked by Paleozoic and Mesozoic sedimentary rocks. Four basins are included in this study. Each have headwaters within the Precambrian rocks and drain eastward through canyons and steep-sided valleys where the streams cut into progressively younger Paleozoic rocks including the Mississippian- and Devonian-age Madison Limestone and the Pennsylvanian- and Permian-age Minnelusa Formation. Small caves and alcoves develop, particularly along the carbonate-bedrock canyon walls, thereby producing many sites for deposition and preservation of fine-grained slackwater sediments from floods. Micaceous sands derived from headwater areas preserved as flood slackwater deposits are easily distinguished from intervening layers of carbonate materials derived from caves and alcoves walls. Observational records of flooding for the four basins in the Black Hills have been supplemented with stratigraphic evidence of large floods occurring over approximately the last 2,000 years. Ages of flood events were determined by radiocarbon analysis, optically stimulated luminescence, and cesium-137 analyses. Flood magnitudes were estimated through hydraulic analyses. Long-term flood chronologies for study reaches were developed from evidence at multiple sites at various elevations relative to channel thalwegs. The resulting chronologies and flood-frequency estimates showed that (1) numerous large and extreme floods have previously affected all four stream reaches, and (2), incorporation of the paleohydrologic information substantially reduced uncertainties, up to 99-percent in some cases, in the estimated recurrence of exceptionally large floods.