Late-Holocene Flood History, Flood-Frequency, and Paleoclimate Analysis of the Central Black Hills, South Dakota

Stratigraphic records in four basins in the central Black Hills in combination with hydraulic calculations show that all basins have experienced multiple large floods in the last 2,000 years with flow rates substantially larger than those gaged historically. Flood-frequency analyses for the study reaches account for 29 paleofloods inferred from interpretation of stratigraphic records locally extending back 1,000 to almost 2,000 years. The addition of paleoflood data to the gaged and historical data significantly reduced uncertainties related to flood-frequency. For all study reaches the 95-percent confidence intervals about the low-probability quantile estimates (100-, 200-, and 500-year recurrence-intervals) were reduced by at least 78 percent relative to those for the gaged records only. In some cases, 95-percent uncertainty intervals were reduced by 99 percent or more. Additionally, a stratigraphic record of 35 large paleofloods and four large historical floods during the last 2,000 years (including several floods not used in the frequency analyses due to age constraints) reveal four flooding episodes at A.D.: 130-40, 640-670, 900-1290, and 1410 to present. During the Medieval Climate Anomaly (~A.D. 900-1300) the Black Hills experienced 13 large floods compared to nine large floods in the previous 800 years. This high concentration of large flooding events were likely caused by: 1) instability of air masses caused by stronger than normal westerlies; 2) larger or more frequent hurricanes in the Gulf of Mexico and Atlantic Ocean; and/or 3) reduced land covering vegetation and an increase in forest fires caused by the severe drought. By examining the response of streamflow to the MCA, it seems likely that if severe long-term drought conditions persist for the Black Hills region, an increase in the frequency and magnitude of large floods can be expected. The Black Hills paleofloods represent some of the largest known floods, relative to drainage area, for the United States. Many of the other largest known United States floods are in areas with physiographic and climatologic conditions broadly similar to the Black Hills--semi-arid and rugged landscapes that intercept and focus heavy precipitation from convective storm systems