Paleoflood studies collect and investigate a broader range of information related to the hydrologic history of an area, and thereby significantly increase the potential for improving flood frequency analyses in the Black Hills area. In essence, Paleoflood hydrology is the study of previously unmeasured floods, whether fairly recent in occurrence or even ancient in time. A paleoflood survey typically consists of: 1) identifying paleostage indicators (PSIs), which are indicators of flood stage associated with a previous, large hydrologic event; 2) estimating the peak discharge for that event; and 3) estimating the age of the event. A variety of approaches can then be used to incorporate the paleoflood data (number of floods, magnitude, and associated timeframes) into flood frequency analyses. Paleoflood techniques have proven to be very useful for improving the flood frequency estimating of relatively recent events where recurrence intervals are in the range of 50- to 500-years, and are also applicable to studies involving much older floods when the flood events have been more significant.
Improved confidence in flood frequency estimates for the Black Hills area would initially require paleoflood surveys at a number of sites. The surveys would, at the least, need to include paleohydrologic investigations of distinctive sedimentary deposits, botanical evidence of past large flows, erosional indicators of paleostage on channel margins, and indicators on landforms in streams and floodplains. The paleohydrologic data gathered during the surveys would then require careful analysis prior to inferring the maximum stage of past floods or developing estimates on the age of those floods. If the surveys and analysis can be successfully accomplished, the resultant data would be highly useful to improving the flood frequency estimates of Black Hills watershed areas.