Holocene Paleofloods And Their Relevance To Flood Mitigation, Risk Assessment, And Policy.

Abstract

Paleofloods are past floods that occurred without being recorded by direct hydrological measurement. Paleoflood evidence can tell us about older, and often much larger historic flooding events than those of the instrumental record. For the past 40 years, paleoflood hydrologists have made the argument that geological or botanical flood evidence can help us better understand the true risks of extreme floods. However, because paleoflood studies are typically presented as individual works for single rivers, it is not known how often this kind of work produces information about floods that are equal or larger than the flood-of-record in conventional hydrological gage records. In my research, I created a new synthesis of all available paleoflood case studies on river systems in the U.S. spanning all available proxy records during the Holocene. My synthesis compared the largest paleoflood event in an area against the flood of record reported by the nearest stream gage, and also evaluated the relevance of paleoflood hydrology to natural hazards research. In order to be included in my analysis, studies had to have taken place on a river system in the continental U.S. during the Holocene, and also had to report a variable of flood magnitude. In the strong majority of cases (70%), Holocene paleoflood events are larger than the largest flood as recorded by instrumental river gages. In the most extreme case, a paleoflood event on Elk Creek, SD had a magnitude 12 times larger than the flood of record. While there is great potential for using data from paleoflood studies in flood frequency analyses, paleoflood hydrologists do not have a standard approach to reporting their results. In my review, I will present a list of suggestions for reporting the location, date, and magnitude in future paleoflood studies to comply with new reporting standards outlined in USGS Bulletin 17C.