WHAT HISTORIC FLOODS IN CENTRAL APPALACHIAN CANYONS TELL US ABOUT PALEOFLOOD RECONSTRUCTION (Invited Presentation)

Several extremely large floods in Central Appalachian canyon reaches between November 1985 and June 2016 have provided experiments to test how accurately and precisely paleoflood tools can retrodict the physical characteristics of documented floods in this region. Where a sufficient range of clasts sizes is available and actual bedload transport can be determined, the size of transported boulders and blocks are consistent with empirical equations to reconstruct flow attributes published in the 1980s. However, many extremely large (5-20 m) imbricated sandstone blocks unmoved by historic flooding suggest either quite extraordinary floods occurred in the distant past, or that imbrication is not a sufficient criterion on which to interpret bedload transportation. Open-air clastic slackwater deposits tend to lie 2-4 m below flood trim lines and flotation deposits along canyon walls and tributary confluences, leading to discharge retrodictions that may be less than two-thirds of nearby USGS flow calculations. This underestimation stems from the nature of “Eastern U.S.” floods, which contrast with typical floods in regions where slackwater deposits have proven to be precise flood stage indicators. Foremost in this nature is the relatively low suspended sediment concentrations that occur in the well-armored Appalachian river channels during floods, except those associated with widespread debris flows and other slope failures. Video and field investigations show that the high-water environment along steep Appalachian canyon walls is dominated by breaking waves generated by the collapse of large channel hydraulics, so there is neither a low-energy environment nor a suitable low gradient surface for fine-grained deposition. Slackwater deposits at tributary confluences tend lie well blow other high-water marks and may be short-lived because of the active nature of tributaries. In canyons that include carbonate bedrock, river-derived slackwater deposits in caves provide the most precise match to other high water marks, but river-level caves tend to be quite active and these sediments may be significantly eroded with a few years.