GEOMORPHIC RESPONSE TO EXTREME FLOODS IN THE APPALACHIANS

The geomorphic impact of extreme floods in the Appalachian Mountains has been a focus of study for decades. Based on data from suspended sediment transport, the seminal paper on magnitude and frequency by Wolman and Miller (1960) concluded that most geomorphic work is accomplished by the summation of frequent low magnitude floods. The definition of geomorphic work was altered by Wolman and Gerson (1978) to emphasize landform change and also noted the importance of recovery time between successive floods. Some high-magnitude floods such as Tropical Storm (TS) Agnes (Mid-Atlantic 1972) produced record discharges but nominal geomorphic impacts. In sharp contrast, other floods such as Little River (Virginia 1949), Hurricane Camille (Nelson Co., Virginia 1969), Cheat River (West Virginia 1985), Rapidan River (Virginia 1995), TS Irene (Vermont 2011), and TS Lee (Pennsylvania 2011), resulted in catastrophic geomorphic erosion and deposition in channels and on floodplains.

Analyses of the impacts of extreme Appalachian floods reveal a wide range of geomorphic responses depending on the complex interaction of many factors, such as basin morphometry, channel slope, bedload, bed and bank cohesion, unit stream power, up-basin mass wasting, event ordering, and land-use history. Although geomorphic response to extreme floods has been complex, there are some significant trends emerging. Major fluvial response has occurred downstream from areas of widespread hillslope activity (especially debris flows) that yielded large volumes of coarse sediment (1949, 1969, 1995, 2011); even while minor changes occurred at sites with high flow discharge. Major geomorphic change has also been observed along reaches where stream power exceeded resistance thresholds in channels, especially where gradients were high, valleys were narrow, and flow depths great. In north-central Pennsylvania (2011), catastrophic channel and floodplain changes were widespread in Plateau watersheds with high ruggedness numbers, especially where they were significantly influenced by the history of land-use (logging). Geomorphic response there was large in terms of gravel transported and changes in channel pattern. In sharp contrast, Ridge and Valley and Piedmont watersheds with even higher discharges experienced negligible geomorphic change.