Geomorphic response to catastrophic flooding in north-central Pennsylvania from Tropical Storm Lee (September 2011): Intersection of fluvial disequilibrium and the legacy of logging

Abstract

More than 25 cm of rainfall from Tropical Storm Lee (TS Lee) over 2 days in September 2011 resulted in catastrophic flooding (U.S. Geological Survey estimated recurrence interval >100 yr) on several Susquehanna River tributaries emanating from the Appalachian Plateau in north-central Pennsylvania (USA). Helicopter photography and field work were used to prepare a detailed geographic information system database of geomorphic response to the flood along ~250 km of Loyalsock, Muncy, Lycoming, and Fishing Creeks. Unlike the response of many streams to previously described Appalachian floods, fluvial response to the TS Lee flood was extensive in these gravel bed streams, characterized by (1) large-scale avulsions and chute development on the insides of meanders, (2) erosion of gravel from channel margins and transport downstream in large pulses, (3) headwater landslides and alluvial fan activation, (4) major floodplain erosion and deposition, and (5) breaching of anthropogenic berms and reconnection of the main channel to prehistoric floodplain anabranches. Geomorphic work, expressed both as bedload sediment transport and landform change (geomorphic effectiveness) was significant: as much as 55,000 m 3 /km of gravel was transported within a single watershed. Landform changes included erosion of chutes (to 500 m long), gravel bars (point bars and mid-channel bars), channel widening (in places >100%), and reoccupation of former multithread channels previously cut off from the mainstem by historic channel straightening, berming, and dredging.Streams in this region appear to be in a phase of disequilibrium largely in response to major shifts in sediment delivery from their watersheds caused by historic logging and a series of floods ~100 yr ago. Widespread clearcutting (A.D. 1850-1920) contributed large volumes of sediment to these streams. Dendrogeomorphic data bracket a period of aggradation of these logging legacy sediments between the 1870s and 1930s, creating a significant low terrace inset into Pleistocene outwash and glacial sediments. Recent floods in 1972, 1996, 2004, and especially TS Lee in 2011 initiated an enhanced phase of disequilibrium as a geomorphic threshold was crossed, resulting in widespread erosion of logging legacy sediments deposited nearly 100 yr ago. The change in sediment load (increased coarse bedload) as a result of widespread bed and bank erosion caused a change in channel pattern from single thread to multithread. Pattern change was facilitated by aggradation of gravel bars above floodplain elevations which promoted avulsion and chute formation. Based on preflood and postflood geomorphic mapping, >6,700,000 m 3 of gravel were mobilized during the flood across 4 watersheds. Mobilization of logging legacy sediment is occurring as pulses of gravel move downstream episodically. This paper demonstrates the important influences of drainage basin morphometry (e.g., ruggedness number) and fluvial history (land use and geomorphic) in understanding current channel dynamics and basin response to heav...

Effective Hydrological Events in an Evolving Mid‐latitude Mountain River System Following Cataclysmic Disturbance—A Saga of Multiple Influences

“…There are many factors that influence geomorphic effectiveness of events. They include (1) magnitude and frequency of events relative to system recovery times (Beven, 1981; Costa, 1974, 1978; Newson, 1980; Phillips, 2009; Wolman & Gerson, 1978); (2) stream power and spatial gradients in stream power (Baker & Costa, 1987; Gartner et al., 2015; Magilligan et al., 2015; Righini et al., 2017; Scorpio et al., 2018); (3) event duration (Costa & O'Connor, 1995; Magilligan et al., 2015; Newson, 1980); (4) fluvial history, including event sequencing (Beven, 1981; Kochel et al., 2016); (5) threshold conditions or tipping points (Bull, 1979; Kochel et al., 2016; Magilligan et al., 2015; Ritter et al., 1999; Schumm, 1979; Sidle et al., 2013); (6) local topographic, geologic, and hydraulic conditions (Costa, 1974; Kochel et al., 2016; Lisenby & Fryirs, 2016; Miller, 1990; Rathburn et al., 2017; Righini et al., 2017; Scorpio et al., 2018; Wolman & Eiler, 1958); (7) strong channel–hillside coupling (Hassan et al., 2019; Rathburn et al., 2017); and (8) surface resistance and interactions with vegetation (Bull, 1979; Diehl et al., 2017; Graf, 1979; Gran et al., 2015; Phillips, 2009; Wolman & Eiler, 1958).…”

“…The concepts of geomorphic work done by, and the geomorphic effectiveness of, specific hydrological events have been documented and discussed for decades (e.g., Baker & Costa, 1987; Beven, 1981; Brunsden & Thornes, 1979; Bull, 1979; Costa & O'Connor, 1995; Fryirs, 2017; Gartner et al., 2015; Kochel et al., 2016; Lisenby et al., 2018; Magilligan et al., 2015; Miller, 1990; Newson, 1980; Phillips, 2009; Phillips & Van Dyke, 2016; Righini et al., 2017; Ritter et al., 1999; Schumm, 1979; Scorpio et al., 2018; Wolman & Gerson, 1978; Wolman & Miller, 1960). In prior studies, investigators have looked at the effectiveness of hydrological events of varying magnitudes and frequencies spread broadly over geographic regions (e.g., Miller, 1990; Wolman & Gerson, 1978), of single events along relatively short reaches of channel length (e.g., Costa & O'Connor, 1995; Magilligan et al., 2015; Pitlick, 1993), of single events widespread across different physiographic terrain (e.g., Kochel et al., 2016; Magilligan et al., 2015; Righini et al., 2017; Scorpio et al., 2018; Wolman & Eiler, 1958), and, less commonly, of multiple rare events affecting the same basin and river system (e.g., Newson, 1980). In many cases, these studies have examined geomorphic responses and effectiveness within the context of differentially stressed systems—namely fluvial systems affected by a variety of anthropogenic manipulations such as dams, confining berms, levee systems, channel straightening, and removal of large wood and other roughness elements (e.g., Kochel et al., 2016; Llena et al., 2020; Wohl, 2019).…”

“…In many cases, these studies have examined geomorphic responses and effectiveness within the context of differentially stressed systems—namely fluvial systems affected by a variety of anthropogenic manipulations such as dams, confining berms, levee systems, channel straightening, and removal of large wood and other roughness elements (e.g., Kochel et al., 2016; Llena et al., 2020; Wohl, 2019). Prior studies have also examined fluvial systems affected by anthropogenically induced disturbances (e.g., Kochel et al., 2016; Walter & Merritts, 2008). Underrepresented in the literature are long‐term analyses of expansive reaches of a single fluvial system which document recurring responses (annual to every few years) to varied hydrological events following a substantial natural disturbance (e.g., Moody & Meade, 2018), especially in systems that are relatively free of substantial anthropogenic stresses.…”

“…different physiographic terrain (e.g., Kochel et al, 2016;Magilligan et al, 2015;Righini et al, 2017;Scorpio et al, 2018;Wolman & Eiler, 1958), and, less commonly, of multiple rare events affecting the same basin and river system (e.g., Newson, 1980). In many cases, these studies have examined geomorphic responses and effectiveness within the context of differentially stressed systems-namely fluvial systems affected by a variety of anthropogenic manipulations such as dams, confining berms, levee systems, channel straightening, and removal of large wood and other roughness elements (e.g., Kochel et al, 2016;Llena et al, 2020;Wohl, 2019). Prior studies have also examined fluvial systems affected by anthropogenically induced disturbances (e.g., Kochel et al, 2016;Walter & Merritts, 2008).…”

Dendrogeomorphological Evidence of Flood Frequency Changes and Human Activities (Portainé Basin, Spanish Pyrenees)

“…There are very few dendrogeomorphological studies investigating the effects of human impact on flood regimes. Some studies have analyzed the effects of river-regulating dams (St. George and Nielsen 2003), land-use changes linked to forest fires in a catchment (Brown and Sieg 1996), logging (Kochel et al 2016) and in-channel structural defensive measures (Ballesteros-Canovas et al 2016). Currently, studies exploring the effects of land-use changes on flood regimes in mountain areas (e.g.…”