MEGA CURRENT BEDFORMS OF THE SUSQUEHANNA RIVER: FEATURES THAT FORM FROM MAJOR FLOOD EVENTS

Major recent flood events resulting from Tropical Storms Ivan (2004) and Lee (2011) produced mega current bedforms that can be observed on both the North and West Branches of the Susquehanna River. These bedforms have wavelengths ranging between 7 to 20 m, wave heights of 25 to 30 cm (but possibly up to 1 m), and they are composed of coarse gravel to fine cobble sized sediments. Though they have been observed on both the North and West Branches, the glacial sediments from the North Branch produce the best examples of these features in many places. The bedforms occur on point bars, islands, and in midstream where flow regimes dramatically fluctuate. They were identified and analyzed using Google Earth Pro, a drone, and through direct field examination.

They can not be observed easily at ground level. Ideal conditions are necessary to observe them using remote sensing. These large structures have eluded previous detection because ideal conditions are necessary for their formation, their size, and because they may easily erode between flood events. They have been observed on some historical aerial photographs in the Susquehanna River substrate. Strong downpours, ice jams, disturbance by lithophilic fish, and subsequent high energy flood events may modify the bedform structures.

The mega current bedform structures are significant because they represent the movement of massive amounts of coarse sediments from tributaries into and along the river substrate. The downstream movement of this sediment influences benthic macroinvertebrates (BMI) both during and following major flood events. Mega current bedforms may also contribute to alluvial bar island formation. Large bedforms such as these represent a disequilibrium in the river system as mostly unconsolidated Pleistocene glacial outwash and recent sediment is transported downstream during high-energy events.