HIGH-RESOLUTION SUBSURFACE (500 MHZ GEORADAR) CHARACTERIZATION OF A CREEK TERRACE WITH INTENSE ZOOGEOMORPHIC ACTIVITY, LANGHORNE, PENNSYLVANIA

The biogeomorphic elements (tree roots and zoogeomorphic activity) in riparian settings are rarely considered when addressing slope stability and flood impact. High-resolution ground-penetrating radar (500 MHz GPR) was used to map the subsurface conditions along the northern bank of Neshaminy Creek (Bucks County, PA), a site of a historic grist mill, with nearby evidence of past beaver activity (relict chew stumps). Since 2019, this site has been heavily impacted by resurgent beaver population (tree cuts, bark peeling, bank lodges, and bank burrows). A large active beaver burrow is located ~1.2 m below the creek terrace (2.3 m above the mean water level). The geophysical records show clear hyperbolics in the shallow subsurface, related to both tree roots and biogenic voids. Several dipping (5.0-8.7°) reflections beneath the terrace and adjacent to the burrow likely represent lithological contrast that may contribute to a future slope failure, especially in light of bark-stripping of large stabilizing trees. In addition, white-tailed deer, Canada geese, common carp, white sucker, and freshwater turtles are involved in riparian zoogeomorphic cascades. Several months following GPR surveys, a record flood (> 5 m) associated with the remnants of Hurricane Ida (September 2021) spread a layer of re-deposited legacy mud and exposed other biogenic structures to erosion due to vegetation removal. Our findings highlight the need for detailed investigations of biogenically affected streambanks and provide a valuable reference dataset of pre-flood conditions.