CONTRASTING BEDROCK-GEOMORPHIC SETTINGS CONTROL BEAVER IMPACT ALONG THE PENNYPACK CREEK, PENNSYLVANIA

We present a new database of the geomorphic impact of resurgent activity of the North American Beaver (Castor canadensis) at Pennypack Ecological Restoration Trust in suburban Philadelphia. The bedrock consists of Early Paleozoic metamorphic rocks (schist, mafic and felsic gneiss), Triassic terrestrial rift-basin sedimentary rocks (mostly sandstones of Stockton Fm.), and localized intrusions of Jurassic diabase. The sandstone consists of >80% of oxidized quartz, muscovite, and minor calcite cement, making it more susceptible to weathering than metadiabase. The latter contain 35% plagioclase feldspar, 25% clinopyroxene, and traces of orthopyroxene, biotite, amphiboles and iron oxides. Two sites with contrasting bedrock, geomorphology, and beaver impact included: 1) upstream site with a low terrace incised into weathered sandstone and 2) steep hillside of diabase above a pond containing a large active beaver lodge. At Site 1, just upstream of an abandoned mill dam, beaver impact during the winter and spring of 2021 included a number of cut 4-22-cm-diameter trees (mostly beech) within 3-10 m of the northern streambank, as well as numerous gnawings on exposed stabilizing roots. Continuing observations show degradation in slope stability and exposure of fresh Triassic sandstones. Small sections of beaver trails coincided with white-tailed deer trails. On a steep (45-60°) slope at Site 2, several stumps cut during 2019-2020 show clear evidence of relative aging. Here, beaver impact extended >15 m uphill of the pond lodge, likely due to the assistance from the gravity to the felling and transport of small-diameter trees. Our study demonstrates a strong control of antecedent bedrock geology on riparian geomorphology, which in turn exerts an influence on the types and magnitude of zoogeomorphic impact of the beaver.