BIOGEOMORPHIC EFFECTS OF SEMI-FOSSORIAL MAMMALS ON THE RIPARIAN ZONE OF A SMALL FORESTED CREEK, SOUTHEASTERN PENNSYLVANIA

A 300-m-long reach of westward-flowing Mill Run, a small forested tributary of Neshaminy Creek (Buck County, Pennsylvania) was surveyed in order to assess the distribution of large mammalian bioturbation structures. Burrows excavated, and many still occupied, by groundhog (Marmota monax) are some of the most prominent biogeomorphic features along creek banks and terraces. Entrance diameters (average: 25.9 cm) and distances to stream cut bank (maximum: 20 m) were recorded for 32 burrows, with georeferencing provided by a handheld GPS. A cluster analysis was implemented by plotting circles (radius: 5 m) around burrows to test a relationship between the number of burrows in a cluster and distance to stream bank. The recent retreat of the southern bank resulted in an average of 3.8 m distance of the groundhog burrows, compared to 6.9 m along the north margin. Single-entrance burrows tended to be father away from the stream, some utilizing large tree root systems as entrance canopies. Large inclined shafts near Mill Run and joining groundwater sapping gullies likely contribute to more enhanced localized hydrological exchange during rainstorms. Because event-scale slope failure may be partially exacerbated by burrowing activity, it is important to observe the formation of new zoogeomorphic features in order to determine the influence of channel dynamics on their position.