BIOGEOMORPHIC AGENTS ALONG A SMALL FORESTED CREEK, TYLER STATE PARK, PENNSYLVANIA

Biogeomorphic impact on fluvial and riparian geomorphology increases with increasing magnitude and rate of biogenic activity and decreasing stream size. This relatively understudied aspect of forested streams has ben investigated along the Mill Dam Run, a small tributary of the Neshaminy Creek (lower Bucks County, Pennsylvania), which flows westward across the sandstone-mudstone contact within the Upper Triassic Stockton Fm, with the upper 1.0-1.5 m dominated by weathered bedrock and alluvium. The long-term stabilizing action of tree roots is punctuated by massive bank collapse due to treethrows, particularly during storm wind stress and flood-induced undercutting. In adition to several drainage control structures and foot traffic by park visitors, zoogeomorphic impact is exemplified by numerous groundhog (Marmota monax) burrows, ranging from active (some re-occupied by other animals) to abandoned and partially filled. A suite of parameters (burrow entrance diameter, minimum shaft length, depth, inclination, exposure azimuth, and distance from cutbank) were measured on 15 burrow complexes along north and south banks of the stream, with some complexes containing up to 5 entrances. Overall, burrow diameter increases and inclination slightly decreases away from the cutbanks, with the majority of shafts inclined away from the stream at 10-65° (mean: 29º). The minimum volume of excavated sediment ranges from 5,000-160,000 cm³, with much of the spoil pile material redistributed locally and entering the stream. Ongoing erosion will eventually intersect the groundhog galleries, accelerating retreat of the upper sections. Our study demonstrates that cumulative effects of discrete events, such as treethrow and bioturbation by semi-fossorial rodents, exert a key role in geomorphic evolution and sediment budget of small forested biotopes. Therefore, recognition, mapping, and quantification of the rates and extent of biogenic activity must be incorporated into morphodynamic models.