INTENSE ZOOGEOMORPHIC ACTIVITY ALONG A HEAVILY DEVELOPED STREAM REACH, LITTLE NESHAMINY CREEK, PENNSYLVANIA

Biogeomorphic activity was examined along two reaches of a 500-m-long section of Little Neshaminy Creek, Warwick Township (Bucks County, Pennsylvania). This 15-20-m-wide, W-E-flowing channel has been modified by several engineering structures (bridges, streamside roads) and is floored by heterogeneous sediment ranging from gravel to clay. Along the north terrace of the upstream (western) reach, several openings (width: 0.15-0.26 m) located within 1.5 m of a 1-2-m-high bluff are interpreted as bank burrows excavated by a beaver (Castor canadensis). Along the opposite side of the creek, a temporally persistent population of large invasive common carp (Cyprinus carpio) acts as the second important zoogeomorphic agent. Along the downstream (eastern) reach, the north terrace is presently under heavy development, whereas recent beaver activity is exemplified by characteristic sign between the south bank and nearby road. Seventeen chew stumps, some with nearby felled trunks, are located 2.3-12.0 m (mean: 4.2 m) from the steep 1.4-m-high bank. Anatomically constrained stump height ranges between 20-58 cm (mean: 33 cm), with diameters of 4.7-13.3 cm (mean: 8.9 cm). The majority of the asymmetric chew-surface indicators show that trees were felled northward toward the stream (312-040°). Several trunks fell in a landward direction, which suggests that they may have been pre-positioned to facilitate dragging and also cautions against the significance of a mean treefall azimuth for a bidirectional set of values. Here, similar to upstream reach, trees submerged due to beaver activity and other mechanisms (flooding, windthrow) provide long-term (months to years) shelter for bottom-feeding fish and reptiles, thereby adding to a local zoogeomorphic cascade through macrophyte removal, bank undercutting, and substantial sediment redistribution during normal flow conditions.