ASSESSING THE EFFECTS OF BIOTURBATION ON RIVER TERRACE STABILITY, BUCKS COUNTY, PENNSYLVANIA

In addition to fluvial and slope processes, the post-depositional modification and stability of river terraces are affected by biogenic activity, especially at higher elevations. Two 10-m-long stretches along the west bank of the Delaware River (Washington Crossing Historic Park, Pennsylvania) were used to evaluate the size and distribution of biogenic structures (burrows and treethrows) and to assess their relationship to terrace geomorphology. Upstream site is a steep, 5-m-high bluff (mid-Holocene terrace riser), with a number of large (up to 140 year old) trees toppled by Hurricane Sandy (October 2012). There are seven active rodent burrows penetrating into the bluff face, with four largest structures (entrance diameter >10 cm; tunnel length > 2m) located within the upper 2 m. The second locality (100 m downstream) contains five small burrows within the upper terrace riser (diameter: ~5 cm; length <1.2 m). An additional lower terrace of lesser relief has a younger tree stand and is bioturbated by five small burrows (diameter: 3-5 cm; length <1.0 m). Elevations and distribution of exposed biogenic structures, including their extension into the slope, were compared with a ground-penetrating radar profile collected along the upper terrace edge (> 4 m penetration). Our findings show that field observations greatly underestimate the complexity of biogenic activity represented by hyperbolic diffractions in geophysical images. This is expected, as subsurface images contain a compound assemblage of older bioturbation structures, roots (upper 1.0-1.5 m), and occasional gravel and artifacts. Whereas vegetation results in long-term stabilization of the upper terrace, a combination of deep bioturbation and occasional treefall has led to episodes of massive localized erosion, with potential future impact on important cultural landmarks. In contrast, younger terraces are affected primarily by fluvial processes (floods and ice scour), with greatly reduced treethrow potential by young trees and fewer burrows compromising bank stability. The present study emphasizes the importance of evaluating biogenic activity (pedoturbation, trampling, grazing, root propagation, treethrow) for a more accurate assessment of river terrace modification through sediment redistribution and profile adjustment.