CLAST DISTRIBUTION AND FLOW INDICATORS ALONG A GRAVEL BAR, NESHAMINY CREEK, TYLER STATE PARK, PA

Clast distribution on a gravel bar along the south-flowing Neshaminy Creek at Tyler State Park (Bucks County Pennsylvania), was investigated in order to assess the impact of the weir dam on bedload dynamics. The purpose of this study was to determine clast textural characteristics and map key flow indicators. Thirteen sites were sampled along four transects across bar. Maximum clast diameters decreased downstream from 106-148 mm to 76-80 mm. The unexpected upslope coarsening along each transect is likely related to flow amplification through a narrow bypass channel along the eastern bank of the creek. Blades and discs dominated the clast shape, reflecting the original bedding characteristics of the Triassic sandstone and mudstone. Flow indicators, such as imbrication, tilted trees, and stems of bottom vegetation wrapped around small bushes show that the downstream transport varied within the azimuth of S34E to S70E, with flow dispersal along the downstream flank of the bar. The combined dataset highlights the need for considering the influence of multiple channel effects on flow routing over, through, and around the dam during floods, especially where small-scale bedforms are masked or absent due to large sizes of available gravel clasts.