35-18 Drainage reversals, knickpoints, and vascular plant species richness legacies of Laurentide ice sheet advances into Pennsylvania

Session: Ice sheets, glaciers, and landscapes, oh my! (Posters)

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Abstract:

Glaciers have dramatically shaped the landscape of the northeastern United States with particular impacts concentrated along the glacial margin in Pennsylvania. Among the impacts are the reversal of drainages that breached pre-glacial divides and carved deep, steep gorges decorated by hanging valleys, knickzones, and retreating waterfalls. This study seeks to link the impacts of knickzone formation and the subsequent ecology of these glacially reversed stream valleys, many of which are known to host exotic species to Pennsylvania. Our research design leverages ecological diversity and geomorphic data sets from well-visited State Parks and natural areas distributed along the glacial margin in the northern tier of the state, as well as those further south, but still impacted by proglacial lake formation and drainage reversal. We find that rock erodibility (K) in the study’s knickzones vary according to bed thickness, fracture density and frequency, bed dip, and rock elasticity. To a first order, variations in field determinations of K are poor predictors of a comparatively wider range of normalized channel steepness (k_sn), consistent with the idea that the knickzones are transient and propagating.  We use the simple stream power model and a detachment limited assumption (n=1) to calculate a range of K values for a site where we independently know erosion rate, stream steepness, drainage area, and time of impulsive base level fall. Here, the K values span several orders of magnitude from 8.00E-07 to 5.41E-05, which when applied to the other study sites predicts knickpoint propagation rates ranging from ~1 to ~500 mm/yr. A series of four knickpoints propagating up tributaries of the Youghiogheny River in Ohiopyle State Park, constrained by other geologic and stratigraphic data, suggests that the smaller K and slower propagation rates are more likely. The reconstructed base level fall histories vary according to the type and drivers of the base level fall, but in all cases help refine the time of drainage reversal induced by glaciation. In general, vascular plant species richness increases as a function of knickzone proximity, age, and warmer (south-sourced) stream headwaters.  

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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