DRAINAGE EVOLUTION OF MID-ATLANTIC SLOPE TRANSVERSE STREAMS: KNICKPOINTS, WIND GAPS, AND DRAINAGE CAPTURES OF THE DELAWARE AND LEHIGH RIVERS

The origin and evolution of transverse drainages in the post-orogenic Appalachians remains a topic of spirited discussion particularly when viewed through the lens of novel DEM-based topographic analyses. We present new results extracted from the long profiles and network topology of the Delaware-Lehigh river drainage basin and compare them to recently published and historic studies of adjacent drainages. Specifically, we address the contrasting drainage network of the Delaware-Lehigh network with respect to Kittatinny Ridge, the first ridge in the Appalachian foreland underlain by the thick, resistant, NW-dipping Shawangunk Conglomerate. As long noted, channels in the foreland are mostly graded to rock type and structure. In contrast, the Delaware and Lehigh trunk channels and their tributaries downstream of Kittatinny Ridge, are transverse to bed strike, cutting wind gaps and water gaps in resistant rock types en route to the Fall Zone. There are two distinct knickpoints on the trunk and tributary channels of the Delaware-Lehigh Rivers at ~300-400 m and ~100 m, elevations comparable to knickpoints documented for the adjacent Susquehanna River basin (Miller et al., 2013). Relict channel segments upstream of these knickpoints are graded to higher paleo-base levels. Accordingly, our downstream projections of the relict channel segments above the 300-400 m knickpoint intersect Kittatinny Ridge and Blue Ridge at elevations preserved by the floors of Wind Gap and Lanark Gap respectively, consistent with a paleo-Atlantic slope drainage characterized by consequent streams. Accounting for flexural isostatic bending of the lithosphere, the projected profiles of the 300-400 m knickpoint intersect Miocene-aged (15-6 Ma) upland gravels of the Inner Coastal Plain, whereas the profiles of the 100 m knickpoint projects below the Miocene, suggesting that base level fall occurred in at least two pulses. Upstream passage of the older knickpoint resulted in drainage captures, a process still reflected in the modern χ map of the now subsequent drainages, shaped by variable rock-type and structure (Epstein, 1966). These results place some broad constraints on the origin of the master mid-Atlantic slope transverse rivers, abandonment of the wind gaps, and longevity of the current water gaps.