Disequilibrium profile of the Potomac River near Washington, D.C. – A result of lowered base level or Quaternary tectonics along the Fall Line?

Longitudinal profiles of major rivers in Maryland and Virginia are generally closely adjusted to the resistance of the rocks over which they flow, but they all show major discontinuities at or just above the Fall Line. The amount of discrepancy at the Fall Line between the actual profiles and the projection of the profiles of the upstream reaches increases from about 27 m at the James River to about 50 m at the Susquehanna River. At the Potomac River it is about 45 m. Detailed study of the terraces along the Potomac in the Piedmont above Washington, D.C., suggests that the discontinuity in longitudinal profile is the result of Quaternary downcutting in response to a lowered base level. Changes in base level during sea-level fluctuations in the Pleistocene must have been an important factor, but strong circumstantial evidence suggests that the discontinuities in river profiles may in part be due to differential uplift of the Piedmont with respect to the coastal plain along a zone of flexure or distributed faulting near the Fall Line.