Knickpoint evolution in a vertically bedded substrate, upstream-dipping terraces, and Atlantic slope bedrock channels

We conducted flume experiments to investigate the nature of knickpoint and bedrock channel longitudinal profile evolution for a channel reach underlain by a vertically bedded substrate of alternating resistance. This research was motivated by the presence, behavior, and apparent persistence of impressive knickpoints formed, in steeply dipping or steeply foliated bedrock in Atlantic slope drainages of the Appalachian Mountains. The experiments were carried out in a 10-m-long wooden-box flume filled with a very fine- to medium-grained sand alluvial substrate and a single, 30-cm-wide ridge of vertically oriented, lacustrine siltclay varves designed to simulate a bedrock channel reach. Numerous control experiments established a stable meandering channel form and transport gradient, from which knickpoints were produced by a single, instantaneous base-level fall at the channel mouth. Knickpoint evolution in the alluvial material downstream of the bedrock reach was dominated by inclination of the knickpoint face; this resulted in a rapid transition from a waterfall at the point of base-level fall to a broad, convex zone spanning the entire lower alluvial channel reach. A waterfall and plunge pool reformed at the contact between the lower alluvial reach and the simulated bedrock ridge where the knickpoint shortened and steepened. The knickpoint then migrated upstream through the bedrock reach by a combination of parallel retreat and vertical channel incision. The knickpoint evolution process resulted in the formation of upstream-dipping strath terraces, an uncommon landform possibly present in the Holtwood Gorge of the Susquehanna River. As base-level fall effects were transmitted to the alluvial channel above the bedrock reach, a complex channel response, accompanied by pulses of sediment, alternately buried and excavated the bedrock reach. These results illustrate the complex behavior associated with knickpoint evolution, the unsteadiness of the bedrock channel erosion process, and the significant lag times that may exist in natural fluvial systems that are in the process of adjusting to base-level fall.