KNICKPOINT RETREAT AND LONG PROFILE EVOLUTION THROUGH A VERTICALLY BEDDED SUBSTRATE: A FLUME STUDY

We have designed and run a flume experiment to investigate the nature of knickpoint retreat and long profile evolution through a simulated vertically bedded reach of bedrock in an otherwise alluvial channel. The study is motivated by the presence and behavior of spectacular knickpoint propagation in Atlantic slope drainages where long term base level fall must dominate over the tectonic uplift of rocks as a dominant process driving Appalachian unroofing. The channel alluvium consists of very fine to medium sand and the material used to simulate bedrock is a block of varved glacial lacustrine sediment set into the flume such that the 1-2 mm thick alternating beds of clay and silt are vertical. Numerous calibration experiments were conducted to find the optimum combinations of discharge (~0.1 cubic meters/minute) and slope (~ 0.003) so that a stable meandering channel could be established. The simulated bedrock reach began with a thin alluvial cover atop a 30 cm wide valley bottom of a pre-existing canyon. Upon establishment of a stable meandering channel, base level was dropped 6 cm at the channel mouth, immediately generating a knickpoint that propagated upstream. The process of knickpoint migration was dominated by rotation of the knickpoint face, the result being a rapid transition from a steep waterfall-like knickpoint to a broad, convex knickzone. When the knickzone reached the simulated bedrock situated 3 m upstream of the base level fall, a plunge pool immediately formed and the knickpoint shortened and steeped. The migration process changed to a combination of parallel retreat and rotation through the simulated bedrock. The bedrock canyon was incised and attained a new valley bottom width of ~ 8.25 cm. Upstream dipping strath terraces were left in the wake of the migrating and down wearing knickpoint. As the base level fall signal was transmitted to the alluvial channel above the bedrock reach, a complex response of incision, accompanied by pulses of sediment alternately buried and excavated the bedrock reach. These results dramatically illustrated the non-linear behavior associated with knickpoint migration and underscore the significant lag times associated with incision that may exist in natural fluvial systems, especially those in tectonically stable settings, that are in the process of adjusting to base level fall.