Achieving equilibrium as a semi-alluvial channel: anthropogenic,
bedrock, and colluvial controls on the White Clay Creek, PA, USA
Abstract
Based on well-developed hydraulic geometry relations for width and
depth, classic studies initially interpreted the Mid-Atlantic White Clay
Creek (WCC) as a quasi-equilibrium, alluvial channel. Subsequent studies
document the legacy of colonial-age watershed disturbances and urban
development, confounding earlier classifications. To investigate this
matter, we contribute new data from reach-scale geomorphic mapping, and
observations and modeling of bed material transport. WCC’s longitudinal
profile reflects a history of bedrock incision, while hydraulic geometry
equations for width and depth indicate quasi-equilibrium cross-sectional
adjustment. Alluvial landforms such as pools and riffles, bars, and
actively forming floodplains occur at all 12 study sites, but exposures
of bedrock and colluvium are also common. The ratio of bankfull to
threshold Shields stress averages 1.41 (range 0.41-2.63), suggesting
that WCC is an alluvial, threshold, gravel-bed river. However, a
numerical model of WCC bed material transport and grain size, calibrated
to bedload tracer data, demonstrates that 22% (range 8-73%) of bed
material is composed of immobile, locally sourced cobbles and boulders,
while the remaining bed material represents mobile, sand to cobble-sized
alluvium; this leads us to classify WCC as a semi-alluvial river.
Additional computations suggest that channel morphology is insensitive
to bed material supply. Field observations imply that bankfull Shields
stresses do not represent channel adjustments to achieve stable banks;
rather, width adjustment likely reflects cohesive bank processes.
Despite the numerous and contradictory labels applied to WCC (i.e.,
quasi-equilibrium, Anthropocene, bedrock, semi-alluvial, gravel-bed),
each term contributes insight that any single conceptual model would be
unable to provide alone.