Gravel-bed rivers are often interpreted as equilibrium, near-threshold channels (Parker, 1979), where channel morphology is adjusted to transmit the supply of coarse bed material with the given discharge. Theoretical analyses based on this concept predict bank sediments at the threshold of motion with bankfull Shields stresses on the bed (based on the D50) slightly in excess of this threshold, such that the bed material is fully mobile at bankfull stage. Surveys of 13 sites around the White Clay Creek, however, provide observations that are inconsistent with this concept. Bedrock is exposed along the channel and the longitudinal profile is controlled by migrating knickpoints, suggesting that the slope is imposed by bedrock erosion. Moreover, up to 50% of the bed material is immobile at bankfull stage. These observations suggest an alternate hypothesis to threshold channel theory: immobile cobble-boulder bed material is supplied locally by colluvial processes and bedrock incision, with a throughput load of sand-pebble-sized sediment readily transported by the river that is primarily stored in bars rather than on the bed. An approximate threshold condition based on the D50 of the streambed arises by averaging the grain size distribution over the immobile bed material and the finer throughput load, but this averaged bankfull Shields stress does not provide a useful measure of the mobility of all size fractions on the bed. These observations suggest that the channel morphology of the study site is decoupled from the supply of bed material, and that the White Clay Creek should not be considered an equilibrium, near-threshold channel. To test our hypothesis, we attached radio frequency identification (RFID) tags to 50 clasts in a 100 m reach. The RFID tags were installed with the gravel in situ on the bed at randomized locations in the channel; the distribution of tagged grains mirrors the grain size distribution of the bed. Since the deployment of tagged clasts in June 2019, six surveys have been accomplished and four significant flow events have occurred with the gage height reaching 2/3 of bankfull stage. Afterwards, 77% of tagged gravel remained in place during a given event, supporting our hypothesis. Numerical modeling of bed mobility under a variety of sediment supply scenarios allows us to generalize our field observations.

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.

The Shields parameter based on median grain size D50 and bankfull depth is often used to interpret river morphology, but it may not always be a useful index of sediment transport processes. At 12 sites of the White Clay Creek (WCC), PA, the ratio of bankfull Shields stress to threshold Shields stress averages 1.41 (range 0.41-2.63), suggesting that these channels are alluvial near-threshold gravel-bed rivers. However, field mapping indicates confinement by bedrock and colluvium, and a channel slope dominated by bedrock incision and knickpoint migration. A numerical model of WCC bed material transport and grain size, calibrated to bedload tracer data, demonstrates that 22% (range 8-73%) of the bed material is composed of a population of immobile cobble and boulder-sized sediment supplied through local colluvial processes and bedrock erosion, and a separate population of mobile sand, pebble- and cobble-sized alluvium. Computations also suggest that channel morphology is only weakly coupled to upstream sediment supply. Additional analyses further imply that width adjustment may reflect a balance between cohesive bank erosion and floodplain deposition, though channels nonetheless may be closely scaled by cohesive bank erosion thresholds. WCC represents an example of a continuum of underappreciated, but relatively common, threshold alluvial-colluvial-bedrock rivers with partially immobile beds and widths scaled by cohesive bank erosion thresholds. Fluvial geomorphologists will need to look beyond simple sediment transport metrics to fully understand and classify these stream channels.