Achieving Equilibrium as a Semi‐Alluvial Channel: Anthropogenic, Bedrock, and Colluvial Controls on the White Clay Creek, PA, USA.

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, and gravel‐bed), each term contributes insight that any single conceptual model would be unable to provide alone. Plain Language Summary: Geomorphologists rely on conceptual models to categorize fluvial forms and materials. Here, we present observations from the White Clay Creek (WCC), a Mid‐Atlantic stream, and argue that a multi‐faceted conceptual framework is needed to appropriately understand current processes and conditions. Early studies suggested that Mid‐Atlantic channels represent an equilibrium state controlled by hydrology, but recently the influence of humans has been emphasized. We add to past observations through geomorphic mapping, monitoring, and modeling of bed sediment movement. The WCC displays typical landforms created through erosion and deposition, such as bars, pools and riffles, and floodplains. Width and depth are well‐correlated with water discharge, suggesting hydraulic equilibrium. However, the channel is also bordered by bedrock, gravel derived from adjacent hillslopes, and engineering structures. Furthermore, the WCC longitudinal profile reflects bedrock erosion. Sediment transport monitoring and computations indicate that much of the gravel bed remains immobile when the channel is filled with water, suggesting that the river is not fully capable of sculpting its own channel through erosion and deposition. The WCC reflects many controls, each of which is important: past and ongoing human actions, erosion and deposition of riverine sediment, and occasional exposures of immobile boulders and inerodible bedrock. Key Points: Bankfull Shields stresses are near threshold, but tracers identify abundant immobile clasts derived locally from colluvium and bedrockChannel form is insensitive to sediment supply and channel width reflects cohesive bank processes, rather than bankfull Shields stressesPrevious studies identify fluvial equilibrium and anthropogenic and bedrock controls, but semi‐alluvial is a more applicable term [ABSTRACT FROM AUTHOR]