Morphological characteristics and evolution of stream potholes in the Quanfengtan reach of the Lower Zijiang River, Hunan Province, China

This study investigates the morphological characteristics and evolutionary mechanisms of stream potholes in the granodiorite riverbed of the Quanfengtan reach, located in the lower Zijiang River, Hunan Province, China. Field surveys and statistical analyses reveal that these potholes exhibit diverse geometries (calibers: 0.2–2.5 m; depths: 0.03–1.98 m; depth-to-caliber ratios: 0.08–1.63), predominantly elliptical and shallow, reflecting distinct developmental stages. These potholes show no spatial correlation with bedrock joints, and their long-axis orientations diverge from dominant joint trends. Instead, they exhibit consistent alignment with the river’s prevailing flow direction. Furthermore, pothole dimensions (e.g., depth and diameter) strongly correlate with gravel size, indicating sediment-driven abrasion as the primary formation mechanism. The uplift of the Xuefeng Mountains relative to the Dongting Basin since the Late Quaternary, which drove the incision of the lower Zijiang River, transforming it from an alluvial channel to a bedrock-dominated channel, is the primary precondition enabling pothole development in this region. Additionally, the influence of climatic factors and human activities cannot be overlooked. The progressive formation, coalescence, and evolution of potholes into grooves constitute the primary mechanism driving riverbed erosion by the Zijiang River at Quanfengtan.