Scour holes commonly form on the gully headcut walls and their induced overhanging mass failures play critical roles in headcut retreat in the Yuanmou Dry-hot Valley region of Southwest China, where soils exhibit a distinct layered stratigraphy. However, the morphological characteristics and the dominant controlling factors of the headwall scour holes in this area remain unclear under natural conditions. Therefore, a field survey was conducted to investigate the morphological characteristics of the headwall scour holes under the 17 natural gully headcuts and to identify the dominant influencing factors in the Yuanmou Dry-hot Valley region. Seven morphology parameters of headwall scour hole and three categories of potential influencing factors (i.e. upstream terrain, gully-head morphology, soil stratification and properties) were determined. Results showed that scour holes developed with a constant self-similar geometry on the active gully headcut walls in this area, and their shape universally appeared as irregular semi-ellipsoids. The maximum width, depth and height of headwall scour holes exhibited significantly positive correlations with each other. Soils exhibited distinct stratification with notable variations in properties across different layers. The top dry-red soil layer had significantly higher levels of organic matter content, cation exchange capacity, Fe content, aggregate stability and soil cohesion compared to the underlying soil layers, which indicated much greater soil erosion resistance. The special stratification differences in soil properties facilitated the development of scour holes on the gully headcut walls, however, the size of these scour holes was primarily influenced by the upstream terrain and gully-head morphology, which reflected erosive forces. Specifically, the upstream drainage area and gully headcut height were found to significantly contribute 70.32%~90.67% of the total variations in the morphologies of headwall scour holes. These findings have important implications for the modeling and management of gully headcut erosion process, particularly in regions with stratified soils.

gully erosion; gully headcut retreat; gully head morphology; drainage area; stratified soils