Formation of the Hukou waterfalls by entrenchment due to a downstream integration process

Waterfall represents a pulse of erosion by lowering the local base level, and produces fluvial terraces that serve as an agent to transfer tectonic, climatic, or autogenic signals upstream through a catchment. Although widespread creation, well-dated waterfalls in the trunk of world’s large river are numbered. This has led to rarity of trunk waterfall case studies, obscuring the resulting landform impacts during waterfall migration and therefore, hindering the exploration of their origins and mechanism. The Hukou Waterfall, situated downstream of the Jinshaan Gorge in the middle reach of the Yellow River, is a unique trunk waterfall. With its homogenous tectonic and bedrock conditions, no anthropogenic activities, and preserved fluvial terraces by retreating waterfall, the Hukou Waterfall provide opportunities to replicate the upstream migration process and associated landform response. Herein, we applied detailed field and DEM-based measurements and age constraints to construct dated longitudinal profiles throughout the Hukou Waterfall and downstream Jinshaan Gorge. We replicate two paleo-trunk in longitudinal profiles: (1) relatively low diachronous trunk (Ta) aged headward from ca. 245 ka to present-day with an average retreating rate and incision rate of 24.5 cm/a and 27.5 cm/ka, representing waterfall migration; and (2) relatively high isochronous trunk (Tb) aged ca. 2.5 Ma throughout the downstream of the gorge with a slow incision rate (8.0-8.7 cm/ka) occurred between Ta and Tb, suggesting a slow slip rate of bounding fault at the outlet of the Jinshaan Gorge. Replication of Ta shows analogous slope of riverbeds with Tb, implying that no waterfall commenced until ca. 245 ka in the downstream of Jinshaan Gorge. This study hypothesizes the Hukou Waterfall to have formed as a mid-Pleistocene rapid base-level-lower event. This event is likely ascribed to the entrenchment due to an integration process between the Fenwei Basin (local base level of the Jinshan Gorge) and the Sanmen Gorge (further downstream of the Fenwei Basin), which exposed the subsurface bedrock scarp produced by the faults accumulate slip.