Abstract: The section from Gaocun (GC) to Taochengpu (TCP) of the lower Yellow River (LYR) is a typical wide-floodplain transitional reach, featured with a complex channel-floodplain system and severe development into a secondary perched channel. This study conducts a detailed investigation of the relationship between spatiotemporal water-sediment variations and geomorphological evolution observed in this reach, using its historical bathymetric data and the cross-section method. The results show that during the operation of Xiaolangdi reservoir, the main channel was continuously scoured, causing narrower and deeper main channels, a decrease in river cross-sectional geomorphic coefficient, and an increase in the bankfull area mostly by more than 200%. In recent years, however, these trends have been significantly weakened－the main channel was slightly widened horizontally and deepened vertically between GC and Sunkou (SK), while mainly deepened vertically between SK and TCP. In the GC-TCP reach, the channel-floodplain system has gradually switched from sedimentation to erosion patterns, with the resulted erosion rate of roughly 0.07 m/a of its main channel and 0.002 - 0.008 m/a of its floodplains; its cumulative erosion volume during 2000 - 2016 was around 2.251×108 m3. The lateral slope of floodplains increased in time, and its spatial distribution was concentrated between 0.5‰ and 2‰.

Key words: lower Yellow River, wide-floodplain transitional reach, scouring and silting changes, main-channel geometry, floodplain geomorphology