Abstract: The Tongguan section serves as a crucial node for water and sediment transport in the joint operation of reservoir groups in the middle Yellow River. Based on the data measured in the Sanmenxia reservoir since implementing its operation of storing clear water and discharging sediment, this study examines the spatiotemporal distribution characteristics of scour and silting in the Longmen-Sanmenxia reach and the evolution of the Tongguan riverbed elevation. The results show that this reach’s upper section (Longmen-Tongguan) experiences scour in non-flood season and silting in flood season, whereas opposite is the pattern of scour and silting in its lower section (Tongguan-Sanmenxia). Specifically in the study reach, an inflection point occurs for transition between scour and silting. In the non-flood seasons of 2010-2021, the reach’s scour-to-silting inflection points were concentrated in the tail of Longmen-Tonguan upstream of the Tongguan station. Their locations were mainly related to flow energy at the Longmen station－they shifted downstream by roughly 19.45 km for every 1×106 J/m increase in flow energy. In contrast, in the flood seasons, the silting-to-scour inflection points were concentrated in the reservoir tail of Tongguan-Sanmenxia downstream of Tongguan; they shifted upstream by 32.10 km for every 1×106 J/m increase in flow energy. Within the same hydrological year, the non-flood season and flood season inflection points serve as the endpoint of along-channel scour in the wandering river reach and the starting point of scour in the reservoir, respectively. Between these two points, slight scour or slight silting occurs. Tongguan is located exactly right between them, and its bed elevation is proportional to the distance between them. A larger distance indicates a wider range of the river reach that stays in the state of slight scour and slight silting, which means it is more difficult to effectively lower the Tongguan riverbed elevation. To overcome the difficulty, an effective measure should be to enhance flow energy in the study reach through optimizing the water-sediment relationship in the middle Yellow River.

Key words: Longmen-Sanmenxia reach, erosion and deposition inflection point, backwater length, water flow energy, riverbed elevation at Tongguan