Quantifying transverse gradient effects on hydrological connectivity and optimal thresholds in the lower yellow river floodplain

The lower Yellow River floodplain, as a vital ecosystem supporting millions of livelihoods, faces escalating flood risks due to the formation of a “secondary suspended river”. Using cross-sectional analysis and entropy-weighted modeling (1986–2020), this study quantifies the relationship between transverse gradient and lateral hydrological connectivity in the Hedao reach. Key findings include: (1) Xiaolangdi Dam operations increased the pre-flood and post-flood transverse gradients by 51.64 % and 46.74 %, respectively; it reduced pre-flood connectivity by 15.33 %, yet increased post-flood connectivity by 19.33 %; (2) A 0.07 % transverse gradient in the study area was associated with the decline in connectivity; (3) An optimal transverse gradient threshold of 0.032 % during the pre-flood season and 0.034 % during the post-flood season is proposed to balance flood control and ecological maintenance. This threshold synergistically mitigates “secondary suspended river” risks while sustaining riparian vegetation, thus providing a regional reference for the management of sediment-rich floodplains worldwide.