Abstract: The regimes of flow and sediment entering the Lower Yellow River (LYR) have changed significantly since the Xiaolangdi Reservoir started to operate, causing a great reduction in high flood frequency and enhancing the role of small and medium discharges. To reveal the bed evolution of the LYR braided reaches under small and medium discharges, this study conducts physical model tests on the Heigangkou-Jiahetan Reach using constant discharges of 500, 1000, 1500, and 2000 m3/s. The results reveal three key conclusions. (i) Vertically, riverbed erosion and deposition are distributed alternately along the channel, manifesting a typical evolution of braided reaches. (ii) Laterally, considerable lateral migration of the main channel can also occur in the LYR braided reaches under small and medium discharges. With the discharge increasing, the bending coefficient decreases from 1.18 to 1.13 and the main channel develops from winding to straight. (iii) Longitudinally, the adjustment trends in thalweg profile under all the test discharges are basically consistent with the variations in the amounts of sediment erosion and silting along channel. This paper examines the spatiotemporal adjustments in the riverbed near training works and a bridge downstream, and presents the results of numerical simulations using a stochastic model. It reveals that temporally, vertical adjustment is relatively fast in early stage and then slows down rapidly; spatially, the accumulated riverbed degradation downstream of the bridge decreases nonlinearly along the channel until it diminishes, exhibiting significant spatial decay characteristics. Applications show that our stochastic model has good adaptability to spatiotemporal adjustments of typical short reaches, and that the correlation R2 between measured and calculated results can be up to 0.74. The results of this study would help the future channel regulation and protection of the LYR braided reaches under small and medium discharges.

Key words: braided reach in Lower Yellow River, river channel evolution, physical model test, lateral migration, local adjustment