Abstract： Sandy braided river sedimentation is characterized by high-frequency migration, rapidreworking, and multi-stage superimposition, with complex hydrodynamics driving frequent channel changes thatproduce intricate internal architectures. To reveal the sedimentary architecture and evolution of sandy braidedrivers under seasonal flow control, this study integrated trench profile measurements with UAV-based obliquephotography to systematically document geomorphic changes in the Ba River across different periods. Analysisof trench profiles enabled examination of sedimentary processes and architectural characteristics, leading to theestablishment of a sedimentary model. The results show that: ① Discharge variation is a key control onsedimentary evolution. During flood periods, wide channels cover nearly the entire riverbed, driving downstreamsand migration and rapid deposition to form lobate or linguoid bars. During normal flow periods, decreased waterlevels confine flow to inter-bar channels, slowing bar evolution and transforming bar morphology into irregularcomposite forms. ② Under seasonal hydrodynamic fluctuations, the scale and lithofacies characteristics of sandybraided river deposits show significant seasonal differences. The flood period develops four main lithofacies:pebbly sandstone (with abundant gravels) (Gm), large-scale planar cross-bedding sandstone (Sp2), and large-scaletrough cross-bedding sandstone (St2). The normal water period develops three main lithofacies: small-scale planarcross-bedding sandstone (Sp1), small-scale trough cross-bedding sandstone (St1), and parallel-laminated sandstone(Sh). During the flood period, deeply incised main channels (CHm) develop, with bar accretion dominated bydownstream accretion, forming thick linguoid or lobate bars over 1 m thick. During the normal water period, narrowand shallow secondary channel bars (CHs) develop, with multiple bar accretion styles including topset progradation,downstream accretion, and lateral accretion. Individual layers are less than 0.5 m thick and often appear as multistagesuperimpositions in profiles. ③ Based on the relationship between discharge and sedimentary architecturecharacteristics, a two-stage “construction-reworking” evolutionary model for seasonal sandy braided rivers has beenestablished. During flood period, characterized by high-energy flows transporting coarse-grained sediments to formlarge lobate bars through downstream and vertical accretion, accompanied by intense incision of main channels.During normal water period, characterized by falling water levels that expose previously formed bars: bar headsundergo erosion and retreat, bar centers experience multi-stage “cutting-filling-recutting” modifications, while bartails undergo continuous accretion. Under cyclic flow variations, the geomorphic morphology of sandy braidedrivers exhibits periodic changes, resulting in a complex sedimentary architecture characterized by interbedded andamalgamated flood-season accretion units and normal-flow-season reworking units.