Natural fluctuations in river flow are central to the ecosystem productivity of basins, yet significant alterations in daily flows pose threats to the integrity of the hydrological, ecological, and agricultural systems. In the dammed Lancang–Mekong River (hereafter LMR), the attribution of these large daily flow changes to upstream regions remains mechanistically unexamined, a factor blamed on challenges in estimating the time required for large daily shifts in upstream river flow to impact the downstream stations. Here, we address this with a newly developed subbasin modeling framework that integrates 3D hydrodynamic and response time models, together with a hydrological model with an embedded reservoir module. This integration allows us to estimate the time required between two hydrological stations and to distinguish between the contributions of subbasins and upstream regions to large daily river flow alterations. The findings revealed a power correlation between upstream river discharge and the time required to reach downstream stations. Significant fluctuations (greater than 1 m) in the river's daily flow were evident before the advent of the era of human activities, i.e., before 1992, with around 92 % of these fluctuations occurring during the wet season, particularly in June, July, and August. This pattern persisted throughout subsequent periods, including the growth period (1992–2009) and the mega-dam period (2010 to 2020), with minimal variation in the frequency of events. The Lancang basin contributed approximately 33 %–42 % of these large river fluctuations at the Chiang Saen station. We found that daily-scale water level and runoff might not fully capture dynamic river flow changes, as significant differences were observed between daily and subdaily river flow profiles. Subbasins significantly contributed to mainstream discharge, leading to substantial shifts in mainstream daily river flows. The outcomes and model derived from the subbasin approach have significant potential for managing river fluctuations and broader applicability beyond the specific basin studied.