Satellite evidence of managed-regime hydrologic stabilization and upstream-control instability in sequential river weir systems

River weirs can transform seasonally variable river reaches into more persistent impounded water surfaces, yet multi-weir evidence of such hydrologic changes under managed post-construction regimes remains limited. We used 22 years (2000-2021) of Joint Research Centre Global Surface Water Yearly History data to evaluate 16 multifunction weirs across four major Korean rivers. Fifteen of 16 treatment reaches had lower inter-annual variability in annual water-classified area fraction under the managed post-construction regime, with a median variability ratio (VR) of 0.158. With equal-length nine-year windows, 15 of 16 sites retained VR < 1, and 9 of 16 median-centered Brown-Forsythe tests remained significant after Benjamini-Hochberg adjustment (q < 0.05). The direction persisted after logit transformation (15/16; median VR = 0.356), although the attenuated magnitude indicates sensitivity to the bounded response scale. Mean log(VR) was negative in all four river blocks, although the exact test across only four blocks gave a two-sided p-value of 0.125. Fourteen of 16 upstream comparators increased in water-classified area fraction, with a mean shift of +6.0 percentage points (site-level p = 0.005; Cohen's d = 0.82), but only three of four river-block mean shifts were positive (exact two-sided p = 0.625). Together, the results quantify widespread but heterogeneous hydrologic stabilization associated with the managed regime, provide a uniform satellite-based baseline against which future changes in weir operation can be evaluated, and show why comparator stability and within-river dependence must be tested explicitly when BACI designs are applied to connected river infrastructure.