From Summer Peak to Winter Release: Nine Decades of Flow Regime Alteration by the Lower Naryn Hydropower Cascade, Central Asia

How far can reservoir operation displace a river's calendar? We address the question with 92 years of monthly discharge on the Naryn, the transboundary artery of the Syr Darya basin, together with 46 years of release data and two years of daily reservoir balances. Annual flow shows no trend, yet the centre of timing of flow moved from late June to mid January, a displacement of 165 days (bootstrap 95% interval 158 to 173) that inverts the hydrograph and far exceeds the week-scale timing shifts reported for climate-driven change. A phase and concentration decomposition separates this rotation from an earlier flattening; change points near 1986, 1990 and 1994, confirmed by independent segmentation methods, coincide with the documented transition from irrigation-oriented to energy-oriented operation. Roughly 4.8 km³ per year (4.0 to 5.6) now arrives outside the growing season. The evidence identifies reservoir operation as the dominant driver of the inversion and quantifies the seasonal redistribution at the centre of a thirty year allocation dispute.