The Sefidrud irrigation network, covering 238,000 hectares, is the vital lifeline of agriculture in Gilan province and is under severe pressure due to the reduction of the Manjil Dam's effective storage volume from 1765 to 1158 MCM. This study investigates the long-term dynamics of water delivery to the four main canals (Sangar Left, Sangar Right, Goleroud, and Fumanat Tunnel) over a 16-year period (2009–2025), focusing on identifying structural change points and seasonal signal evolution. Daily data from 5,842 days were used. Complementary methods including STL decomposition, modified Mann–Kendall test for autocorrelation, Pettitt (1979) test with Bonferroni correction, Continuous Wavelet Transform (CWT) with cmor1.5-1.0 wavelet, and red-noise significance testing were applied. The 2015–2016 water year was identified as a structural change point (p = 0.0002). The total mean daily discharge decreased by 32% (from 10.1 to 6.9 m³/s), and the annual delivery volume dropped from an average of 1472 to below 1100 MCM. The Goleroud Canal (GC) remained the only stable component (CV = 52.4%, p > 0.25), reflecting the managerial policy of maintaining base flow (0.3–1.2 m³/s) for the Gilan drinking water treatment plant. Wavelet analysis confirmed a more than 90% reduction in annual signal power and the emergence of short-term managerial signals (30–128 days). After 2015–2016, the Sefidrud network transitioned from a "predictable and seasonal" regime to an "emergency and reactive" one. Therefore, revising the irrigation scheduling calendar based on actual storage levels is essential.