Manganese (Mn) often has to be removed at treatment plants that draw water from lowland rivers and canals, but particle-bound Mn and dissolved Mn(II) require different treatments, and only total concentrations are reported by routine monitoring. It was examined whether records already kept by utilities can separate turbidity-associated from warm-season Mn patterns and support daily raw-water decisions. Routine data (2021–2024; 3,451 Mn and 2,022 iron determinations) from seven intakes on four sources in Hai Phong City, Vietnam, were analyzed with fixed-effects, generalized additive, and segmented regressions and gradient-boosted trees. Total Mn exceeded 0.1 mg L−1 in 47.6% of samples (10.7% in February, 87.5% in September). Mn and iron (Fe) rose almost equally with turbidity, leaving the Mn/Fe ratio, on average, unchanged (standardized β = +0.004, 95% CI −0.017 to 0.023). The ratio increased instead with a 30-day water-temperature index (T30) marking the warm season (β = +0.036, 0.019 to 0.055) but not with temperature anomalies, so the pattern is seasonal rather than demonstrably temperature-driven. Above a transition zone near 25 °C, Mn rose 14.2% per °C of T30. Five routine variables reproduced Mn at withheld intakes, sources, and years (log-scale Nash–Sutcliffe efficiency 0.73–0.79, linear model). In leave-one-year-out tests, same-day alert rules combining a seasonal term with turbidity raised the critical success index by 0.08–0.09 over a fixed calendar. As only routine variables are needed, likely exceedance of the 0.1 mg L−1 limit can be flagged in a spreadsheet on the sampling day, before the laboratory Mn result is reported.