Despite having offered important hydroclimatic insights, streamflow reconstructions still see limited use in water resources operations, because annual reconstructions are not suitable for decisions at finer time scales. Attempts towards sub-annual reconstructions have relied on statistical disaggregation, which uses none or little proxy information. Here, we develop a novel framework that optimizes proxy combinations to simultaneously produce seasonal and annual reconstructions. Importantly, the framework ensures that total seasonal flow matches annual flow closely. This mass balance criterion is necessary to avoid misguiding water management decisions, such as water allocation. Using the framework, and leveraging a multi-species network of ring width and cellulose δ18O in Southeast Asia, we reconstruct seasonal and annual inflow to Thailand’s largest reservoir. The reconstructions are statistically skillful. This work is one step closer towards operational usability of streamflow reconstruction in water resources management.