The utility of optimal‐control theory for the deterministic operation of very large multireservoir systems is demonstrated for a real situation, the complex multireservoir Mahaweli system in Sri Lanka. This system includes 19 nodes, or reservoirs, and 35 release links. The model developed for the Mahaweli is designed to minimize a hydroelectric energy shortage objective and satisfy prespecified irrigation demand constraints. Two alternative approaches are explored for optimal operation of the Mahaweli system. The first involves monthly application of the optimal‐control algorithm to find an optimal policy for the next year, based on current storage and forecasted or historical inflows and demands. The second alternative is an implicit stochastic approach, in which linear operating rules are derived using deterministic optimal control and historical data. Both of the alternatives give reasonable and comparable results. The implicit stochastic optimization alternative has a great advantage regarding computer time and storage requirements. This makes it usable on a small personal computer, providing the system operator with decisions in a few seconds, even with very large systems.