The paper discusses the problems of optimal water management in a distribution system. The main technical elements of the considered water-economic system include: retention reservoirs, among which water transfer is possible, and a network of connections between the reservoirs and water treatment plants (WTP). The system operation optimisation involves specifying proper water transport routes and the rates of water flow from the retention reservoirs to the WTPs, and them volumes of possible transfers among reservoirs, so as to ensure that total system operation costs as specified by the assumed quality coefficient are minimal. The analytic solution of the formulated optimisation task has been obtained as a result of employing Pontryagin’s maximum principle with reference to the assumed quality coefficient. The researchers have assumed fixed initial and end conditions in reservoir level trajectories. Optimisation start and/or end time is free to choose. The solutions obtained gave grounds to develop a simulation computer model representing the system’s operation. The analysis of of the results obtained may affect decisions supporting control of water-economic systems existing in reality.