A technique known as potential cost, used by Faddy [3] for assessing the operation of a dam is seen to be capable of extension to allow for
(i) a very general cost function, as is required for a multipurpose reservoir (the norm nowadays) and
(ii) the use of discounting of future costs, a very widespread accounting procedure.
Numerical results are obtained for an optimal policy based on such an assessment, and demonstrate the need for an accurate specification of the costs associated with the operation of a reservoir. As a by-product a very full description of the steady-state stochastic behaviour of the dam is obtained.