Storage capacity, yield, and reliability of a reservoir are random variables. Conditional distributions of these variables are studied by Monte Carlo simulation for operating periods up to 100 years. It is shown that these distributions are affected by the length of the period of operation due to the assumption of an initially full reservoir. Results of the study will aid in the reliability based reservoir design by clarifying the connection among the storage variables. The relationship between the steady-state reliability and the probability of failure-free operation over an N-year planning period is also investigated, and it is shown that the effect of the initial storage can be neglected only when the ratio of regulation is low.