Seasonal wastewater discharge programs employ different effluent standards during different times of the year to take maximum advantage of a receiving water's natural capacity to assimilate pollutants. The rational design of the programs should try to achieve the maximum economic benefits possible without increasing the risk of water quality impairment. A method is developed for designing seasonal programs for individual dischargers that limits the risk of one or more water quality standard violations in any year. The key elements involve: (1) treating seasonal receiving water assimilative capacity as a random variable; (2) assuming Markov-like behavior of these random variables between seasons; and (3) using a nonlinear programming model to find seasonal discharge limits that minimize waste treatment efforts while maintaining an acceptable annual risk of water quality violation. A case study of controlling ammonia toxicity is presented as well as a comparison of the potential savings available from seasonalization for several pollutants on two rivers with differing seasonal regimes.