The dependence of trihalomethane concentrations on temperature has traditionally been interpreted by linear regression analysis or by an exponential model for the rate of CHCl3 formation. Although mechanistically interesting, these models do not give an explicit description of CHCl3 concentrations and temperature as a function of time. The objectives of this paper are: to determine the statistical significance of seasonal variations in water temperature and CHCl3 concentrations in finished drinking water; to examine the phase of seasonal variations in water temperature and chloroform concentrations in a finished river-water supply, and the time when alternate technologies can best be implemented to meet drinking-water standards for trihalomethanes; to quantify the reduction in surface-water temperatures associated with stream infiltration and evaluate the potential for a corresponding reduction in chloroform formation.