A standard operating procedure (SOP) is presented for calculating the date of freeze-up on the St. Lawrence River at Massena, N.Y. The SOP is based on two empirical temperature decline equations developed for Kingston, Ontario, and Massena, N.Y., respectively. Input data needed to forecast freeze-up consist of the forecast December flow rate and heat flux for the St. Lawrence River and the water temperature at Kingston, Ontario, on the forecast initiation date. The material can provide useful learning activity as well as a research tool. (Author/RH)