A unified degree-day method for simulating the thermal growth, decay, and breakup of river ice covers is developed and applied to ice covers in the St. Lawrence River between Lake Ontario and Cornwall, Ontario. In the simulation model, the variation of the ice cover thickness is related to the ambient air temperature by the formulain whichh = ice cover thickness;h0 = initial ice cover thickness;S = degree-days of freezing since the formation of the ice cover;t = number of days since the formation of the initial ice cover; and α, β, θ = empirical coefficients.Key words: breakup, degree-day method, ice cover thickness, river, simulation, St. Lawrence River.