A one-dimensional hydrothermal model is used to forecast the impact of a proposed hypolimnetic discharge of treated municipal wastewater on stratification and mixing in Onondaga Lake, NY. Important simulated impacts are increased temperatures in the hypolimnion, reductions in density stratification, increased mixing and homogeneity within the hypolimnion, and reductions in the duration of summer stratification. Predictions from this analysis serve as input to water quality models that simulate related impacts of this management action. Features of the hydrothermal model include simulation of entrainment associated with plunging inflows, capability for simulations during ice cover, and a submodel to simulate the near-field mixing associated with a multiport diffuser. The model successfully simulated six consecutive years of historical stratification conditions. The model performs well in simulating the dimensions and temperatures of layers, and the timing/duration of stratification. The model is less successful in simulating the more subtle effects of dense saline inflows that linger from a recently closed soda ash/chlor-alkali facility, such as intermittent formation of chemical stratification during spring and fall mixing.