Mechanistic modeling facilitates rational crystallization engineering and design space screening. For an accurate model, the dominant growth mechanism operating on each face must be determined, which is highly dependent on supersaturation. Considering the case of centrosymmetric growth units, we developed and connected existing mechanistic expressions for spiral and two-dimensional-nucleation growth regimes, through application of stationary nucleation rate theory. Our approach enables calculation of crossover supersaturations and forms a framework to model the specific mechanism operating on each face under given crystallization conditions. Increasing supersaturation can change the crystal morphology; as face families switch growth mechanisms, they may grow out of the steady-state shape, or influence its aspect ratio. Application of the model to naphthalene, biphenyl, pentaerythritol and b-HMX shows the ability to capture experimentally observed examples of such supersaturation-dependent crystal habits. This methodology broadens the applicability of mechanistic crystal growth modeling to include higher-supersaturation industrial processes. (C) 2017 American Institute of Chemical Engineers AIChE J, 63: 1338-1352, 2017