Transverse drainages (water & wind gaps) have been proposed as a criterion for determining the Flood/post-Flood boundary, but what are the limits of the conventional formation mechanisms? In this study I establish an index for comparing transverse drainages of differing depth scales. An index value near zero implies that the stream passes through the lowest divide, and is associated with plausible lake spillover or tectonic and erosional changes to the hydraulic system. An index value near one implies that the transverse drainage is hydraulically close to the same configuration as when it was carved. This index is used to select the Shoshone-Rattlesnake and Wheeler Ridge transverse drainages for modeling their formation by antecedence. Linear and logarithmic uplift models are used. The maximal depth of an antecedent or superimposed transverse drainage is determined to be around 200 ft. without enhanced erosion. Enhanced uplift rate at the end of the Flood is found to necessitate higher erosion rates, and sets a minimum catastrophe size required to generate a gap. For the Shoshone-Rattlesnake transverse drainage, the necessary erosion strongly implies a late-Flood origin, whereas for Wheeler Ridge, the required erosion is of the same order of magnitude as modeled, and could plausibly be generated in post-Flood time by antecedence.