The most common methods used to evaluate the potential hydraulic erosion of rock are index-based methods, which correlate the force of flowing water and the capacity of a rock to resist erosion. This capacity is evaluated using erodibility indices, which combine a set of specific geological parameters. Nonetheless, there exists no clear consensus in regard to the relative importance assigned to the geological parameters. Our study proposes (i) a review of the existing index-based methods used to evaluate the hydraulic erodibility of rock and (ii) a method to determine the relative importance of the geological parameters governing the erodibility of rock. The developed approach relies on a large dataset of case studies providing details of unlined spillways subjected to erosion. We demonstrate that the analysed geological parameters can be classified according to their relative importance – from highest to lowest – as follows: (1) joint shear strength, (2) nature of the potentially eroding surface, (3) rock block volume, (4) joint opening, (5) rock block's shape and orientation relative to flow direction and (6) the rock mass deformation module. This ordering of the relative importance of the geological parameters agrees largely with previously established orderings that were based on field observations.