Rock mass erosion in unlined spillways causes significant structural damage and necessitates expensive repairs. The rock mass is made up of blocks formed by various arrangements of joint sets. The volume and the protrusion of these blocks, as well as the orientation, opening and roughness of the joints, are all features that can affect rock erodibility. Most of these features are incorporated in parameters developed for rock mass characterization. Three joint orientation parameters are compared in this article using a database containing geological and hydraulic information on scoured spillways. According to the detailed methodology, data is first classified according to rock quality using the GSIchart index. Then, for each GSIchart class, data is distributed according to the damage level, stream power and joint orientation parameter chosen. This study shows that no joint orientation parameter is currently able to accurately represent the effect of joint orientation on erosion of excellent- to poor-quality rock mass. Moreover, this study shows that the GSIchart index is not a rock quality index that completely evaluates rock erosion, since some relevant parameters for evaluating rock erodibility are not considered.