Technical Abstract: The ability to reliably design auxiliary spillways has become a pressing requirement because of the very large number of dams involved. An empirical classification index representing the erosional resistance of the overall range of particulate, jointed, and intact earth materials is presented and correlated for a number of practical cases from Arkansas and Kansas in the United States of America and from South Africa with the specific stream power of flows observed to be associated with various degrees of erosion. The United States Department of Agriculture data base and the format in which the data were prepared for the purposes herein, are described in detail. The modes of erosion that occur in spillways are defined. Guidelines are provided for evaluating the failure status of an erosion damaged spillway that enables determination of whether the incident specific stream power exceeded, equaled, or fell below the erosional resistance of various sections of the ground profile affected. The correlation function between specific stream power and erodibility index provides the required design criterion.