Hydraulic erodibility of natural and engineered earth materials, including both soil and rock, can be evaluated in terms of a rational correlation between rate of energy dissipation of flowing water and an erodibility classification of the materials. Earth materials ranging on a continuum from cohesionless granular soil through massive hard rock can be characterized in terms of an erodibility index,Kh. The parameters of the index include key material and mass properties that can be measured in the field and at low cost. These properties include earth mass strength, block/particle size, discontinuity/inter-particle bond shear strength, shape of material units and their orientation relative to the flow. The relative magnitude of erosive power of flowing water is represented by its rate of energy dissipation for a variety of flow conditions, including head cuts, hydraulic jumps, grade changes, and open channel flow. A log-log plot of experienced rate of energy dissipation versus the erodibility index of the studied materials demonstrates a correlation from which the critical threshold to initiate erosion of a material can be predicted for any given set of hydraulic conditions. The method is based on an analysis of 137 field observations of spillway performance collected by the U.S. Department of Agriculture, observations at Bartlett Dam, Salt River Project, Arizona, at four South African dams, and published data pertaining to initiation of sediment motion.