Rock erosion is a rock-water interaction phenomenon. Erodible rock material exposed to flowing water wears gradually and progressively. Submerged tumbling response of rock fragments in a modified slake durability test was expressed in units equivalent to scour depth and stream power. Standard test modifications include saturated-surface dry in lieu of oven drying for sample weight, extended duration and increased number of test increments, and normalized initial sample weight. Sample loss is expressed as a linear dimension based on rock unit weight. Average sample weight during test increments is expressed as stream power using equivalent distance “traveled”. Slopes of approximately linear result trends plotted as equivalent scour depth versus equivalent stream power define the “geotechnical scour number” of degradable rocks. Rapidly wearing rocks have higher geotechnical scour numbers than more durable rocks; geotechnical scour numbers for six rock types are consistent with observed channel behavior and hydraulic loading, indicating that the modified slake durability test may be valuable for predicting scour at bridge sites, as well as for a general index parameter for weak rocks.