A differential equation for degradation is derived by considering variations in friction velocity, critical friction velocity, sediment size, depth of flow, and river width. The armoring effect for nonuniform bed material, which is an important factor in the analysis of degradation problems, is included in the theoretical analysis. Methods of predicting the river-bed degradation below dams are developed by using a differential equation. Numerical examples for a final equilibrium profile demonstrate applications of the method to rivers.