A general description of the power flow in real multimode waveguides is presented. In the time domain, an integrodifferential equation is derived and solved in three ways. For a given input power excitation, the derived approximate techniques make it possible to calculate an impulse response of the waveguide. A general property of the coupling function, describing arbitrary stochastic mode conversion causes, is given and in the case of microbendings and small scatterers, the coupling function is specified. For waveguides showing some mode degeneracy, the integrodifferential equation is modified and a transformed coupling function is described.