A simplified proof is given that the linear dispersion relation for gravity waves is conerted into a nonlinear dispersion relation by making the replacement (&ohgr;k-k⋅u0)→Z(&ohgr;k⋅u0-idw), where &ohgr;k is the wave frequency, k is the wave vector, u0 is the mean flow, and dw is a nonlinear damping rate explicitly determined by the rms wave velocity. The derivation differs from the previous one in its brevity as well as relative simplicity. The purpose is to present a derivation that does not require previous familiarity with the statistical theory of strong nonlinear interactions and to reveal more explicitly the underlying assumptions and approximations. A substance difference is that a spacetime average is used instead of an ensemble average, and the theory is no longer restricted to random phase waves. The physical significance of d is then elucidated and related to recent experimental observations. Limitations of the derivation are to a quasi-stationary, homogeneous state in which u0 and other average quantitites vary only slowly, to wavelengths satisfying 2k,H≫1, and to wave amplitudes that are too large to be treated by weak mode-coupling theories.