The linear theory of internal gravity waves predicts an exponential growth of the velocity amplitudes with altitude. At high enough altitudes, the wave amplitudes become so large that nonlinear effects and turbulent breakdown prevent the velocities from increasing further with height. When this happens, the wave is said to be saturated. In this paper, it is shown that the wave amplitude and amplitude growth factor can be accurately computed from measurements of the sodium layer spatial power spectrum. Lidar measurements are presented which show gravity wave growth lengths that are nearly twice the unsaturated value of 12 km.