It is proposed that the dominant nonlinear effect of low‐frequency instabilities is an incoherent scattering of particle orbits by waves. The scattering is considered to be incoherent even when only a single mode is present. Because it is incoherent, one can include wave scattering in the theory just as collisional scattering is included in the linear wave theory. A nonlinear dispersion relation is obtained from the linear dispersion relation simply by adding wave scattering terms to the collisional scattering terms. The wave scattering causes particle diffusion over the transverse wavelength and in the theory appears as an enhanced viscosity. This wave induced viscosity can be computed from the amplitude of the wave. Waves which are linearly unstable grow until the enhanced scattering causes their nonlinear growth rates to vanish. Theoretically computed values of density fluctuations, phase shifts, and flux are in good agreement with experimental values in the cases considered.