  We propose a procedure to evaluate the impact of nonlinear couplings on the
evolution of massive neutrino streams in the context of large-scale structure
growth. Such streams can be described by general nonlinear conservation
equations, derived from a multiple-flow perspective, which generalize the
conservation equations of non-relativistic pressureless fluids. The relevance
of the nonlinear couplings is quantified with the help of the eikonal
approximation applied to the subhorizon limit of this system. It highlights the
role played by the relative displacements of different cosmic streams and it
specifies, for each flow, the spatial scales at which the growth of structure
is affected by nonlinear couplings. We found that, at redshift zero, such
couplings can be significant for wavenumbers as small as $k=0.2\,h$/Mpc for
most of the neutrino streams.
