  We investigate the presence of an acoustic-quasi-elastic interaction
contribution in the IXS spectra of liquid water at 301K using inelastic x-ray
scattering with sub-meV energy resolution at momentum transfers 0.77<=Q<=4.20
/nm. The contribution appears due to the overlap the acoustic mode with the
tail the quasi-elastic mode and is fully consistent with hydrodynamic theory.
Incorporating this interaction allows us to describe the dynamic structure
factor, S(Q,w), without introducing an extra mode, and may help explain earlier
contradictory interpretations. The sound velocity, and relative intensity of
the quasi-elastic and acoustic mode, plateau for Q>2/nm at values consistent
with a viscoelastic generalization of the Landau-Placzek relation.
