The interaction of ripplons with ripplons and phonons is usually formulated, in quantum hydrodynamics, in terms of creation and anihilation operators. This formulation corresponds, in classical fluid mechanics, to the description of resonant nonlinear interaction of waves in terms of wave amplitudes. This correspondence proves to be valuable for calculation and insight in ripplon physics. It is shown how the existing literature on wave interactions gives us, e.g., the three-ripplon Hamiltonian. Calculations of the lifetime of ripplons on liquid helium at finite temperature are presented. Both ripplon-ripplon and ripplon-phonon processes are discussed. The connection between ripplon-phonon interactions and geophysics is pointed out.