A variety of nonlinear effects are observed in plasma wave resonances of positive ions held just below the liquid helium surface. At high drive amplitudes the resonance curves “backbend” to lower frequencies, and become hysteretic. There is also a nonlinear increase in the dissipation resulting in a broadening of the tuning curves. The generation of second and third harmonics of the fundamental have been observed using FFT analysis, with the third harmonic being unexpectedly strong. Over narrow ranges of drive amplitude and frequency there are sharp onsets of sideband formation, where frequencies differing by a few kilohertz from the drive appear in the response.