The tidal forcing of ice streams at their ocean boundary can serve as a natural experiment to gain an insight into their dynamics and constrain the basal sliding law. A nonlinear 3-D viscoelastic full Stokes model of coupled ice stream ice shelf flow is used to investigate the response of ice streams to ocean tides. In agreement with previous results based on flow-line modelling and with a fixed grounding line position, we find that a nonlinear basal sliding law can qualitatively reproduce long-period modulation of tidal forcing found in field observations. In addition, we show that the inclusion of lateral drag, or allowing the grounding line to migrate over the tidal cycle, does not affect these conclusions. Further analysis of modelled ice stream flow shows a varying stress-coupling length scale of boundary effects upstream of the grounding line. We derive a viscoelastic stress-coupling length scale from ice stream equations that depends on the forcing period and closely agrees with model output.