We use a physical model to investigate how changes in subglacial hydrology affect ice motion of Antarctic ice streams. Ice streams are modelled using silicone polymer placed over a thin water layer to mimic ice flow dominated by basal sliding. The model ice-stream force balance is calculated and compared directly to the observed force balance of Whillans Ice Stream (WIS). Dynamic similarity between the model and WIS is achieved when their force balances are equivalent. The WIS force balance has evolved over time owing to increased basal resistance. We test two hypotheses: (1) the subglacial water distribution influences the ice-flow speed and thus the force balance; (2) shear margins are locations where transitions in water layer thickness occur. We find that the velocity and force balance are sensitive to pulsed water discharge events and changes in lubrication that result in sticky spots, and that model shear margins tend to overlie water lubrication boundaries. We conclude that local changes in basal lubrication near margins (possibly as a result of the presence of sticky spots or subglacial lakes) influence the stability of ice-stream margin position and may be responsible for large and rapid shifts in margin location.