The seismicity rate beneath the downglacier 85 km of ice stream C, West Antarctica, is modulated by the tide. The tide beneath the Ross Ice Shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude. This tidal forcing travels up the ice stream as an attenuating wave at approximately 1.6 m s-1 and is detectable 85 km from the grounding line. We successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity &eegr; and thickness hb and calculate that the substrate has an apparent stiffness &eegr;/hb of O(108) Pa s m-1. This finding suggests that the conditions of the till layer at the bed of ice stream C are similar to those of ice stream B and that the reason for the recent stagnation of ice stream C is other than loss of till. We further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress.¿ 1997 American Geophysical Union