We measured the surface velocity field during the summers of 1999 and 2000 on the 7 km long, 185 m thick Bench Glacier, Alaska, USA. In the spring of both years, a short-lived pulse of surface velocity, 2-4 times the annual mean velocity, propagated up-glacier from the terminus at a rate of ~200-250md-1. Displacement attributable to rapid sliding is ~5-10% of the annual surface motion, while the high-velocity event comprised 60-95% of annual basal motion. Sliding during the propagating speed-up event peaked at 6-14 cm d-1, with the highest rates in mid-glacier. Continuous horizontal and vertical GPS measurements at one stake showed divergence and then convergence of the ice surface with the bed as the velocity wave passed, with maximum surface uplift of 8-16 cm. High divergence rates coincided with high horizontal velocities, suggesting rapid sliding on the up-glacier side of bedrock steps. Initiation of the annual speed-up event occurred during the peak in englacial water storage, while the glacier was entirely snow-covered. Basal motion during the propagating speed-up event enlarges cavities and connections among them, driving a transition from a poorly connected hydrologic system to a well-connected linked-cavity system. Sliding is probably halted by the development of a conduit system.