Gornergletscher, Switzerland, is located adjacent to the marginal lake Gornersee, which periodically drains. We measured glacier ice-flow velocities during two drainage events of the lake, in 2004 and 2006. The common feature of these events was that, during both, Gornersee filled to its maximum level and then overflowed. The events differed in that in 2004 Gornersee rapidly drained via a sudden subglacial connection, whereas in 2006 the lake water continued to overflow and slowly discharged into a nearby moulin. We analysed the changes in ice-flow velocities in the vicinity of Gornersee during the two drainage events, using a three-dimensional ice-flow model which is able to (1) simulate locally variable enhanced basal motion of the ice and (2) account for the load and release of water pressure exerted on the ice margin. We demonstrate that the key features of the observed flow changes can be reproduced adequately in the numerical model by considering these two effects as the main mechanisms. We interpret the 2006 flow changes to be dominated by the release of lake water pressure acting on the ice during the lake drainage. The 2004 ice-flow changes can be explained by enhanced basal motion, and the impact of the lake water pressure provides certain clues to some observations insufficiently explained by enhanced basal motion.