We have measured the surface flow rate of the large east Greenland glacier, Sortebr¿, through both the initiation and termination of a major surge using synthetic aperture radar (SAR) feature tracking, optimized to minimize error. The Sortebr¿ surge began between November and January 1992--1993, after at least 6 weeks of subfreezing temperatures over the whole glacier, and propagated rapidly up-glacier from a central nucleus. Sortebr¿ reached sustained fast flow rates of up to 24 m d-1, and the active phase lasted for 28--32 months before terminating in June 1995. Termination was abrupt, coinciding with the arrival of the spring thaw and the apparent release of a large volume of stored water from a single outlet at the front. The surge mechanism is interpreted as a switch from channelized to distributed drainage, which at present is best explained by Kamb's linked cavity sliding model.