[1] Trapridge Glacier, a polythermal surge-type glacier located in the St. Elias Mountains, Yukon Territory, Canada, passed through a complete surge cycle between 1951 . Air photos (1951 -1981 and ground-based optical surveys are used to quantify the modifications in flow and geometry that occurred over this period. Yearly averaged flow records suggest that the active phase began $1980, and lasted until $2000. The average velocity in the central area of the glacier went from 16 m yr À1 in 1974 to 39 m yr À1 in 1980; it peaked at 42 m yr À1 in 1984, and remained above 25 m yr À1 until 2001. Over that interval, the flow decelerated by steps, in 4-year pulses. After a particularly vigorous acceleration in 1997-1999, the glacier gradually slowed to presurge velocities. In 2005, the flow was less than 9 m yr À1 . Digital elevation models are generated by stereographic analysis of air photos for 1951, 1970, 1972, 1977, and 1981. These models are updated annually using ground-based survey data and a novel implementation of Bayesian kriging. Over the course of the surge, the front of active ice advanced 450 m and the glacier area increased by 10%, with an associated thinning of the ice. The previous surge of Trapridge Glacier, starting before 1939 and ending before 1951, led to a terminus advance of $1 km. Comparison of the two surges suggests that the 1930s surge started with a slow progression similar to what we observed in the 1980s and 1990s, and switched to a faster flow mode after 1941. This second phase was never attained in the recent surge, probably owing to a lack of mass.