Slow surge of Trapridge Glacier, Yukon Territory, Canada

Abstract

Trapridge Glacier, a polythermal surge‐type glacier located in the St. Elias Mountains, Yukon Territory, Canada, passed through a complete surge cycle between 1951 and 2005. Air photos (1951–1981) and ground‐based optical surveys (1969–2005) 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−1 in 1974 to 39 m yr−1 in 1980; it peaked at 42 m yr−1 in 1984, and remained above 25 m yr−1 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−1. 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.