Velocity Record of the 2020-22 Muldrow Glacier Surge, Denali National Park, Alaska

Surge-type glaciers are characterized by long-term quiescent phases during which ice flows relatively slowly. This quiescent phase is then followed by a short period of significantly higher velocity after enough ice mass has accumulated at high elevation. The recurrence interval of these surge events varies from years to decades for different glacial basins. Given this long recurrence interval between surges, observing and quantifying a glacier's velocity field from the onset to the termination of a surge has proved challenging. In late 2020, the Muldrow Glacier in Denali National Park, Alaska began to surge for the first time since 1957. This surge continued until its termination in July of 2021, with portions of the glacier reactivating and continuing to surge into 2022. Over the course of this surge, a number of datasets were collected to document this once in a ~60 year event. The surface velocities of the glacier's surface were observed using satellite imagery, feature tracking of repeat, high-resolution aerial photos, on-ice GPS stations, and a ground-based interferometric radar. During the peak of the surge, observed velocities reached 20-25 m/day throughout much of the Muldrow basin. These velocity datasets, combined with digital elevation models produced from airborne photogrammetry throughout the surge, a seismic dataset recorded by an array of seismometers deployed along the moraines, and time lapse photography produce a rich dataset. The velocity record shows an onset of higher-than-usual velocities several years before the full surge started. This velocity event traveled down a major glacier tributary, the Traleika branch. After it reached the main branch, it triggered a full surge, which propagated both up- and down glacier, where it reactivated stagnant ice from the previous surge. After surge termination in summer 2021, higher than usual velocities persisted in the Traleika branch. This challenges commonly held notions about surge initiations and terminations.