The 17 december 2024 Takhini River landslide and river-ice tsunami, Whitehorse, Yukon, Canada

Abstract

On 17 December 2024, a 118,000-m3 landslide on a cut bank of the Takhini River, Yukon, broke through the river-ice and generated a tsunami. The tsunami transported blocks of river-ice up to 250 m from the landslide deposit and destroyed vegetation on the opposite riverbank splintering and uprooting trees more than 20 cm in diameter. River-ice blocks in the tsunami deposit commonly exceed 4 m2 and 30-cm thickness. Snapped branches and impact scars on the trunks of four surviving trees in the tsunami impact area record damage from rafted ice up to 2.5-m height. The landslide-generated tsunami increased the hazard-affected area by 260% from 2.8 ha to 7.2 ha. The ice cover on the river effectively concentrated the tsunami hazard by reducing its overall footprint while increasing the damage in affected areas through entrainment of ice blocks; this effect is an important consideration for future landslide and tsunami hazard assessments in cold regions. Residual hazard at the site includes additional landslides from the landslide scar and altered river dynamics due to obstruction by the landslide deposit. Erosion rates of a previous smaller landslide suggest that the obstruction in the river is likely to remain for at least a decade. The lack of conspicuous triggers for this landslide points to the importance of progressive failure processes in a dry brittle soil slope over-steepened by fluvial erosion.