Glacial lake outburst floods (jökulhlaups) are challenging to study because they often take place below hundreds of meters of ice, and the complex interplay between lifting of the glacier and melting of the flood path is not well understood. Here, we provide unique insights into the mechanics of jökulhlaups and how they affect three-dimensional glacier motion using satellite synthetic aperture radar (SAR) and field observations, acquired before, during and after two recent jökulhlaups (in 2021 and 2022) from the subglacial lake Grímsvötn in Iceland. The results indicate that the same event may alternate between channel flow and sheet flow and that ponding of water along the subglacial flood path can play a major role in jökulhlaup dynamics. This has important implications for physical jökulhlaup modeling and for better understanding the role of subglacial hydraulics on glacier dynamics, required for improved predictions of the future evolution of glaciers and ice sheets.
Magnússon et al. (2026) studied this question.