The jökulhlaup Mýrdalsjökull ice cap, S-Iceland, in July 2024. Insights from radio echo sounding and mapped surface elevation changes

Jökulhlaups in wide range of net volume and discharge drain from Mýrdalsjökull ice cap in S-Iceland. The largest ones are related to eruptions of the underlaying volcano Katla and can drain cubic kilometres of flood water with peak discharge on the order of 10⁵ m³ s^–1. The last such jökulhlaup occurred during the eruption in 1918. Most of the jökulhlaups from Mýrdalsjökull are however related to geothermal activity beneath ice cauldrons located near the rim of the Katla caldera. These jökulhlaups are much smaller, typically with flood volume between 10⁵ and 10⁷ m³ and peak discharge between few m³ s^–1 and few hundred m³ s^–1. Larger events, with net flood volume exceeding 10⁷ m³ and peak discharge exceeding 1000 m³ s^–1, occurred in 1955, 1999, 2010 and 2024. The source of these jökulhlaups was in all cases beneath known ice cauldrons but what exactly causes them is debated. They have both been attributed to small subglacial eruptions and to powerful events in the geothermal system beneath the glacier. The jökulhlaup in 2024, drained on July 27^th from ice cauldrons located in the northeast part of the caldera. These cauldrons had since the start of regular surface elevation monitoring in 1999, been very stable features in the glacier surface. The lack of topographical surface changes indicated insignificant storage of meltwater beneath these cauldrons, likely caused by persistent leakage from them. This approved also with repeated radio echo sounding (RES) profiling carried out annually over these cauldrons since 2012 with the aim of detecting water chambers beneath the cauldrons. Water draining these cauldrons was expected to drain southwards into the river Múlakvísl, but the jökulhlaup in 2024 drained towards east into the river Leirá at the glacier margin, and from there into the river Skálm. The only hydrological monitoring in the flood path were at the bridge passing Skálm on the primary road in S-Iceland. The swift jökulhlaup had already flooded over the bridge and the road next to it before the road had been officially closed. Here we analyse the cause of this jökulhlaup by studying: a) Elevation changes, deduced from Pleiades satellite images, at the jökulhlaup’s source and path both before and during the July 2024 jökulhlaup as well as during the subsequent period of repeated smaller jökulhlaups still ongoing. b) Extensive and detailed glacier bed mapping with RES carried out in the spring 2025 over the jökulhlaup’s source and flood path, as well as comparison with RES profiles measured in this area prior to the 2024 jökulhlaup.