jökulhlaup

A jökulhlaup is an Icelandic term for a glacier-outburst flood, a sudden, large-scale release of water held back by a glacier or by an ice dam, typically originating from a subglacial or englacial lake. The word combines jökull (glacier) and hlaup (flood or run). The phenomenon is a major component of glacial hydrology and is often linked to volcanic or geothermal activity beneath the ice.

Most jökulhlaups occur when meltwater accumulates in subglacial lakes or channels and is temporarily trapped by an ice dam. If the dam breaches or a surge in meltwater increases pressure, water rapidly drains downslope, sometimes carrying significant sediment, rocks, and ice blocks. Volcanic eruptions beneath ice can rapidly increase meltwater production, also triggering a breach and a flood. In some cases, the failure of an ice dam due to overfilling or structural weakness initiates the event.

Characteristics of jökulhlaups include a rapid onset, hours to days of high flow, and discharge that can be substantial enough to erode channels, alter river courses, and form or modify outwash plains. The floods can pose serious hazards to nearby communities and infrastructure, especially in Iceland where many glaciers abut populated and road networks.

Notable examples have occurred in Iceland, particularly associated with eruptions and geothermal activity under Vatnajökull and Grímsvötn, producing floods that have affected downstream valleys and coastal areas. The term is commonly used in Icelandic glaciology, while similar processes in other regions are generally described as glacial outburst floods (GLOFs).