Geomorphologic and Hydraulic Reconstructions of Early Holocene Jökulhlaups along the Hvítá River and Gullfoss Waterfall, Iceland

Abstract:

Glacial lake outburst floods or jökulhlaups have been one of the most significant Earth surface processes throughout the Quaternary, impacting landscapes far beyond the source region and leaving a geomorphologic legacy that can persist for millennia. Iceland experiences more frequent jökulhlaups than nearly anywhere on Earth, though most research focuses on subglacial volcanic floods that drain across sediment-laden outwash plains. However, abundant evidence also exists across Iceland for non-volcanogenic floods in bedrock landscapes, which may serve as a better analogue for most global jökulhlaups.

As the Icelandic Ice Sheet retreated during Late Pleistocene-Early Holocene deglaciation, an ice-dammed lake formed in the Kjölur highland region and periodically drained in jökulhlaups across southwestern Iceland. These floods left extensive erosional and depositional evidence along the modern-day course of the Hvítá River, including canyons, bedrock channels, boulder deposits, and Gullfoss—Iceland’s most famous waterfall. The largest events reached an estimated peak discharge on the order of 10⁵ m³ s^-1, ranking them among the largest known floods in Iceland and on Earth. Yet, all our evidence for the Kjölur jökulhlaups comes from only one publication from a quarter-century ago.

This project employs new methods to better constrain flood timing, routing, magnitude, and recurrence interval at this underexplored site. Specifically, this presentation focuses on links between geomorphology and hydrology. It compares mapped field evidence with reconstructed flow conditions from HEC-RAS hydraulic models and paleohydraulic calculations, framing findings in the context of process-form relationships, geomorphologic thresholds, and magnitude-frequency regimes. It also situates results within Icelandic Ice Sheet deglaciation chronology—derived from ongoing cosmogenic nuclide exposure dating and previous paleoenvironmental studies—to reconstruct a series of glacial lake evolution and drainage scenarios. Finally, it assesses the Kjölur jökulhlaups for use as an analogue to past and contemporary glacial lake outburst floods in Arctic and alpine regions worldwide.