Two Centuries of Ice-dammed Glacier Lake Outburst Floods at Rembesdalskåka, Norway.

Using an ensemble of diverse data sources, including digital elevation models, lake bathymetry, mass balance records, historic texts, and regional meteorological records, we present a detailed assessment of glacier geometry and Glacier Lake Outburst Flood (GLOF) mechanism changes. The analysis focuses on GLOFs documented over the past 200 years at Nedre Demmevatn, an ice-dammed lake formed by the outlet glacier Rembesdalskåka of Hardangerjøkulen, Norway.

Our results reveal variability in glacier extent, with pronounced thinning over the past 25 years. This accelerated thinning coincides with the near-annual recurrence of outburst floods since 2014. Hydrograph data from the past decade, characterized by rapid glacier melt, indicate a shift in outburst mechanisms from overspill, partial flotation, and channel enlargement to predominantly channel enlargement. We propose that this transition is driven by seasonal temperature fluctuations, evolving lake bathymetry, and progressive thinning of the ice dam.

In total, we document 26 GLOF events over two centuries. Historical records indicate that drainage mechanisms in the 1900s alternated between supraglacial overspill and partial ice uplift with channel enlargement. The former occurred predominantly during periods of glacier advance, while the latter was more common during phases of glacier retreat, often accompanied by a reduction in ice velocity. The study reveals a link between evolving bathymetry, drainage mechanisms and glacier responses to climate warming. This knowledge is important for communities and infrastructure near rapidly changing glaciers, supporting effective adaptive strategies.