Three decades of high-Arctic river monitoring revealed no trend in runoff and a regime shift in glacial lake outburst floods

Study region Zackenberg River Catchment, NE-Greenland. Study focus Arctic freshwater runoff has increased due to rising air temperatures, and associated increases in precipitation and ice melt. While the runoff of river catchments connected to the Greenland Ice Sheet has increased, there is no clear signal for catchments disconnected from the ice sheet. Here, we analyze the Zackenberg River discharge record, the longest record from a Greenlandic river catchment disconnected from the ice sheet. New hydrological insights for the region We find no significant long-term trend in river runoff and explain this by the limited glacier coverage of the catchment, while precipitation as the primary driver shows no trend. Since monitoring began in 1996, recurrent flood waves originating from the outburst of a glacier-dammed lake have been observed. Their already moderate contribution to annual runoff has recently begun to diminish, accompanied by a change of flood characteristics. Continued glacier mass loss acted as long-term preconditioning factor for the shift in outburst characteristics. We show that prior to the shift, rapid subglacial lake drainages dominated, with timing linked to the seasonal development of the subglacial drainage system. The onset of ice dam disintegration in 2019 initiated a transition to ice-marginal dominated flood routing with declining lake drainage rates and volumes. We present a conceptual model for this new drainage dynamics and propose that it will ultimately lead to ice dam collapse and the cessation of the outburst floods.