Declining discharge of glacier outburst floods through the Holocene in central Patagonia

Glacier outburst floods are a major hazard in glacierized catchments. Global analyses have shown reduced frequency of glacier floods over recent decades but there is limited longer-term data on event magnitude and frequency. Here, we present a Holocene palaeoflood record from the Río Baker (Chilean Patagonia), quantifying the discharge and timing of glacier floods over millennial timescales. A catastrophic flood of 110,000 m3/s (0.11 Sv) occurred at 9.6 ± 0.8 ka, during final stages of the Late Glacial Interglacial Transition, followed by five flood-phases coeval or post-dating Holocene neoglacials. Highest flood frequencies occurred at 4.3–4.4 ka, with 26 floods of minimum discharges of 10,000–11,000 m3/s, and 0.6 ka with 10 floods exceeding 4600–5700 m3/s. The largest modern outburst flood recorded surpassed ∼3810 m3/s. Thus glacier flood magnitude declines from the order of 0.1 to 0.01 Sv over the Early to Mid Holocene, and to 0.001 Sv in the instrumental record. •First bedrock gorge palaeoflood record of Holocene Glacial Lake Outburst Floods.•GLOF frequency increased at the end or post-dating three Holocene neoglacials.•Flood discharge (magnitude) declined from 0.1 Sv to 0.001 Sv through the Holocene.•The decrease in peak GLOF magnitude reflects an indirect control of ice-sheet mass.•Modern discharges are not exceptional in terms of the Baker valley Holocene record.