New geophysical surveys and sediment cores constrain past dynamics of the Amundsen Gulf ice stream of the northwest Laurentide Ice Sheet (LIS), Arctic Canada, during the last glacial episode. An ice-rafted debris (IRD) unit and its stratigraphic relationship to former grounding lines record the withdrawal of the ∼60,000 km2 marine-based ice stream. Calibrated (cal.) radiocarbon ages from the IRD unit and from ice-contact raised marine sediments indicate that the ice stream retreated ∼250 km over a few centuries or less during the early Younger Dryas, ca. 12.80 ± 0.15 cal. kyr B.P. Despite a likely cooler paleoclimate, ice-marginal recession occurred at rates of at least 1 km/yr, triggered by grounding line retreat from a bathymetric sill and by a concomitant increase in calving margin length. Sediment cores from the adjacent continental slope confirm that the IRD-rich unit was also deposited in the Beaufort Sea as a chronostratigraphic marker bed. Such new evidence for widespread ice rafting establishes Amundsen Gulf as a discrete source area for early Younger Dryas sediment in the Arctic Ocean, including Fram Strait (between Greenland and Svalbard). The timing and scale of iceberg discharges show that Amundsen Gulf was a major conduit for fresh water to the Arctic Ocean during the Pleistocene-Holocene transition.