Lake Agassiz was the largest lake in North America during the last period of deglaciation; the lake extended over a total of 1.5 × 106 km2 before it drained at ca. 7.7 14C ka (8.4 cal. [calendar] ka). New computer reconstructions—controlled by beaches, isostatic rebound data, the margin of the Laurentide Ice Sheet, outlet elevations, and a digital elevation model (DEM) of modern topographic data—show how variable the size and depth of this lake were during its 4000 14C yr (5000 cal. yr) history. Abrupt reductions in lake level, ranging from 8 to 110 m, occurred on at least 18 occasions when new outlets were opened, reducing the extent of the lake and sending large outbursts of water to the oceans. Three of the largest outbursts correlate closely in time with the start of large δ18O excursions in the isotopic records of the Greenland ice cap, suggesting that those freshwaters may have had an impact on thermohaline circulation and, in turn, on climate.