Fluctuations in the level of Lake Agassiz are dated at two sites in northwestern Ontario. A radiocarbon date on a modern shell sample indicates dates on freshwater molluscs from the area are about 440 ± 100 years (GSC-3281) too old due to the hard-water effect. An adjustment of 400 years to two fossil freshwater mollusc dates of 11 400 ± 410 (GSC-3114) and 10 400 ± 100 years BP (GSC-2968) makes them compatible with radiocarbon dates on wood from deposits in other parts of the Lake Agassiz basin. The two new dates indicate the beginning of the low-water Moorhead Phase of Lake Agassiz started about 11 000 years BP. The high-water Emerson Phase started when the water level rose to form the Upper Campbell beach approximately 10 000 years BP. The red clay widely distributed throughout northwestern Ontario was deposited during the Emerson Phase when the ice margin lay along the Hartman, Dog Lake, and Marks moraines.