A series of airgun single-channel seismic-reflection profiles from Thunder Bay and the adjacent Isle Royal Trough of Lake Superior suggest that water from Glacial Lake Agassiz did not catastrophically drain into Lake Superior at this locality, as has been previously suggested. Thunder Bay is underlain by a relatively smooth bedrock surface, the double bedrock sill at its mouth is not significantly incised, and no subaqueous fans occur at the base of the escarpment into the Superior basin. The Thunder Bay area contains none of the features observed further north in Lake Superior, which have been interpreted as flood-related from earlier seismic surveys. These previously reported northern features are interpreted as resulting from a younger (post-Marquette) catastrophic release of Lake Agassiz water. The absence of such features in the Thunder Bay area suggests that Lake Agassiz did not drain catastrophically through the bay at the beginning of the Younger Dryas cold interval.