When and how did the Champlain Sea basin open to oceanic waters? Analysis of glacial and marine data seems to point to progressive expansion of the west arm of the Goldthwait Sea up the St. Lawrence to the vicinity of Quebec, between about 12.8 and 12.4ka, with the first or Charlesbourg Phase of the Champlain Sea, forming in the Quebec region, about 12.4ka. 
At this same time lakes were developing and extending northward at the southern margin of ice that occupied the main Champlain Sea Basin, but a barrier created by the Sainte-Maurice lobe on the north shore and by an ice remnant in the Bois-Francs region on the south separated the marin from the lacustrine part of the basin until about 12ka.
The areal extent of the Champlain Sea during the Charlesbourg Phase cannot be defined at this time.(Parent & Occhietti, 1988) The opening of the Charlsebourg basin to seawater and the drainage of Lake Beauce possibly occurred around 12.4ka but no later than about 12.2ka. Drainage of Lake Beauce marks the beginning of the Charlesbourg Phase, which was restricted to the easternmost margin of the marine basin. The ice front subsequently retreated northward to the vicinity of Quebec. The extent of the Charlesbourg Phase around 12ka is not known. Based on the dates of 11.6ka at Notre-Dame-des-Laurentides, 11.1ka at Beauport and 11.2ka at Lapointe, it is possible that the edge of the Laurentian Highlands was not deglaciated during the Charlesbourg Phase.