Final answer:
The Ottawa River is the river that defines the border between Ontario and Quebec, flowing into the Saint Lawrence River. It has historical significance for French Canada and contributes to the region's agriculture and trade.
Step-by-step explanation:
The river that flows along the border between Ontario and Quebec and empties into the Saint Lawrence is the Ottawa River. This river is a significant body of water that has played a historical role in the development of French Canada.
The Ottawa River acts as a natural boundary between the two provinces and is part of the watershed that drains into the Saint Lawrence River, which is a vital artery for the region's agriculture, commerce, and transportation.
Notably, it was through the St. Lawrence River and its tributaries that the French and later the British established settlements and conducted trade, contributing to the bilingual cultural landscape seen in areas like Ottawa, where bilingual stop signs are evidence of this historic divide.