The French explorer Samuel de Champlain was the first European to reach the mouth of the river in 1609. Already an important pathway for the Iroquois Indians, it soon became on for French traders. They built forts along its length: Fort Riechélieu at its mouth, Fort St. Louis (or Chambly), Fort Ste. Thérese and Fort Ste. Jean on the way, and Fort Ste. Anne on the Isle La Motte in Lake Champlain near its source. Some early journals and maps refer to the lower river as the Sorel River.