This river flows past the only walled city in the 
This river flows past the only walled city in the Americas north of Mexico. Another city on this river is home to a "Biodome" with replicas of four ecosystems, as well as an Olympic stadium nicknamed the "Big O." This river empties into a massive estuary that contains the Magdalen Islands and Anticosti Island after flowing past to the Gaspé Peninsula. This river flows through the Thousand Islands. The Ottawa River drains into this river. The opening of this river's eponymous Seaway made the Erie Canal completely obsolete. For 10 points, name this river that flows northeast from Lake Ontario through Montréal and Québec City before emptying into its namesake Gulf.
This river flows past the only walled city in the Americas north of Mexico.