Sea level is rising also I thought.
The Great Lakes are connected to the Atlantic Ocean aren’t they?
In the same way as any other lake is: through rivers. Superior is 600 ft asl, Huron and Michigan are ~580 ft, Erie is 577 ft, and Ontario is 480 or so.
After Ontario, there is the Saint Lawrence river that carries the water to the sea.
They spill into it quite spectacularly via Niagara Falls and more gradually via the St. Lawrence Seaway.