[Maine Mariner]

Excellent Observations about the locks at the Welland Canal and Sault Saint Marie, Michigan. There is one further problem. Depth of the lakes and channels. Ships using the Great Lakes whether lake ore boats carrying iron ore or other bulk products and ocean going ships can load to a draft of about 25 feet. About half the draft for ships that call on ocean ports.

[Eric in the Ozarks]

Crude oil was transported by lakes-going tankers from Superior to refineries in the US and Canada up till the early 1950s when the Interprovincial pipe was laid east of Superior. If there was a spill during the early years, it went unreported.
My old time tanker captain remembered the tankers at anchor, waiting their turn in Superior Harbor. I bought heavy oil several times a year in 50,000 bbl tankers from Sarnia, up to my tanks at Superior and Taconite Harbor to fuel steamships.

[Eric in the Ozarks]

I don't know if you saw this development but Algoma Company of Canada has built a new fleet of lakers that are just narrow enough to transit the Welland, yet can call in Duluth-Superior for iron ore. They boats WERE BUILT IN CHINA and have stack scrubbers, allowing the use of heavy bunker. Algoma claims they're 35 percent more efficient than the 1,000 footers now in use on the American side.
Look up “Algoma Equinox.”

[Alberta's Child]

I don’t know if it’s true anymore, but I believe the lock system at Sault Ste. Marie could accommodate larger ships than the Welland Canal further downstream. This meant that many of the ships used in Great Lakes shipping (ore carriers, for example) were basically “captive” to the Great Lakes.

[Colonel_Flagg]

Ships using the Great Lakes whether lake ore boats carrying iron ore or other bulk products and ocean going ships can load to a draft of about 25 feet. About half the draft for ships that call on ocean ports.

Duluth harbor, adjacent to the Superior Entry, routinely handles 1,000-foot vessels that draw 17 meters (56 feet). However, as has been noted elsewhere, those ships can't fit through Welland Canal so they are confined to lake duty. It is also the farthest inland freshwater port in the world and IIRC, also the largest.