Very navigable and useful, but in ways different from today.

The St. Lawrence had a number of areas that were impassable to ships, and Niagara falls is a pretty obvious problem for shipping from lake Ontario to the upper lakes, so in short, they took everything out of their boats and went around obstacles. Small scale, this process of portaging involved taking your canoe and its contents out of the water and physically carrying it all to a place where one can place it back in the water safely, usually along the shortest possible land route. Big picture, people got smart and would construct ships in each body of water to move stuff.

So using Niagara falls as the example, early on to get from lake Ontario to lake Erie you'd row your canoe to the "lower landing" of the Niagara river in modern day Lewiston NY, pull out all your stuff, and carry it up the Niagara escarpment, which the French referred to as the "walk on four legs" because that hill is damn steep. Once at the top you've got a roughly 12 mile walk that roughly follows the route of portage road in Niagara falls (creative name amirite?) Before arriving at the upper landing and getting back in the water to continue out to lake Erie.

This system would be refined and improved over the years. A native community relocated to the top of the Niagara escarpment so they could charge Europeans to carry stuff up the hill, a road was made from the top of the escarpment to the upper landing, forts were built all along the portage route (the largest, fort Niagara defending the mouth of the Niagara river) and eventually the British even tried adding a whacky system of hand cranked elevators to facilitate getting stuff up the escarpment. They also figured out it's way easier to have a larger ship move a ton of goods to Fort Niagara, transport them a few miles to the lower landing by canoe, carry everything up the hill, then put it back on another large ship than it was to personally carry your canoe up the hill.

I leaned heavily on Niagara as a case study because the Old Fort Niagara Association has published tons of information on exactly how that portage route was operated, but there are thousands of portages large and small across the lakes and rivers. The need to be able to carry ones boat out of the water easily is part of the reason Native people in the lakes relied so much more heavily on birchbark canoes than their more southerly counterparts did.

For reading material check out "Rethinking the Fur trade" I think the authors name is sleeper smith. Also "the voyageur" by Grace Ann Nute (this one is slightly dated but she does a better job of discussing the nuts and bolts of how stuff works than most). Finally, Old Fort Niagara's publications touch on this stuff as well.