Just because there's a height difference with locks, doesn't mean some kind of singularity would develop without them. Either the height difference would disappear, or continuous currents through the canal would maintain it.

In the Panama canal, they benefit from keeping the inland lakes above sea level so that they're deep enough. If they were connected to the sea, they'd lower significantly and even more excavation would be needed to make a path for ships.

There would also be fascinating effects of strong currents through it, sea level changes, fresh water lakes becoming salt water (sorry local people!), and fish migrating.