[ceequof]

Gatún Lake of the Panama Canal is also freshwater, at 26m elevation.

Generally, the big risk with elevated freshwater lakes connected to the sea by locks isn't saltwater contamination, but the disastrous flood that would happen if all the lock gates were opened at the same time. A lot of work was put into the safety measures, most of which have been removed due to cost in recent decades: https://en.wikipedia.org/wiki/Panama_Canal_locks#Safety_feat...

If a flight of locks was completely opened, Gatún would promptly drain out of them, wiping out everything downstream. You could imagine the entire canal would be offline for months to years as infrastructure was rebuilt and the lake was refilled by rainwater.