Harbors generally are freshwater, and boundaries between fresh and sea water are innumerable, if you stop and think about it for a few seconds. If this was a real problem it would have contaminated Lake Michigan and the Hudson Valley by the same means.

The reason it hasn't is that water generally doesn't flow uphill.

I guess 32.7 meters above sea level is reasonably uphill.

I'm too lazy to calculate the approximate volume of fresh water that represents, based on the lake's surface area. Pretty safe to say that the rest of the lake's volume, at and below sea level wouldn't be rendered brackish too easily.

Although, given China's penchant for environmental accidents, I wouldn't put it past them, if all that additional lake water above sea level were to get skimmed off the top by an accidental spill into the ocean.

For comparison, the Great Lakes are about 60 m above sea level (Kingston, at the junction of Lake Ontario and the St Lawrence, is at 200 ft). Salt water inundation has not been a problem. However, invasive species (zebra mussels, for example) have been. They are fresh water species that get sucked up into ballast water in other ports (many of which are on rivers or estuaries) and pumped out in other destinations. There are procedures to avoid this (exchanging fresh-water ballast for salt water while at sea, for example) but those procedures are inevitably implemented by humans, who are certain to eventually make a mistake sufficient to cause contamination.

Meh, China may have Three Gorges, but Western corps have plenty of environmental atrocities as well. They can pretty much operate with impunity, even on US soil. Bhopal, Prince Edward Sound, Deepwater Horizon, Niger Delta, fracking, etc etc. I think the only constant is money.

Not hugely likely, the lake is higher than the sea so the salt won't migrate up. The water level in the lake will likely be affected though.

The danger is ballast water held by ships, but like Panama they'll likely forbid all water discharge while in the canal.

I am sure accidents happen. So some contamination will occur regardless.

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.