[bryanlarsen]

The Chicago - Great Lakes connection is man made, so doesn't count per the methodology of the OP.

[Kon-Peki]

The permanently navigable connection is man made.

It was seasonally connected (and navigable by canoe), requiring a very short portage the rest of the year.

[Nevermark]

So North and South America are not the North and South Panama Canal Islands. Got it!