How deep can the river be at that point? Not very, I’d think. Barely enough to conceal a submerged sub. Probably barely enough to float a sub running on the surface.
And just think of what mines could do in such a river.
I read "Iron Coffins" years ago, and I'm pretty sure the author mined the Chesapeake Bay on one mission.
I'd bet mining Canadian ports in the early days of WW II was a priority item.