From what I understand, in areas adjacent to the ocean, slough means something like salt water lagoons next to the ocean, and may be pronounced differently.

We've got a slew of sloughs here in the San Francisco Bay area, and they're pronounced just as you folks in the Great White North pronounce them. Out here, by the Big Water, a slough is a a creek running through a marshland or a tidal flat. In fact, we might use the term to describe the creek so beautifully described by Seņorita de Troy the other day (I didn't comment on it in that thread, Helen, but it was gorgeous).

Note: I just clicked on the link provided by Dr. Bill, and I see that, at no charge whatsoever to you loyal readers, I have explained more than was strictly necessary.