Good points, Paul - I should have thought of the obvious.

The only difference I can think of is that, when swimming in that lake or stream, the opening is, well, closed; when cleaning, maybe not so much, er, closed? (And I do remember a time, in the early 70's, when the Gulf was so polluted at Biloxi, Mississippi, that the Air Force allowed us on the beach, but made the water off limits. (Of course, that's not your typical wilderness lake, either.)

Awkward phrasing, I'll admit; and, again, I'm not informed enough to know how this physical process works.