[Tagging] Waterway direction

[SomeoneElse]

  On 01/09/2010 20:53, Anthony wrote:
>
> Can you pardon my laziness and tell me whether or not the water in
> that part of the canal fills the entire tunnel.

It doesn't - there's a narrow footpath to the side (presumably where 
boats were pulled along manually - the horses had to take the path over 
the top of the hill I think).

However - imagine that such a waterway were being built now and there 
wasn't a requirement for any sort of towpath.  If it were still 
navigable but you could still get a boat down it would it still be a 
canal?  I'd say yes, but others may differ.

Another case - imagine a stream (that runs overground but then dips 
beneath and re-emerges.  Here's one example I think:

http://www.openstreetmap.org/?lat=54.07919&lon=-2.16045&zoom=15&layers=M

If I remember correctly (although it's a few years since I've been 
there) the stream that flows from Malham Tarn at the top goes down a 
hole and re-emerges at Malham Cove at the bottom of the screen.

The "water sinks" node is here - 
http://www.openstreetmap.org/browse/node/12792309 tagged as a 
"cave_of_debouchement" (which is a new one on me but entirely 
plausible).  In this case the underground bit isn't mapped but if it 
was, would it still be a waterway?  I suspect that it'd be down to the 
"duck test" and the mapper on the ground.