I looked at it for a good half an hour.  It looks like a swamp.  I suspect at some point they likely added the ditch to improve drainage and to keep it from overflowing into the neighborhood. 

So I cut out some of the MP and made it a swamp.  I added some streams.  There is so little elevation change it's just a best guess.  I just thought it looked too weird to not have something in that section.  I also very tempted to remove the stream and add some kind of different wetland there instead.  A marsh? A wet_meadow? 

I think this is just one of those cases where the natural world doesn't like fitting into our categories.