I had 4 Black Terns from about 11:45 to 12:15 Saturday while standing at the end of the path you described. They were quite distant, upstream and close to the VA side. It took me 30min before I picked them up by looking through my scope. They eventually headed downstream (before heading upstream out of sight) where I got decent looks, but they still would have been too far to pick up with no scope.  

Here is a quick link to a map where my sighting took place. They stayed in this area for a while. It was basically in between the concrete structure on the VA side and the small island way upstream. 

http://tinyurl.com/nznj4s

Jared Fisher
Bethesda, MD