And then there's where a viaduct becomes a bridge! Queen Ethelfleda viaduct/Runcorn (Railway) Bridge. Majestic in its own right, but never in anyone's lists sadly.
Also, I'd say possibly the best definition would be a viaduct joins 2 locations of similar heights, bridges go up and over something. I.e. a bridge goes from land, gains altitude to traverse say a river, and back down the other side.
A viaduct would be between 2 hills either side of said river.
This is why in my selection, the QE viaduct runs from just outside Runcorn station (at height) until it becomes Runcorn bridge where it goes across the Mersey and then down to land level.