A recent update of a certain flight simulator made me revisit this chart and I had not noticed the lines of triangles (at the Chain Bridge / Roosevelt Bridge etc, but not at the Key Bridge) in a past version, years ago. What are they? I couldn't find the information in the FAA Chart User's Guide.
So, applying some local knowledge, the Key Bridge is a high arch bridge in that it is a good distance above the water level, and the other ones listed, Rochambeau, Memorial , Roosevelt, and Chain Bridge are all "low" bridges, more like causeways.
I tried looking for a chart legend that said this, but I wasn't able to find one.