strictly a bridge is "a
structure that allows people or vehicles to cross an obstacle such as a
river or canal or railway etc"

The bridge I have in mind was built in 2004. It is not over an inlet,
outlet, creek or dock, but is over the main flow of the Thames.

So it allows people or vehicles to cross an obstacle, and it is over
the main flow of the Thames.
It clearly doesn't allow them to cross the Thames, since both its ends
are on the same side, so it must allow people or vehicles to cross
something else.

My guess is there is some section of the Thames Path which is
cantilevered over the river to get round something.