Oh, when you call the barrier - they give you a span to pass through. Travelling upstream, spans are designated from Left to right with A (alpha) being far left and not navigable. B (bravo) 2nd from left etc etc. Upto I think K span.
There is also designation as to which span of each bridge to pass under. Two amber lights indicate which span to use - three red lights closed.
A tall guy I used to know ran a useful website with all this info on it but it's long since disappeared unfortunately.