Make the bridge the image in the event, meaning if it's 2000 or 2003, it has to be on the top layer in the chipset, and you'll have to make multiple events.
Set the event as Over Hero, and after you pass the bridge, make a switch that turns ON. Make in the event when the switch is on, that the event is Below hero, meaning you'll have to do two pages.
Catch what I mean? You can manipulate it like this. I'm sure you're smart enough to figure out esactly how to do it, unless you're completely new, in that case I'll post it step for step, but it isn't that hard really.