Edit: It seems you already have found a solution :).
Cut the river into 3 parts then, part 1 and 3 being solid like now, part 2 not, which is the part where the bridge is overlapping.
At least that's how I would do it (without having seen your river and bridge images) :).
Maybe there is another option to make the bridge overruling the solid behavior of the river, but unfortunately I don't know it as I just started to use Construct2.