looks like the beams have come of their bearing point mate i dont think you can get it back on from the top. Like the lads are saying i think you will need a crane to lift it out if possible , you are gonna need to wait until the water level drops so you can see exactly the damage to the beams. How deep is the river , can you stand in it when the level drops if so maybe get in the river and put in some standards on oversize baseplate say 18 inchs square
you could tack weld them to the standards before you drop them in the river , then you could try and build a 13 foot tower in the centre to re-support it all. The problem is if it goes you wont stop it , last resort push the thing over . :0