Substituting lumber sizes for ground contact bridge sill and ease of handling for one person
I’ve posted several issues with other answers...I won’t repeat here. 

Recommendation:

I prefer the solid 8x8 member idea, but if too difficult for the OP, due to physically trying to move the members around, then laminated members laid FLAT and anchored with straps (as suggested by @Solar Mike) and anchor with straps or hooks over the top , rather than any bolted or nailed connectors. 

Having the members flat will help with the shifting of the bridge, due to movement (dynamic load) or water movement. 

I’d definitely have the straps or hooks installed over the top of the “beam” so it doesn’t float away and doesn’t move off the solid base.....but I’d avoid using bolts to hold everything down.