There's a tradeoff between keeping it small and letting it float up a little vs. larger and STILL maybe having it move up. Think the only thing that wouldn't move would be to build it out of stuff all heavier than water. THEN, I might have trouble with erosion on the ends. This is the problem I encountered at the previous location when I anchored the ends in pretty solid. Ended up with erosion at both end of the bridge.
Tough to know how high to build it. Isabel submerged everything down there under 6' of water. Strangely, enough, the present bridge wasn't moved, or it moved and sat down the same spot.
Tried just putting rocks and bricks in the stream. These get covered with mud and slime very quickly.
Ralph