I build my bridges by placing two core wood poles side by side, then moving them out parallel into the water.

With each span gets a pair of support poles, to clip under the protruding poles, I place a temporary floor board between the two support poles I've just placed. I place this floorboard under water so I can stand below/under my bridge looking outward. I then can snap the next segment from a low perspective. If the poles do not touch the river bottom, then I raise the river bottom with stone just enough to turn the pillars light blue (indicating they are in contact with the ground).

I've managed very huge spans across large rivers doing this. No mods.

I could demonstrate for you in game if it suits you.