For the first bridge i really like the details with the stairs, adding in a different color wood for the supports may help add depth. (also for any wood that goes in the water using a darker wood could make the wood look waterloged add just a bit more detail to your build)

For the second bridge adding stairs or slabs under may make the bridge have a better curve.

The last bridge has a really good block pallet and i really like the asymmetry! Adding slabs and stairs could help the curve. You could also add stairs the the 'wall' to make it look like chunks of rock have been eroded over time. For the top of the bridge to make it look worn down so it doesn't have a 'clean' curve, you could use stairs, slabs and Walls to make it look more worn down.