I spoke before I looked.  I stand corrected.

But, it's still a good way to build semi-realistic looking bridges with ease and flexibility.

Actually, it's not even a meshed sidepiece, and those bridges are easy to create.

Build a road that goes directly through your river to the other side.  Then, click the Grading Road Tool button, click the Flatten Road button on the right side, then click on the road that runs through your river.  Be sure to click on a part of the road that is on the bank of the river and not underwater, as it will flatten the piece of road to the elevation you clicked on.

Then, you'll have a stretch of land that connects both sides of the river, with the road on it.  Use the terrain paint tool to paint that stretch of land rocky or dark sand or whatever to provide the texture of the "bridge."

The good thing about these bridges is that they don't require any snapping or precise terraforming.  Just build a road, flatten it, and paint under it.  These "bridges" can be as long or as short as you need them to be, with no pieces or game objects required.

You can also use this technique to build curved bridges (just remember to lay out your road underwater exactly how you want it before you flatten it to avoid unnecessary terraform detailing later).