A lot of timber piles would be driven into the river bed by pulling up and letting go of a giant boulder onto the top of the pile. Once the timber piles were set, they could then build a wooden bridge deck on top.

It wasn't a permanent bridge and the Romans would usually destroy their bridge after they no longer needed it.

[–] PugJesus@lemmy.world 2 points 1 week ago

It wasn’t a permanent bridge and the Romans would usually destroy their bridge after they no longer needed it.

Minor note: on campaigns in permanently hostile territory, yes, to deny the enemy a means of retaliation, but many wooden bridges served for hundreds of years in conquered territory after being constructed. Bridges were a great way of controlling and monitoring civilian traffic, after all!