Dig to bedrock or drive in pilings to shore up your foundation if you can, it all depends on the local geography. Given the lateral forces on a bridge it seems

Dig to bedrock or drive in pilings to shore up your foundation if you can, it all depends on the local geography. Given the lateral forces on a bridge it seems unlikely that modern builders would float the footings of the arches in the riverbed, but in the case of this bridge I think that was what happened (a guess given the fact that three arches failed during a flood in 1890s when a pile-up of logs and debris forced a lot of water flow to push deeper to pass the bridge.)