[pclmulqdq]

Just use a little bit of magic or a different solution if you want things to go faster. I trust a set of clever players to be able to come up with something to totally fuck up the DM's plans.

Also note that Caesar's crossing of the Rhine (a notoriously tough river to bridge) took 10 days, and several similarly amazing feats of quick river diversion have occurred in Chinese history. All without magic. If you don't care whether your works are permanent, the work can be done very quickly.