[pavel_lishin]

I like that both of your suggestions include spending money and time.

You can flood a dungeon or build a new bridge, but both will take at minimum weeks, if not months.

If the King's daughter was kidnapped by those who live in the dungeon, or if the outcome of a battle hinges on you being able to cross that bridge over a chasm, they're not going to be happy when you come to them with a three-year plan for civic improvements.

[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.