Dredging is so much easier than digging, but it still works as a very conservative lower bound. That's 3km a month, so a diversion of 6km would take at least 2

Dredging is so much easier than digging, but it still works as a very conservative lower bound. That's 3km a month, so a diversion of 6km would take at least 2 months. But earthworks of that size have to be planned and approved and the labor organized. It could never be done in that time frame.