How long do you think it would take to build the Mander-Blackwater canal?

To take a similar example in terms of technological levels and length of canal, the 17th century Canal du Midi in France, which is 150 miles long, took 12,000 men 15 years to build. 

That seems like a lot of workers, but that’s about 1.3% of the Reach’s workforce. And given that 90% of the Reach’s population is essentially unemployed during the winters, you could probably cut down on construction time considerably by increasing your workforce above the historical parallel.