@Dannyalcatraz
I really like aqueduct idea. Unfortunately, I think mischievous goblinoids and bandits would have a field day destroying it since there'd be no way to constantly guard an aqueduct 300 miles long.
Its essentially the same problem faced by oil producers in nations where "freedom fighters" are operating.
Which is to say that it is still feasible if the payoff is big enough. That aqueduct may be extremely long, but it may shave months off of the delivery time. And a shorter delivery route is easier to defend- they may attack the aqueduct, but they'd also go after the river transports, carts or even logs. Or even the quarry itself.
The solution would be regular patrols, extra heavy nearest the goblinoid tribes.
In all likelihood, you'd have a hit here and a raid there, but, for the most part, barring an actual uprising or horde invasion the aqueduct would be "safe" for most of its length.