Previously I have asked about my desert world Skara Could canals solve H G Wells Martians water shortage problems? To recap: For current purposes such a world would have gravity and atmosphere suitable for human life, but would have limited supplies of water most of which would be locked in the ice caps. Each ice cap would be surrounded by a zone with some water but which was too cold for agriculture. North and south of the equator would be a warm but very dry desert zone where it was warm enough for agriculture but without enough water.
Background information: Assume that water was originally more widely distributed, but the planets orbit was disturbed by the passage of a brown dwarf star through or near to its parent star causing a change in its orbit. Although the change in orbit had catastrophic effects, civilization was able to survive, but is faced with the problem that the limited water available is increasingly being frozen out at the poles generating the need to ship water from cold wet regions to warm dry ones on a planetary scale.
I have now decided to base this world in a pre steam technology era, but unfortunately I don’t believe that huge canals (multiple canals 200m x 10m deep running for thousands of miles) would work in this era. One possibility would be to allow magic, but I’m very wary of magic as anything could be possible and it could easily over power the scenario causing unwanted effects.
So how can I arrange for some form of magic to make thousands of miles of big canals possible, whilst preventing it from dominating and distorting the rest of feudal society that lives there? It should still take hundreds of years to build the canals. But I don’t want that much ease to change for example no magical castles easily created in the middle of the desert or easy open pit mining.