What problems am I going to have to address with regards to man-made continent wide/spanning rivers ? Ecological hazard, erosion, pollution ?
CLARIFICATION : This is a completely fantastical setting, magic taking the place of technology for the most part. My purpose is twofold. 1) a steady water supply independent of rain/building aquaduct structures/piping (instead of going out looking for water you make the water come to you); 2) a faster cheaper (something something magic) means of transportation (for example even transporting food to the most remote and desolate places of this mind numbingly / dumb Empire. Those places which would in turn send things back in exchange, things that they specialize in making/harvesting)
CONTEXT (Copypasta of my response to @AtorGade' answer) : The setting is completely wacky. This is very a intentional decision on my behalf, it's just the kind of silly atmosphere I'm looking for. My fictional empire would be an-all encompassing entity, impossibly big in size and scope even if most of it is going to be uninhabited (Forest...etc). The empire and it's governing bodies are going to have a great role, especially with regards to land management. Strategically placing of cities, villages and infrastructure, taking full account of variables such as terrain. (Canals would only be used in flattest regions.)