The Great Aqueduct runs from the Firetip Mountains to the Village of Midhaf. The source of the water is ice melt from the highest peaks of the firetips, where the water fills a series of three massive basins, and then is moved via the power of windmills (and likely a bit of magic) down into the pipes and channels that eventually become the aqueduct.
From the foothills of the Firetips, the aqueduct runs 200 miles to the village of Midhaf, where it empies into a cistern near the ancient dried-up well atop the Shrine of Pelor. The cistern feeds into an ancient but efficient series of pipes and culverts which feeds water for irrigation to the patchwork of farms that supply food to both Midhaf and via trade to the Dwarves of the Firetip Caverns.
Architecturally, the aqueduct is largely unremarkable, except in that it is remarkably long, and oddly, quite flat. It is assumed that some ancient magic assists in driving the water forward along its path and helps to maintain the waters purity. In cross-section, the aqueduct is 30 feet wide, and runs about 50 feet above the largely flat and barren plains. There are cisterns built into the supporting structure every mile or so, which provide water to passing travelers, which is a key reason that the Midhaf to Firetips trade road (more like a well-worn path) runs quite close and parallel to it. Every 4th or 5th major support structure has a narrow winding stair leading to the top of the aqueduct, presumably for maintenance, although until the water stopped it had never needed any.