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The Aqueduct is a district in Civilization VI, which provides early water infrastructure. It requires Engineering and must be placed next to the City Center on one side, and next to a River, Mountain, Oasis or Lake on the other side.
The Aqueduct is not a Specialty District, meaning that you could construct it in any city which has the right conditions without subtracting from your maximum district count, as Population dictates.
The Aqueduct's purpose is to provide fresh water to cities which don't have it. Thus, you have the liberty to found a city not right next to a source of fresh water (River, Lake, Oasis, etc.), but up to 2 tiles away from it. Later, you just construct an Aqueduct between the City Center and the fresh water source, and you will have your Housing bonus! Mountainous regions are ideal candidates for an Aqueduct-supplied city, because you can easily supply fresh water, even though you won't have it initially. Coastal regions are good candidates too, but you must make sure there is a fresh water source close enough.
For cities already supplied with fresh water, the Aqueduct is less useful. You will get a +2 Housing, but you risk losing a tile which you could put to a more productive use. Still, it could be worth it, under the right circumstances.
The remains of aqueducts – man-made watercourses – have been found scattered about ancient settlements around the world ... Egypt, India, Persia, Greece, Azteca, and especially across the once-Roman lands. Over 415 kilometers (about 258 miles) of aqueducts brought fresh water to the metropolis of Rome for drinking and bathing. These Roman aqueducts were marvels of engineering (considering the times) and often roofed, so also serving as bridges where they crossed ravines and waterways. Although there were some health issues involved in the design of aqueducts (notably the sometime use of lead to line them), in general a supply of relatively-clean water was a boon to any town hoping to grow into a city.