What are the full rules for aqueduct placement restrictions?

In Civ6, what are the full rules (including any undocumented ones) for where aqueducts can be placed?

See the below screenshot. It seems like I should be able to place an aqueduct between the city and the river/mountain. Ignore the tooltip which is for a different square.

It has all of the requirements: It is next to the city center, and it is next to a source of fresh water. Two, in fact, since if you look closely there's also a river behind the mountain. When I hover over the square the tooltip also acknowledges that that square is next to a river. And yet, I cannot build an aqueduct because no suitable locations for it can be found.

I saw some comments on Reddit about the same water source not being usable twice, and while I haven't verified that, it also isn't applicable in this case either.

1 Answer

Is that tile a floodplain? You can't place districts on floodplains (unless you're eqypt)

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