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I wrote red soil, but more specifically, where I lived there was Terra Roxa (purple soil?), which seems to be a kind of red soil according to the English Wikipedia page: https://en.m.wikipedia.org/wiki/Terra_roxa

And it is the prevalent soil on the north of the state of Paraná, regarded as Brazil’s agricultural barn: https://en.m.wikipedia.org/wiki/Paraná_(state)

So it does confuse me that the state’s soil would be unfertile, as I grew up learning how good it was and surrounded by prosperous farms.

The Portuguese Wikipedia page does talk about it being fertile (no English translation): https://pt.m.wikipedia.org/wiki/Terra_roxa

So maybe it isn’t a type of red soil in the end; or there are some types of red soil that are (very) fertile.

Brazilian here. Perfectly safe (color-wise; of course it can be polluted as hell despite its color, just like any other river).

Our ground/mud has a different color. Some areas on the south even have a red soil (very fertile, but makes everything about ground level look dirty very quickly): https://en.m.wikipedia.org/wiki/Red_soil

There’s great variety of water colors even in the same area, just search for images “meeting of the waters Manaus”: