i'm not so big on bounding imagination with 'reality', since it's fantasy anyway. to flesh it in if you want, of course massive scale for a flooded/shallow river more than a lake helps.. there could also be high mineral content in the water.

i have seen in certain undisclosed locations waterfalls and rivers form 'walled pools', almost exactly like a series of hot tubs squished together with 2-3' smooth walls built around them.. let's see if i can find a pic.. i expect one could conceive of a similar thing happening to a wide waterfall, forming a retaining lip.

ahh, a painting, even more anonymous..

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