river of life

In the box with the Mercury Mysteries was a 1976 edition of The How and Why Wonder Book of The Human Body published by Grosset and Dunlap. In case you have concerns about the plate above, rest assured, it was approved of by Oakes A. White of the Brooklyn Children's Museum. Now there's an arborial name.

So I'm not sure I understand how this diagram works but it looks like the midpoint where all the blue turns to red is something like a jubilee zone. A magnification would certainly reveal drunk fish flipping in the currents.