The three rivers estuaries are all 60° apart, coinciding with the convection rolls system. A river flows on the surface, so how does it connect with the mantle? The answar is that water is the best geologist. It always finds the weakest spots, wherever they are, in order to make a path to flow along. The annual isostatic fluctuation at these estuaries is of global geophysical scale, especially for Amazon with 8 cm vertical movement of the crust every year.
The convection rolls exist because heat must constantly leave the inner part of the Earth. It has been shown with experiments that mantle material heated so that it starts convecting, and not made accelerating any further than that, does form convection rolls. It is logical that exactly those conditions prevail inside the Earth. These rolls have to extend from equator and sway in a regular way. The weather system and the ocean current system form curved paths as they move along, and so does the mantle. The theory fits everywhere, no one can deny it. And here we have one more manefistation. The three estuaries are all 60° apart as compared with the relevant central point on the 32nd parallel, of a circular path revealing the division lines between the main convection rolls of the lower mantle. It is even so that Mississippi and Nile are very clearly 120° apart, because the estuaries happen to be very close to the critical 32nd parallel, one flowing from the north, the other from the south.
In the drawing, the main upwelling lines of large scale lower mantle rolls are shown, and the relevant backflow lines of horizontal flow. These black lines are only relevant for the upper mantle, and in a way also for the core. The effect of those lines is so big that they are responsible for the location of all those three rivers. Newly, BBC made a TV series about exactly these three rivers. They are very special.