The meandering courses of waters, which are caused by rebounds from the percussions made by the water into the banks, will deepen the bed beneath the river more than at any other part, and they strike at great depths in their percussions.1  And the water turning through2 these depths3 is cause for the hollowing and destruction of the banks that are struck.4

And it is clearly seen and recognized that the waters that strike the banks of rivers act like balls that strike against walls, which depart from those at angles equal to those of their percussions and go to strike against the opposite walls.  Thus, these waters, having made their percussion first against one bank, rebound to the opposite, and there they make a great percussion and hollowing,5 because there is a greater confluence6 of water in that place.  The reason is that the water, rebounding from one bank to the other, digs away at the bottom of the river,7 at the part that is beneath itself.  But the other water of the river, which cannot be received at this lowness, gets pushed and driven somewhat along the direction of the river;8 then, because flight is lacking in it,9 it returns to its natural course.  That is, since the bottom of the river is lower under the twisted ways made by the percussions of the waters mentioned previously, this second water,10 which has lost its accidental means of flight, resumes its natural course and falls into the low places of the river, striking the banks in the same place as the percussion of the rebounds noted before.  Since this bank is buffeted, then, by two different percussions,11 a greater concavity is caused there, because one strikes the bank up above12 and the other, of greater declination, erodes and undermines the bank at the base.  And this is the cause of the ruin and devastation13 of the banks, noted above.