When the fulness of rivers is diminished, then the acute angles formed at the junction of their branches become shorter at the sides and wider at the point; like the current a n and the current d n, which unite in n when the river is at its greatest fulness. I say, that when it is in this condition if, before the fullest time, d n was lower than a n, at the time of fulness d n will be full of sand and mud. When the water d n falls, it will carry away the mud and remain with a lower bottom, and the channel a n finding itself the higher, will fling its waters into the lower, d n, and will wash away all the point of the sand-spit b n c, and thus the angle a c d will remain larger than the angle a n d and the sides shorter, as I said before.
[Footnote: Above the first sketch we find, in the original, this note: “Sopra il pote rubaconte alla torricella”; and by the second, which represents a pier of a bridge, “Sotto l’ospedal del ceppo.”]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.