A schematic map of the Arno valley, on a scale of about 1:200,000, showing the route of a proposed canal to bypass the Arno between Florence and Fucecchio or Vico Pisano (two possible routes are shown at that point). Some of the towns are labelled; the map is finely pricked for transfer along its major outlines. 

The Arno is not navigable as far as Florence, due to rapids in the river ten miles west of Florence. Leonardo's proposed route for a navigable canal passes north from Florence (at far right), via Prato and Pistoia, through a deep cut in the mountain pass at Serravalle, and then through two marshy lakes before rejoining the Arno near Pisa. Leonardo first sketched this route c.1490, when he was still in Milan (on a sheet in the Codex Atlanticus, fol. 46r-b); around the same time he proposed canalising the Arno either side of Florence (CA fol. 289r-e). But there is no evidence that his plan for a bypass canal was ever seriously considered by the Florentine government.

A second, unrelated scheme seems to be indicated at the left of the map. The Arno is not inked beyond the dark circle that represents Pisa, then under siege by Florence. Leonardo visited the Florentine camp in July 1503 and, with the support of Niccolò Machiavelli, proposed a scheme to divert the Arno and thus cut Pisa’s access to the sea. Diversion channels seem to be drawn here as rough diagonal lines from the Arno south-west towards Livorno. As recorded in Landucci's Diario, digging of a channel actually began on 22 August 1504, but was abandoned on 26 October.

Leonardo's notes:
Upper centre: 'cor[so dac]que torbida e / adûque [l]acqua.darno nô uale.'
To the left: 'Il minor fiume che mette nel magore / isscarica nella perchussione dj quella il suo ca/richo e cosi spigne la sua argine inver/rso il mezo del fiume magore ella [ôdadjtio/ne] perchussione desso fiume mjnore fa piegare / lacque del magore alloposita riua e cosi la / piega e consuma'
Top left: [Richter 1006] +++
Bottom centre: 'da firenze a fucecchio migla 40 per acq^a darno [and a calculation]
To the left of that, a scale of miles.
Bottom left: [Richter 1006 +++]