Leonardo, as Vasari wrote, 'was the first, while still a young man, to discuss the question of deviating the Arno to channel it from Pisa to Florence'. The deviation of the Arno and the digging of a canal was one of Leonardo's most grandiose projects. He worked on it for over 40 years, with various objectives: reorganising the water courses in Tuscany, from the Val di Chiana to the lakes of Fucecchio and Bientina, draining the marshes, irrigating the fields, preventing floods and hydro-geological imbalance, procuring energy and water resources for production activities, creating a great throughway of fluvial communication and building dams that could be utilised to disperse an enemy's army or to harass a city without having to use soldiers.

Accordingly, Leonardo studied two possibilities for making the Pistoia Canal flow into the Arno, and then on to the sea: through the Padule di Fucecchio marshes, or, further downstream, those of Bientina.

From his boyhood Leonardo had known the best viewing points for observing the territory involved. From the ridge of Montalbano, from Artimino to Monsummano, and in particular from the heights in the vicinity of the Abbey of San Giusto and the Hospital of Sant’Alluccio, he could study the winding course of the Arno from Florence to Fucecchio, the plain of Florence-Prato-Pistoia, Serravalle, the Val di Nievole and the Padule as far as Ponte a Cappiano.

During the periods spent in Florence, he studied the course of the Arno with its bridges in the section traversing the city and its uncontrolled course upstream and downstream, with the Bisarno, the breaks in the embankments, the mills and the sandbanks.

He decided to delineate geometrically, with the compass, a semicircular canal running from Florence to Serravalle, traversing Prato and Pistoia, thus anticipating the present-day route of the Firenze-Mare superhighway.

The new course of the river would not only be navigable, but would be only apparently longer, since it cut out the loops in the natural riverbed in the Arno Valley, and not only between Signa and Empoli.

In preparation for building the canal Leonardo studied excavating machines, designing a powerful innovative type.

He also studied systems for levelling off or tunnelling through the hill of Serravalle and for building locks that would raise or lower boats at the different altitudes along the river's course. He also designed a way in which, on different levels, the canal could intersect with other existing rivers in the plain of Prato and Pistoia.

The deviation of part of the water from the Arno into the Pistoia Canal diminished the flow in the natural course of the river, and kept the water in the navigable channel at a constant level.

In case of flood, the canal could serve as overflow ditch to keep Florence from being inundated. In times of draught the water reserves created upstream of Florence toward the Val di Chiana could be used to raise the level of the canal.

Furthermore, Leonardo planned to deviate the Arno into a canal between Cascina and Riglione, upstream of Pisa, toward the Livorno marshes. His purpose was that of depriving the city, at war with Florence, of the river and its resources, and thus forcing it to surrender. Work on this project began in August 1504 but the initiative was then abandoned: "The river laughed at he who tried to tame her" (Ludovico Antonio Muratori).