The author is Leonardo da Vinci and the study dates back to 1504, a period during which Leonardo worked for the Florentine Republic (he was most probably invited to go to Florence by Niccolò Machiavelli , the same who will sign as Secretary of the Second Chancellery of the Florentine Republic, the painter’s tenancy agreement for the “Battle of Anghiari” which was supposed to be painted in Palazzo Vecchio). This study is related to the project of diversion of the river Arno: on the top center of the drawing we can read the following: “shifting the river Arno in a canal”. The canal that we see (always on the top of the drawing) was supposed to allow the Florentines not only to bypass the tortuous river course between Florence and Signa but also – and above all – to cut the city of Pisa out – with whom Florence was at war – from the course of the river Arno, depriving it of the of the river and its resources.
Pisa had gained its independence in 1494 with the descent of Charles VIII. Pisa surrendered again to Florence in 1509 and Niccolo Machiavelli himself had a leading role in the regaining of Pisa.
Regarding the project: the diversion of the river Arno to cut Pisa out of its course is specifically documented in a drawing inside the Madrid Code II, in some letters written in July 1503 and in a payment of the Signoria of Florence to Giovanni Piffero, also known as Giovanni Cellini (the father of Benvenuto Cellini). One of these states:
“Spent on cars with 6 horses and on food in order to go to Pisa with Leonardo da Vinci to level the river Arno and shift it from its bed“.
The project, however, will be abandoned almost in the bud (August 1504): as Emanuele Repetti reported in 1839, citing an older text, “in that same period  the Florentines tried nothing less than diverting the whole river Arno from Pisa to cause more devastation to its people. They dug two deep and wide canals near the tower of Fasiano, and 4 more canals above the city, the aim was to shift the course of the water and move it towards Coltano and Calambrone; to do so, they built a large dam on the river bed where 8000 workers were already engaged. All of a sudden, a river flood destroyed everything, and they had to abandon such a risky project. A letter the Commissioner Francesco sent to the Council of the Ten in Pisa, informed the war magistrate that he was there with Alessandro degli Albizzi, one of the Ten Magistrates of Balìa, the engineer Leonardo da Vinci, and other people including the governor. After studying and discussing the project, they came to the conclusion that it was an interesting work, and they had to decide whether it had to be shifted there or if it needed a canal able to prevent any attack from the enemies”.
For the transcription of these (and other) documents concerning Leonardo click HERE.