The weir is an artificial barrage on the river that made it possible to properly channel and convey water in order to exploit it to produce energy for hydraulic factories. The Santa Rosa weir was located in the area of Cascine Park (the other large weir of Florence was the one of San Niccolò, situated in the area of the homonymous gate). It took the water to a fulling mill outside the city. Its structure is still visible today.

The Arno’s complex characteristics have often been the object of studies aimed at solving, or at least checking, the problems periodically posed by its waters. In 1631, Galileo Galilei presented a report in the form of a letter to Grand Duke Ferdinando II, concerning the proposal of Sigismondo Coccapani to transform the Arno into a canal, making it navigable from its mouth until Florence, and freeing the countryside from the frequent floods. The idea was not new, as it had already been advanced in 1458 during the Republic of Florence. Moreover, in the times of Ferdinando I, Antonio da Sangallo had designed a possible canalization of the River Arno from Livorno to the swampland known as "Chiane" to the Tiber.

A few days before Galileo’s report, Coccapani had obtained the so-called "privilege", that is to say the license that authorised him to proceed with the work. Galileo’s approval would have permitted the architect to obtain the financial aid indispensable to get work started. Galileo expressed a positive evaluation of Coccapani’s project, though underlining the many years necessary for its realisation. However, for many reasons, first and foremost Galileo’s condemnation, the project was never carried out.

The River Arno was often taken as example in the studies on the velocity of river waters conducted by Galileo and Benedetto Castelli who, from Rome, also expressed his desire to return to Florence and to eat «with greater pleasure the little fish of the Arno than the sturgeons of the Tiber».