Water law in medieval Lombardy

"Diploma" of Frederick Barbarossa granting navigation rights to the monastery of
San Carpoforo di Como (1159)

I recently came across Acque della Lombardia Medievale, apparently the catalog for an exhibition held by the Biblioteca Ambrosiana di Milano in 2015. (If you're ever in Milan, don't miss the associated Pinacoteca Ambrosiana.) The editors, Federico Gallo and Rita Pezzola, write:

According to Roman law, one defines every permanent water-course as "publicum" whether it was navigable or not, and only rivulets and streams were considered private. During the Middle Ages and in particular in the 10th and 11th centuries, we find more and more imperial and royal diplomas giving grants and donations related to stretches of rovers: they refer to the construction of ports and mills and to fishing and navigating rights. Thus the principle, or better the custom, of considering some parts of a river as capitalized (today we would say "privatized") was established, and more and more we find that the water-course was at the disposition - more or less in their possession - of the people who owned the adjacent land. During the Dieta di Roncaglia (Piacenza) in 1158, Federico I, called Barbarossa - assisted by lawyers from the school of Bologna - redefined the legal status of rivers. Referring to Roman law, the Emperor inserted the "flumina navigabilia" in the regalie (royal prerogatives), so that the "flumen publicum" no longer referred to permanent water-courses, but only to the navigable ones.

For a similar development in 19th-century Canadian water law, see here.