Santerno

Santerno
The Santerno river
Country	Italy
Basin features
Main source	Tuscan Apennines, Italy
River mouth	Reno River
Basin size	about 700 km2 (270 sq mi)
Physical characteristics
Length	103 km (64 mi)
Discharge	Average rate: 16 m3/s (570 cu ft/s)

The Santerno is a river in Romagna in northern Italy. It is the major tributary of the Reno River. In Roman times, it was known as the Vatrenus (small Renus), although, in the Tabula Peutingeriana, it was already identified as the Santernus.

It rises near the Futa Pass, at 1,222 metres (4,009 ft) of elevation, in the Apennine ridges facing the plateau of Firenzuola in the province of Florence. Beyond Firenzuola, it flows northeast through the province of Bologna near Castel del Rio, where it is crossed by a famous medieval bridge, the Ponte degli Alidosi. It then flows past Fontanelice, Borgo Tossignano, Casalfiumanese, and, once in the Pianura Padana (the Po River's valley), Imola. The river forms the border between the province of Bologna and the province of Ravenna for a distance before entering the province of Ravenna. It then empties into the Reno, 7 kilometres (4 mi) from Bastia.

It is probable that, in ancient times, the river flowed eastward from Bagnacavallo, as confirmed be the existence of a frazione of Ravenna called Santerno. In those days, after Imola, the river divided into two branches, one moving northeast toward Ravenna, the other continuing northward to join the Po. The Ravenna branch disappeared, most likely in the eighth century.