In the hamlet of the same name of San Benedetto Val di Sambro, the small lake of Castel dell'Alpi has the Savena torrent as both its tributary and emissary. Unlike the nearby lakes of Suviana, Brasimone and Santa Maria, this one was formed naturally.
Castel dell'Alpi lake is the only natural basin in the province of Bologna. It was formed in 1951 following a landslide which blocked the course of the Savena stream, partially destroying the village of Castel dell'Alpi, now rebuilt on the right bank of the lake.