Ostia was founded along the Tiber river and close to the sea. Each of these strategic conditions was lost over the centuries: the position near the sea through the gradual growth of the dune belt (see www.ostia-foundation.org/coastline/), and the position along the Tiber on the 15th of September 1557, as the result of an exceptionally high flood, which shortened the river course by cutting off the narrow meander. On this occasion the castle of Ostia, constructed only 60 years earlier by pope Julius II, became suddenly isolated from the Tiber.
The remains of the cut-off river curve is called the Dead River (Fiume Morto) and can still be seen on an aerial photograph of 1911, and – if you know where to look for it – even on modern Google Earth images. The event was somewhat predictable, given the sharp curve of the river course near the castle in the decades before the catastrophe.
Until the end of the 19th century the remains were still visible in the landscape as a small lake. It was later filled in with materials from the excavations. Nowadays it requires some imagination to envision in the grassy fields one of the major rivers of Italy flowing alongside the Castle and the excavations of Ostia.