The difficulties of the harbour were increased by the continued silting up, produced by the enormous amount of solid material brought down by the river.
5, p. 231) the harbour of Ostia had become dangerous: he speaks of it as a "city without a harbour owing to the silting up brought about by the Tiber.
103, owing to the silting up of the Claudian harbour, and the increase of trade, to construct another port further inland - a hexagonal basin enclosing an area of 97 acres with enormous warehouses - communicating with the harbour of Claudius and with the Tiber by means of the channel already constructed by Claudius, this channel being prolonged so as to give also direct access to the sea.
In the middle ages Ostia regained something of its importance, owing to the silting up of the right arm of the Tiber.
About 1395 this channel began to show signs of silting up, and during the next hundred years the process proved rapid.