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.
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.
The slope of the river bed diminishes until the plain compels the river to move slowly, swinging in meanders proportioned to its size, and gradually, controlled by the flattening land, ceasing to transport material, but raising its banks and silting up its bed by the dropped sediment, until, split up and shoaled, its distributaries struggle across its delta to the sea.
Here, in fact, lay some of the oldest and wealthiest towns, the sites of which have, however, been removed inland by the silting up of the shore.
