The Tiber is the third-longest river in Italy, rising in the Apennine Mountains in Emilia-Romagna and flowing 406 kilometres through Tuscany, Umbria and Lazio, where it is joined by the river Aniene, to the Tyrrhenian Sea, between Ostia and Fiumicino. It drains a basin estimated at 17,375 square kilometres. The river has achieved lasting fame as the main watercourse of the city of Rome, founded on its eastern banks.
The river rises at Mount Fumaiolo in central Italy and flows in a generally southerly direction past Perugia and Rome to meet the sea at Ostia. Popularly called flavus, in reference to the yellowish colour of its water, the Tiber has heavily advanced at the mouth by about 3 kilometres since Roman times, leaving the ancient port of Ostia Antica 6 kilometres inland. However, it does not form a proportional delta, owing to a strong north-flowing sea current close to the shore, to the steep shelving of the coast, and to slow tectonic subsidence.