The Tirso () is the most important river of Sardinia for its length and the size of its drainage basin. It starts on the mountains of the centre of the island and ends in the Mediterranean Sea.
Name
The river was known in Sardinia as riu de Aristanis ("river of Oristano") or, simply, riu Mannu (the "big river"). The name Tirso began to be used in modern times following Ptolemy that named it in ancient times as "Thyrsos" (, "mouth of the Tirso river").
Geography
The Tirso river has a length of , and a drainage basin with an area of approximately .
Its discharge (volume of water which passes through a section of the river per unit of time) depends of the season; its average discharge is but is very low during summer.
Course
The Tirso river starts in the Buddusò plateau, on the slopes of Punta Pianedda, at an elevation of about , to the north of the comune of Bitti. From here, the river flows to the southwest.
Finally, after , it flows into the Gulf of Oristano in the Sea of Sardinia (Mediterranean Sea). The river enters the sea at about to the north of the city of Oristano.
The river forms the "Omodeo lake", an artificial lake that is long and wide; it is one of largest artificial lake in Italy.
It flows through 81 comuni.
Main tributaries
Most tributaries of the Tirso are small streams; the main tributaries are: Taloro, the main tributary, long and a drainage basin of ; and Massari, the second largest tributary, long and a drainage basin of .
Both rivers are left tributaries of the Tirso.