Its principal tributaries are - from the right hand the Jalon with its affluent the Jiloca, the Huerva, Aguas, Martin, Guadalope and Matarrana; from the left the Ega, Aragon, Arba, Gallego, and the Segre with its intricate system of confluent rivers.
The Ebro is the principal river, and receives from the north, in its passage through the province, the Arba, the Gallego and the united waters of the Cinca, Esera, Noguera Ribagorzana, Noguera Pallaresa and Segre - the last three belonging to Catalonia.
It is watered by many rivers, the largest of which is the Segre, a left-hand tributary of the Ebro.
South of the point at which the Segre is joined on the right by the Noguera Pallaresa, the character of the country completely alters.
Seo de Urgel, near the headwaters of the Segre, is a fortified city which has been an episcopal see since 840, and has had a close historical connexion with Andorra (q.v.).
(See CATALONIA.) Lerida, the capital of the Spanish province of Lerida, on the river Segre and the Barcelona-Saragossa and Lerida-Tarragona railways.