Gran Valira

The Gran Valira is the biggest river in Andorra. It flows through the capital, Andorra la Vella, and exits the country in the south near the Spain–Andorra road border crossing.
The Gran Valira is a tributary to the Segre, a tributary to the Ebro. It flows into the Segre in La Seu d'Urgell, Spain.
Geography.
The Gran Valira river has a length of , only outside Andorra, and a drainage basin with an area of for the whole Valira system including the Valira d'Orient and the Valira del Nord rivers. 
Its average yearly discharge (volume of water which passes through a section of the river per unit of time) is at La Seu d'Urgell in the Catalonia, Spain.
The Valira river has the shape of "Y". The upper branches are the "Valira del Nord" an the "Valira d'Orient" rivers. They join in Escaldes-Engordany and, from here, the river is called the "Gran Valira".
Valira del Nord.
Le Valira del Nord river is also called "Valira de la Massana", "Valira de Ordino" or "ribera de Ordino". It starts in the "Estanys de Tristaina", a group of 3 lakes in northwestern Andorra, in the Ordino parish at an altitude of above sea level. From here, the river flows to the centre of the country, flowing through the villages El Serrat, Les Salines, Llorts, Arans, La Cortinada, Ansalonga, Ordino, La Massana, Anyós and Engordany (part of Escaldes-Engordany). Its main tributaries are the "Sorteny" and "Arinsal" rivers.
Valira d'Orient.
The Valira d'Orient river is also called "Valira de Encamp", "Valira de Canillo" or "Valira de Soldeu". It starts in the "Cirque des Pessons" in northeastern Andorra, in the Encamp parish. From here, the river flows through the villages Soldeu, El Tarter, L'Aldosa, El Vilar, Canillo, Molleres, Les Bons, Encamp et Escaldes (part of Escaldes-Engordany). Its main tributary is the "Madriu" river.
Gran Valira.
The Gran Valira river starts with the confluence of the two previous rivers in Escaldes-Engordany. It flows through the towns Andorra la Vella, Santa Coloma, La Margineda, Aixovall and Sant Julià de Lòria. Then it flows into Spain where it is joined, by the left side, by its main tributary, the Runer river.
Finally, it flows into the Segre river in La Seu d'Urgell.