The rivers of Spain, even those regarded as the main ones, have a very modest flow owings to the fact that they drain away only rainwater, and rainfall is scarce an irregular.

The rivers drain into the Mediterranean and the Atlantic. There are nine major rivers:

• the Mino (310 km.), the Duero (895 km.), the Tajo (1,007 km.), the Guadiana (778 km.) and the Guadalquivir (657 km.), each flow into the Atlantic.
• the Segura (325 km.), the Jucar (498 km.), the Turia (280 km.) and the Ebro (910 km.) which empty into the Mediterranean.

In the north the so-called mountain rivers have a short course, owing to the nearness of their source to the sea. The main ones are the Bidasoa, the Nervion, the Sella, the Nalon, the Navia and the Eo.