The River Ebro is 578 miles (930 km) in length, which makes it the longest river that's entirely in Spain. It rises in Cantabria, near Spain's northern (Atlantic) coast, and flows into the Mediterranean at the southernmost extremity of Catalonia.
The longest river in the Iberian peninsula is the Tagus, which rises to the north–east of Madrid and flows through Portugal into the Atlantic. The Tagus is 626 miles (1,007 km) in length; it flows for 716 km in Spain, 47 km along the border, and 275 km in Portugal.
The only great navigable river in Spain (according to Wikipedia) is the Guadalquivir. It flows through Cordoba and Seville, and reaches the Atlantic at Cadiz; its name is derived from the Arabic for 'great valley'. It's 408 miles (657 km) long.
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