If you ask different people what is the name of spain's longest river, you will likely get many different answers. This is because the name of Spain's longest river depends on whether you take into account a river's length in its entirety or only its route within Spain's borders. Taking into consideration the latter, the Ebro would be considered the country's longest river. It is fed by several tributaries from the Sistema Iberico and the Pyrenees and drains a sixth of the country. It rises in the Cantabrian Mountains at Fontibre and empties via a large delta into the Mediterranean Sea at Tarragona, Catalonia, near Barcelona.

The third longest Spanish river is the Douro, which runs for over 557 kilometers. It is also one of the Iberian Peninsula's longest rivers. The Douro starts in central Spain's Sierra de Urbion before crossing into Portugal. The Elsa, Arlanzon, and Pisuerga are the main tributaries of this important river. The Douro is a multitasking waterway, used for drinking, power generation, and even cooling nuclear reactors.

The fifth longest river is the Guadalquivir, which flows over 510 kilometers. It is the second longest river entirely within Spain's borders. The Guadalquivir is also a multitasking river, used for drinking, power generation, and irrigating arid farmland. Its current crisis stems from inadequate water transfer planning going back over a hundred years, which miscalculated the amount of water available and failed to take into account Spain's regular droughts.