Why now the northern rivers of the Iberian Peninsula (Ebro, Duero, Tajo) are called by the same names as in the Roman era

Why now the northern rivers of the Iberian Peninsula (Ebro, Duero, Tajo) are called by the same names as in the Roman era, and the southern rivers (Guadiana, Guadalquivir) by others?

The north of the Iberian Peninsula was not under Muslim rule as long as the South. The names of the southern rivers come from Arabic words, for example: Guadalquivir – from Wad al-Kebir (Big River).

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