During the Roman Warm Period, the city of Amposta was recorded as a seaport on this river despite now being more inland. A city partly named for this non-US river in the Álava province is home to the Apollo Theatre and is served by the Vitoria Airport. The world's first church dedicated to Mary, the Cathedral-Basilica of Our Lady of the Pillar, is found on this river. This river flows through the Campoo district town of Reinosa, built by the same ruler as nearby Torrelavega and (*) Santander. The town of Saguntum south of this river (10[1])was captured in the leadup to the Second Punic War. The Gállego flows into this river (10[1])near a city home to the Aljafería Palace, Zaragoza. This river rises in (10[1])Cantabria and forms a delta in southern Catalonia. For 10 points, name this river that, after the Tagus, is the longest in length on the Iberian peninsula. ■END■