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 (-5[1])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. (10[1]-5[1])The town of Saguntum south of this river was captured in the leadup to the Second Punic War. (10[3])The Gállego flows into this river near a city home to the Aljafería Palace, Zaragoza. This river rises in 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■ (10[1]0[1])