Regular river navigation begins only at Abrantes, a few miles below which the Tagus is greatly widened by receiving on its right bank the impetuous Zezere from the Serra da Estrella.
The principal right-hand tributaries, besides the Gallo and Zezere, are the Jarama, descending from the tableland of New Castile a little below Aranjuez, the Alberche and the Tietar, which collect their head waters from opposite sides of the Sierra de Gredos, and the Alagon, from the rough and broken country between the Sierras de Gredos and Gata.
Lesser ranges, which are included in the Beirene system and vary in height from 2000 to 4000 ft., are the Mesas, between the rivers Coa and Zezere; the Guardunha and Moradal, separating the Zezere from the Ponsul and Ocreza, tributaries of the Tagus; the Serra do Aire, and various ridges which stretch south-westward as far as the mountains of Cintra (q.v.).
The chief Portuguese tributaries of the Douro are the Tamega, Tua and Sabor on the north, the Agueda, Coa and Paiva on the south; of the Tagus, the Ocreza, Ponsul and Zezere on the north, the Niza and Sorraia on the south, while into the Guadiana, on its right or Portuguese bank, flow the Caia, Degebe, Cobres, Oeiras and Vascao.