Tagus, the largest river of the Spanish peninsula, issues from the watershed between the provinces of Guadalajara and Teruel; follows a more or less westerly course across the centre of the peninsula, and, after dividing into two portions below Salvaterra, its united waters enter the Atlantic by a noble estuary 20 m. long; total length 566 m., of which 190 are in Portugal; navigable as far as Abrantes.