Rivers Dome of an old radar station near the highest point of Serra da Estrela on a winter's day. There are three rivers that have their headwaters in the Serra da Estrela: the Mondego, which is the biggest river entirely within Portuguese territory; and the Zêzere, which is a tributary to the Tagus; a tributary of the Mondego, the Alva also has its headwaters in the range. It is 1,691 m (5,548 ft) high. Nature Park.