Nearly half of the territory (43%) is less than 200 m above sea level. Average altitude is 240 m, compared to the average of 640 m for the Iberian peninsula as a whole, and almost all (95%) of the higher areas (over 400 m) are situated north of the Tagus.

The coastal areas are topographically different from the interior since, as a general rule, the low lands stretch inland from the coast.

The Peneda, Gerês, Marão, Alvão, Montesinho, and Nogueira mountain ranges, part of the Galicia-Douro system, are to be found in the north. The CordiIheira Central Ibéria (central Iberian ridge) extends into Portugal via the Malcata, Gardunha and Alvelos ranges, and further north, via Estrela, Açor and Lousã. The Estrela range, the highest point in the country (1991 m), with its cirques, lakes, moraines, glacial valleys, and erratics, bears witness to the last glaciation (Würm).

To the west, the Estremadura Limestone Massif, made up of the Sicó, Aire, Candeeiros and Montejunto ranges, is the area where karst formations are most in evidence (lapies fields, dolines, uvalas, poljes, caves and potholes).

To the south of the river Tagus, there is a large peneplain marked here and there by a few higher features, such as Ossa, Monfurado, Adiça, Grândola, and Cercal, as well as the São Mamede range, the largest in the Alentejo.

Other smaller mountain ranges are worthy of mention, rising above the surrounding countryside, such as Arrábida, situated on the Setúbal Peninsula, which is the best example of a fold belt to be found in Portugal, and the formations of volcanic origin at Sintra near Lisbon, and at Monchique, in the western Algarve.

The relief is based on three main geological formations. The Maciço Antigo (Ancient Massif), covering a large area of the country and made up of metamorphic and igneous rock; the sedimentary margins, thick layers of sedimentary rock found in the triangle formed by Espinho-Tomar-Cascais (the western margin), together with the southern part of the Algarve (the southern margin); and the Tertiary basins of the Tagus and Sado rivers, which present the greatest morphological uniformity in the whole of Portugal’s territory and are composed of sedimentary rock.

To these three units should be added the continents shelf, 30 to 60 km wide, which runs virtually parallel to the coastline, and cut by three underwater valleys, one in front of the Nazaré headland and two others around the Arrábida peninsula.

Finally, linked to the relief is a heterogeneous geology, made up of different kinds of rock, giving rise to considerable variation in types of soil. On the mainland, there are five main groups: rocks which are basically schist-like, found in Trás-os-Montes, Alto Douro, Beira Baixa, the Alentejo and the Algarve; granite and similar rocks, found mainly in Minho, Beira Alta and Trás-os-Montes; sand and sandstone, particularly in the Tagus and Sado river basins; Limestone and marl in the Estremadura Limestone Massif and Arrábida; and alluvium in the lower reaches of the Tagus, Sado, Vouga and Modego rivers.

Some of the biggest rivers in Portugal, the Douro, Tagus, Guadiana and Minho, originate in Spain, where the greater part of their hydrographic basins are to be found, part of their course forming the border between the two countries. The main rivers whose course is entirety within Portugal’s borders are the Mondego, Vouga, Sado and Mira. In general, most watercourses are confined to the north of the country, the south being characterized by a few large river basins, such as those of the Tagus, Guadiana and Sado.

The irregular nature of the hydrological system reflects the rainfall: regime, which is fairly constant in the north, but concentrated in short periods in the south. For example, at times it is possible to walk across the river Tagus at Santarém, the flow of the Mondego in flood can be 3000 times greater than the discharge in summer, and in Mértola, variations of up to 28 m from normal levels have been recorded in the waters of the Guadiana.

Reduced discharges are also linked to the great demand on inland waters, with clear repercussions for the biological communities that inhabit them.

Dams help regulate certain rivers and their reservoirs are a significant feature of Portugal’s hydrographic system. They have a noticeable effect on the economy and the landscape, changing water flows and local climatic conditions.