Tagus Basin
The Tagus Basin is the drainage basin of the Tagus River, which flows through the west of the Iberian Peninsula and empties into Lisbon. It covers an area of 78,467 km2, which is distributed 66% on Spanish territory and 34% on Portuguese land.
It is the third-largest basin in the Iberian Peninsula, after the Douro Basin, with 98,258 km2, and the Ebro Basin, with 82,587 km2.
| Tagus Spain | Total Tagus | % Spain/total | |
| Length | 910 | 1092 | 83.3 |
| Surface area km2 | 55,645 | 83,678 | 66 |
| Population | 7,000,000 | 10,000,000 | 70 |
Physical environment
The Tagus basin is one of the most important in the Peninsula, due to its extension and its flow, being the one with the highest population density in Spain and the peninsula. It is formed by an elongated surface with an east–west orientation, the Tagus River flows from the Sierra de Albarracín, where it has its source, to the estuary, Mar de la Paja, next to Lisbon, through the center of the Hesperian Massif with a length of 910 km, in the Spanish area, 1092 km in total length. The basin is wedged between the Central System, to the north, the Montes de Toledo and Sierra de Montánchez, to the south and the Iberian System,, to the east; bordering to the north with the Ebro and Douro basins; to the south with the Guadiana Basin and to the east with the Ebro and Júcar basins. The western limit, as far as the Spanish area is concerned, is delimited by the Erjas and Sever rivers, which form the border with Portuga]. In the interior of the area defined by these mountains and by the minor reliefs of the Hercynian massif, which completes the closure to the west, it is structured in a graben filled by Cenozoic materials, sands, clays, marls, gypsum and some limestone in the upper levels, which constitute horizons of silting up of the ancient lake that occupied the original depression.The mountain ridges of the Tagus basin only reach high altitudes in the Central System, especially in the middle and eastern sectors, where they frequently exceed 2000 meters above sea level; in the Iberian System, only some peaks of the Montes Universales exceed 1800 m.a.s.l., while in the Montes de Toledo the altitudes are considerably lower. The altitude of the interior depression is much lower, although very variable, decreasing rapidly from the extreme northeast to the western edge, thus, while in the plains of La Alcarria the heights are close to 1000 m.a.s.l, in Aranjuez they drop below 500 m.a.s.l, in Navalmoral de la Mata to 300 and in the lands to the south of Coria to little more than 200 captured part of the original Douro basin by [headward erosion">Stream capture">captured part of the original Douro basin by [headward erosion, favored by the greater gradients determined by the altimetric difference between the two basins, the most characteristic examples being the Alberche and the Alagón.
The network of tributary rivers of the Tagus is very dissymmetrical, those on the right bank are the ones that provide the most abundant flows, as they collect the contributions from the central system and the Iberian system; the left tributaries are generally shorter and have a low flow, especially those that originate in the Montes de Toledo.
The longest rivers in the Tagus basin are:
| Name | Length |
| Tagus River | 910 |
| Tajuña River | 254.1 |
| Alagón River | 208.6 |
| Jarama River | 204.9 |
| Alberche River | 193.8 |
| Tiétar River | 175.7 |
| Hernares River | 172.9 |
| Guadarrama River | 131.8 |
| Salor River | 126.3 |
| Almonde River | 122.9 |
| Guadiela River | 109.5 |
| Gallo River | 107.1 |
| Algodor River | 101.2 |
| Manzanares River | 92 |
Biotic framework
The biotic framework of the Tagus Basin, due to its geology, geomorphology and climatology, presents a great variety of ecosystems that include different habitats and protected species; these ecosystems range from the high peaks of the mountain ranges of the Central System to the river valleys of the Upper Tagus or the alluvial plains of Toledo and Cáceres.The Tagus basin can be divided into two large biogeographical zones: the Luso-Extremaduran province, and the Castilian-Mestrazgo-Manchegan province, with its natural boundary at the confluence of the Alberche. In short, this is a climatic-lithological differentiation: the limestone sector in the center-east and the siliceous soils in the center-west.
Vegetation
In general, the vegetation of the basin is as follows: in the high limestone mountain ranges, Serranía de Cuenca and Upper Tagus, there is a dense vegetation cover of needle-leaved forests dominated by Pinus nigra and Pinus sylvestris, pure or mixed; when climatic conditions are extreme, the pine forest is replaced by paramo vegetation or Juniperus thurifera juniper groves. Descending in altitude and with the alternation of sunny and shady exposures, there are two types of subsclerophyllous forests: Quercus faginea Portuguese oak groves in the valley bottoms and shady slopes and occupying the so-called alcarrias,, and mixed oak groves with Portuguese oaks and/or junipers in the sunny and rocky terrain, the main components of the Tajuña valley. Occasionally, and on sandstone lithology, there are oak or black pine forests, Pinus pinaster, such as in the Gallo canyon, Sigüenza or Poyatos, below these communities there are pure holm oak forests of continental character. The most frequent scrublands in this area are the boxwoods, junipers, replacing the pine forests at the highest or most exposed heights and the erizals. Underneath, there are steppe, Cistus laurifolius, sages and shrub pastures, as well as peat bogs and cervunals of altitude or linked to hydromorphic soils.When thermicity increases, either by the type of substrate or by the altitude, the pine forest becomes Aleppo pine, Pinus halepensis, as in Sierra de Altomira, which is replaced by kermes oak, rosemary or even an esparto grass or esparto grove. To the north, the lithological change makes pine forests of Scots pine, Pinus sylvestris, frequent, as in the Sierra de Pela and Sierra de Ayllón, or the important pine forests of the Lozoya Valley and the upper basin of the Guadarrama and Aulencia rivers. Below, the domain of the Quercus pyrenaica Pyrenean oak groves begins, in Sierra de Ayllón, Sierra de Somosierra, Sierra de Guadarrama, part of Sierra de Gredos, Sierra de Gata and Sierra de Béjar; either by the action of man or by climatic-edaphic conditions, the oak grove is replaced by Erica australis heaths, or by Cytisus scoparius or Genista cinerea broom groves. At altitude, above the forest limit, the scrub is Cytisus oromediterraneus and Echinospartum barnadesii.
To the south, already in the middle of the evaporitic depression of the Tagus, the gypsophilous scrublands appear, so exclusive and rich in endemic and vicariant species, which extend from the foothills of the Tagus in the Almoguera Reservoir and reach past Aranjuez and Borox, being also present in the lower basins of the Jarama, Tajuña or Manzanares. As a link between the gypsiferous substrates and the marly and calcareous ones, we find the esparto-rosemary and kermes oak groves.
Descending in altitude from the Central System or the Montes de Toledo we enter the domain of the holm oak forest and the further west we move, the holm oak forest gives way to the cork oak grove, a typical sclerophyllous community of clear Atlantic influence. These holm oak groves are replaced by various types of scrubland, from the gorse and thyme groves of the Alcarrias, passing through the gum rockrose shrubs with Spanish lavender and thyme of the Sierra de Guadarrama or Montes de Toledo, by the broom groves of Cytisus multiflorus, by the thickets of basil-leaved rock rose and other low rock-roses, or by the extensive retama groves of Retama sphaerocarpa frequent in the plateau of Toledo and southern regions of the Community of Madrid.
The southern slopes of the Sierra de Guadarrama and Sierra de Gredos are home to important pine forests of cluster pine, Pinus pinaster, stone pine, Pinus pinea, and relicts of black pine, Pinus nigra, the latter two species mainly in Gredos and in the Cofio–Alberche basin; among these stands alternate Spanish chestnut forests, Quercus broteroi Portuguese-oak groves.
To the west of Toledo and east of Cáceres, Campo Arañuelo, south of Ávila, Tiétar valley, and almost all of Extremadura, there are holm oak and cork oak dehesas or mixed masses of them. The dehesas with livestock use are a refuge of the floristic diversity, characteristic of the Iberian west. Another type of dehesas, those of ash or oak are located in the foothills of the Sierra de Guadarrama and Sierra de Béjar.
Riverbank vegetation
To describe these plant communities of the Tagus basin, it is convenient to make a zoning that allows grouping areas that, while respecting the hydrological characteristics, present similar phytosociological characteristics.Upper Tagus
The riparian vegetation of the Upper Tagus is characterized by the almost continuous presence of two types of willow groves of calcareous character, one of arboreal size that occupies the fresh banks with good soil in areas of medium and high mountains and another of shrubby character that usually occupies secondary channels, margins and alluvial beds with a high water table in areas of medium mountains and foothills.The first of the willow groves, generally corresponding to the Salicetum purpureo-albae association, is composed of one or two bands of vegetation, conditioned by the profile of the valley, where willows such as Salix atrocinerea, Salix alba, Salix fragilis or Salix neotricha dominate. They are usually accompanied by other tree species such as Fraxinus angustifolia, Populus nigra and alba, Corylus avellana or Cornus sanguinea, always surrounded by a thorny border of brambles and roses; within their courtship appear herbaceous plants such as Equisetum ramosissimum, Brachypodium sylvaticum or Carex acutiformis.
More frequent and widely distributed in the Upper Tagus are the calcareous shrubby willow groves, alliance Salicion discolori-neotrichae. They are generally dense formations, the typical wicker beds, which occupy the secondary channels, banks and alluvial beds of hard water rivers and more or less irregular regime, with floods and low water, withstanding floods well; its characteristic species are Salix purpurea var. lambertiana and Salix elaeagnos subsp. angustifolia, the latter dominating in the cooler areas. It is common to find meadows-rushes and even reed beds of the Molinio-Holoschoenion alliance and isolated specimens of Populus nigra or Populus alba, as well as Berberis vulgaris, Viburnum lantana or Ligustrum vulgare. In other areas of the basin of lower altitude these willows form the first band of riparian vegetation. Examples are some stretches of the Tagus in Morillejo, the Hoz Seca and Upper Cabrillas streams and numerous small riverbeds in this area.