Tagus Basin

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.

[9] 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.

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.

The most frequent type of poplar grove in this area corresponds to the Rubio tinctorum-Populetum albae association, where in addition to the aforementioned species there are other arboreal willows such as Salix fragilis, Ulmus minor or Fraxinus angustifolia.

The edges with temporary waterlogging are where there is the greatest originality and floristic richness with communities of hygrohalophilic quenopodiaceae such as Salicornia ramosissima, Suaeda splendens, Microcnemum coralloides, these examples are located in the Cavina, Salinas and Cuevas streams between Aranjuez and Yepes.

Another increasingly scarce element are the elm trees, the best represented, although in a very precarious state of conservation, are found in the Algodor River, downstream of the Finisterre Reservoir, in the headwaters and slopes of the Martín Román stream where specimens of boxthorn, Flueggea tinctoria endemism luso-extremeño, are beginning to appear.

The shrub stratum, when well preserved, is usually dense and rich in thorny plants such as Rubus spp, Crataegus monogyna, Prunus insititia, Sambucus nigra, etc.

When the direct influence of the altitude and the more humid climate of the headwaters of these rivers is abandoned, the sauceda is impoverished in tree species and if the conditions of xericity increase, specimens of Erica scoparia can be found, as in the Sorbe and the Riatillo.

At lower altitudes ash and poplar groves dominate, the former, with red osiers and dogwoods (Cornus sanguineae-Fraxinetum angustifoliae), are frequent in the lower reaches of the rivers Frío, Hoz, Dulce, Salado, Tajuña and the upper reaches of the Badiel, These ash woods prefer steep topographies that limit the rate of evapotranspiration and when well preserved it is a pluristrative forest with other tree species such as Salix purpurea, Cornus sanguinea, Ligustrum vulgare, Prunus mahaleb, Sambucus nigra, various brambles and hawthorns and other species typical of the surrounding communities such as Quercus faginea, Juniperus thurifera, Bupleurum rigidum, etc.

If it is a typically xerophytic tayaral, it is accompanied by Artemisia campestris, Carlina corymbosa, Asteriscus aquaticus, Glycirhiza glabra, Retama sphaerocarpa or Asparagus acutifolius.

The arborescent Salix salviifolia willow groves of the upper Lozoya, downstream of the Pinilla dam and up to the Puentes Viejas reservoir, Salicito saviifolio-lambertianae subas.

An excellent example is the willow groves on the northern slope of the Sierra de Gredos that flow into the Alberche, such as Garganta Iruelas and Valsaína (they also include Corylus avellana and Ulmus glabra), Lanchamala and La Yedra.

Finally, the Luso-Extremeña alder grove, where two differential species Osmunda regalis and Flueggea tinctoria are frequent, as well as the presence of Viburnum tinus, Erica lusitanica, Genista falcata and Acer monspessulanum.

Where there is an alluvial plain there is also an ash grove (Ficario ranunuculoidis-Fraxinetum angustifoliae), behind it, a series of phreatophytic communities dominated by rushes of roundhead bulrush, Scirpus holoschoenus, and a seasonal sub-humid grassland.

These species are adapted to a series of specific conditions of the aquatic environment, occupying different stretches of the river according to their habitat requirements and water purity.

High mountain rivers and streams, areas of clean, oxygenated and cold waters, present a great variety and taxonomic richness with the usual presence of Plecoptera (families Perlidae, Leuctridae, etc.

The Mediterranean mountain has a lower diversity, although relatively high, being well represented some heteroptera and trichoptera adapted to low current and temperate waters (families Hydroptylidae, Rhyacophilidae, etc.

In the Tagus basin, 29 species of fish belonging to the families Anguillidae, Salmonidae, Esocidae, Cyprinidae, Cobitidae, Ictaluridae, Siluridae, Poeciliidae, Centrarchidae and Percidae have been cited.

Six species of urodele amphibians are distributed, the gallipato, Pleurodeles waltl, is abundant in the basin, although it has populations threatened by isolation such as those in the south and northwest of the Community of Madrid.

The Common parsley frog, Pelodytes punctatus, is distributed in the basin mainly in its eastern zone, Madrid, Cuenca, Guadalajara and Toledo, and is considered a frequent but threatened species.

Also in the gorges, ravines and slopes of river terraces nest large numbers of cock-of-the-rock birds, some of them of great ecological value as the Eurasian griffon vulture (Gyps fulvus), Egyptian vulture (Neophron percnopterus), golden eagle (Aquila chrysaetos), Bonelli's eagle (Hieraaetus fasciatus), Eurasian eagle-owl (Bubo bubo), peregrine falcon (Falco peregrinus), etc..

The mammal species directly related to the freshwater environment due to their aquatic or semi-aquatic nature in the Tagus basin are: the Eurasian otter (Lutra lutra), an autochthonous aquatic mustelid adapted to this environment, distributed throughout the basin, except in unfavorable or polluted waters; another mustelid present is the American mink (Mustela vison), an allochthonous species, which has colonized some areas after escaping from farms or uncontrolled releases.

The Pyrenean desman (Galemys pyrenaicus) is generally found above 700 m a.s.l., in streams and rivers of constant current and steep slope, in well oxygenated and clean waters.

The Mediterranean water shrew (Neomys anomalus), an insectivorous mammal with excellent swimming and diving qualities, associated with permanent and well oxygenated watercourses and in areas of high humidity.

Finally, the Southwestern water vole (Arvicola sapidus), which is linked to permanent aquatic environments and can be found up to 2100 m above sea level, is scattered throughout the basin.

The flora species, endemic, rare or vulnerable, linked to the edaphic-hygrophilous environments: Glinus lotoides, Euphorbia uliginosa, Myosotis lusitanica and Juncus valvatus.

Small sub-basin on the right bank of the Tagus, 274 km2, including the municipalities of Alcanena, Entroncamento, Golega, Porto de Mós, Santarém and Torres Novas.

It consists of fractured calcareous rocks, with eminently subway runoff, contributing as a recharge structure for the free, semi-confined and confined aquifers in the area.

In this sub-basin, the rocky escarpments play an important role, where a large number of rupicolous birds nest, and it is included in the Special Protection Zone of the International Tagus.

This area is rich and varied from the point of view of habitat that enhances the quality of the fauna it shelters, so in the upper courses of the Zézere and its tributaries, the species of Chioglossa lusitanica and Galemys pyrenaicus stand out.