The Sierra de Guadarrama halves the waters of two different large river basins: The Tajo River one at the south and the Duero River one at the north. It is also the administrative borderline of the Madrid and Castilla León Autonomous Communities. The main Sierra de Guadarrama Rivers have their origin at the National Park and are documented on the Natural Systems of the National Parks Law 30/2014 as “Water Courses and Riparian forests”. There are 337 km of fluvial courses (169km at the southern slope and168 km at the northern slope), of which 227 are permanent and 109 temporary, although the small streams, brooks and colluvial soils characteristic of this National Park, would double the mentioned water courses. All of them are classified, according to the hydrologic planning instruction as “Mediterranean siliceous mountain rivers”.
The waters of the Sierra de Guadarrama National Park are oligotrophic, with a very low mineralization, conditioned by the Sierra predominant substrate: gneiss and granite. They are mountain streams and rivers with a high Mediterranean character due to the zone climate, changing the altitude between the Supra, Oro- Mediterranean and Crioro-Mediterranean levels.
This Mediterranean character is defined by a flows system highly fluctuant both within the year and on a year-on-year basis. In addition it is also important the snow influence in those systems and that is why they are mainly rivers and streams rainy and snowy featured.
The water courses are real life niches, both for the aquatic biota they handle, and for their dependant species and conservation condition. We should emphasize in relation with the fauna, the common trout (Salmo trutta), the Iberian frog (Rana Iberica), and the European otter (Lutra lutra) whose populations are recovering over the last years in the National Park. In addition there is a wide diversity of fluvial invertebrate species living at the National Park water courses. It is remarkable the presence of indicators reflecting clean waters with an excellent ecological condition, such as the Plecoptera “Brachyptera arcuata” or the Tricoptera “Allogamus laureatus” both protected by the Madrid Autonomous Community Regional Catalogue, being the Tricoptera group (popularly known as “tubes”) the most biologically diverse and characteristic of the National Park rivers. Other minority groups but not less important are the aquatic Coleoptera, Plecoptera and Diptera. Some of them, endemic of the Sierra de Guadarrama have in its rapid, clean and cold waters, their ecologic niche. Among the invertebrates we should highlight certain dragonflies, such as the damselflies and the golden-plated rings dragonfly (Cordulegaster boltonii).
Due to all these particularities and its fragility, the Sierra de Guadarrama National Park Administration accomplishes a periodic monitoring process of their preservation condition at the main water courses with their source at the Park.
The National Park rivers and streams flora is mainly limited to mosses and liverworts, and rodoficeus algae type of Lemanea, linked to rapid waters zones. On several occasions appear species such as the Montia Fontana in sources areas and the Mentha sp., and as well the Apiaceae Oenanthe crocata, very characteristic of Sub Mountain streams at the spring season. There is as well Ranunculus aquatilis in less gradient areas, even if it is characteristic of rivers and streams middle and low reaches, with certain nutrients contribution. The Carex sp. masses are remarkable for their function in the fluvial ecosystem, even if they are not considered as macrophyte as such but as helophyte.
The riparian forest presence on the Sierra de Guadarrama National Park fluvial courses is restricted to major entity waterways. The willow groves of Salix atrocinera and S. salvifolia dominate, being the last ones at lower zones. It is very characteristic to find at the river banks the alder buckthorn, Frangula alnus, dominant on several occasions in relation with the ashen willow tree. River banks are quite common with willow groves of Salix atrocinera and forest species such as: Taxus baccata, Ilex aquifolium, Populus tremula, Corylus avellana, Euonymus europaeus, Sorbus aria, Sorbus aucuparia, Sorbus latifolia, Viburnum opulus, Viburnum lantana and Rhamnus catharticus among other.
In the Madrid Autonomous Community, in the southern slope of the National Park, there are three different main rivers watersheds: the Lozoya, the Guadarrama and the Manzanares. In the National Park northern slope are present the Cega, the Pirón (Cega’s tributary), the Eresma and the Moros (Eresma’s tributary) rivers watersheds.
The Lozoya River
The Lozoya river source is coming from the fusion of different fluvial courses, in the Rascafría municipal area. In its higher reach it is called La Angostura River, coming from the union of the Peñalara stream, whose source is at the Peñalara Lake, the Guarramillas stream, whose source is at the northern slope of La Bola del Mundo, and Las Cerradillas stream, whose source is at La Cabeza de Hierro Menor.
On its left bank it receives the Garcisancho/ La Umbría, Santa María, Artiñuelo and La Saúca mightiest tributary streams, and other secondary such as Los Pájaros, whose source is at The Birds Lagoon, and Hoyoclaveles. Both together with the Peñalara stream are included on the Peñalara Wetlands Ramsar Site, wetlands list with international relevance.
On its right bank it receives the Barondillo (or Valhondillo) and the Aguilón tributary streams, being at the confluence with the last one when it becomes the Lozoya River as such. The Aguilón stream is remarkable due to its high biodiversity preservation, and as well because it is the mightiest tributary of the Lozoya River.
In its 91 km trail, it harbors several reservoirs created to supply water to the Madrid Autonomous Community, and holding 62% of the total Community water stocking capacity. It finally flows into the Jarama River in the Uceda municipal area.
The Manzanares River
The Manzanares River source is over 2.000 m altitude at the Ventisquero de la Condesa, between the Guarramillas Peak and the Valmartín Mount, on the Cuerda Larga ridge southern slope, opposite to the Lozoya River source, in the Manzanares el Real municipal area.
On its left bank it receives the tributary streams: Valdemartín, La Condesa, Hoyos de la Sierra and Las Majadillas inner La Pedriza area, and outside the Mediano stream. On its right bank, it receives the Navacerrada tributary river, also called Samburiel, whose source is at La Maliciosa in La Barranca area and whose mouth is at the Santillana reservoir, just close to the Manzanares river.
In its 92 km trail, it crosses different spots, such as La Pedriza within the National Park, and El Pardo Mount, being protected in its medium reach by The Cuenca Alta del Manzanares Regional Park.
The Guadarrama River
The Guadarrama River is a direct tributary of the Tajo River. Its source is on the National Park southern slope, in the Cercedilla municipal area, between La Fuenfría Pass and Navacerrada Pass. It has three main tributaries: La Venta River, Pradillo River and Navalmedio River. La Venta river source is at La Fuenfría Pass and it runs parallel to the Roman road known as Vía Antoniana connecting in the 1st-4th centuries AD, Mérida and Zaragoza
The Pradillo river source is at Los Acebos fountain, at the foot of the Siete Picos ridge. Both together with the Navalmedio river (also called Regato del Puerto stream) with its source at the Navacerrada Pass, they constitute Las Fuentes River which joins La Venta river at Cercedilla, just before the boundary line with Los Molinos. Traditionally, from that point it is called Guadarrama River.
After a 132 km trail it flows into the Tajo River, in the province of Toledo. Within the National Park, we should highlight the Roman bridges it dodges such as El Descalzo, EL Reajo and Enmedio at the village of Cercedilla.
The Eresma River
The Eresma River belongs to the Duero river basin, and its source is on the National Park northern slope. Its source is at the confluence of several steams: Minguete whose source is at the Montón de Trigo, El Puerto de El Paular whose source is at Los Cotos Pass and Peñalara Massif northern slopes and El Telégrafo whose source is on the surroundings of the Alto del Telégrafo peak and the Umbría de Siete Picos area. After a 134 km trail it flows into the Adaja River, in the province of Valladolid, crossing historical cities as emblematic as Segovia and Coca (Cauca).
At the headwaters of the Eresma River look remarkable the following streams: Siete Arroyos, Collado de la Flecha, Chorro Chico, Chorro Grande, La Chorranca, Peñalara, Dos Hermanas, Del Infierno, De El Paular, Del Telégrafo, Ventoso and Minguete. But, one of the most important is the Cambrones River, whose source is at the Basardilla area, village close to Torrecaballeros, and after a 14 km trail it flows into the Eresma River at the Pontón Alto level. It is also relevant the Frío River, also called La Acebeda River, whose source is at the opposite slope of the spot called Ojos del Río Moros, joining downwards the Cereceda and Los Horcajos steams. It becomes a tributary of the Eresma River after joining the Milanillos River.
The Moros River
The Moros River is the most important tributary of the Eresma River, among the ones with their sources in the Sierra de Guadarrama. Its source is at Los Ojos del Río Moros (River Moros Eyes), in El Espinar municipal area. It is a spring located on the southern slope, at the foot of Montón de Trigo and west of the Minguete Mount. When it joins the rainy character streams, Los Ojos, Tirobarra, Patarro, Los Horcajos and Tejo they become all together the River Moros.
The Cega River
The Cega River is the most important river of the National Park northwestern section. It is a direct tributary of the Duero River, receiving waters of many streams. Its source is in the Navafría municipal area, at the Navafría Pass. It comes from the fusion of different high-mountain courses such as Las Pozas River and El Chorro, El Artiñuelo, Peñalara, Viejo and El Hoyo streams.
After a 141 km trail it flows into the Duero on its left bank, in the Puente de Duero municipal area in the province of Valladolid. It crosses important cities such as Cuéllar.
The Pirón River
The Pirón River is the most important tributary of the Cega River. Its source is as well in the Sierra de Guadarrama northern slope. It comes from the confluence of several mountain streams with sources close to the Malangosto Pass. After a 90 km trail direction southeast-northwest through Segovia lands, it enters in the province of Valladolid to run its last 5 km and flows into the Cega River on its left bank. The main steams at its source are: Majada del Hidalgo, Pastizal, El Merendero and Majalasana.