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.