The Valmayor dam is located in the municipality of Valdemorillo. The Valmayor reservoir is located in the northwest of the Community of Madrid, in Spain. It belongs to the Guadarrama basin, although it is built on the main tributary of this river, the Aulencia. It is the second largest in the province in terms of the amount of water stored (124.4 million cubic metres), after the El Atazar reservoir (425.3 million cubic metres), in the Lozoya basin. It is managed by the Canal de Isabel II.

The Valmayor reservoir integrates several ecosystems of great environmental value, which have coexisted with various agricultural, livestock and forestry developments. The riverside vegetation occupies a pre-eminent position, together with the holm oak forests that appear in the vicinity, mainly in the form of pastureland.

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The reservoir is protected by the Community of Madrid under Law 7/1990, of 28 June, on the Protection of Reservoirs and Wetlands in the Community of Madrid and also by its inclusion in the Regional Park of the Middle Course of the River Guadarrama and its surroundings.

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This ecosystem is home to different animal species, both amphibians, fish and reptiles as well as aquatic and migratory birds (grebes, teals, herons, ducks...) Fish species include carp and pike. Fishing is allowed at different points in the marsh