The Embalse de Valmayor, or Valmayor Reservoir, is situated to the northwest of Madrid in the municipality of Valdemorillo. It is managed by the Canal de Isabel II, and belongs to the basin of the Guadarrama River. The reservoir, which runs along a length of eight miles upstream of the Aulencia River, occupies an area of 755 hectares and also serves the towns of El Escorial and Colmenarejo. In terms of the amount of water stored, the Embalse de Valmayor is the second largest reservoir in the province of Madrid, with a capacity of 124.4 million cubic meters. The largest is the Atazar Reservoir, located in the basin of the Lozoya River and boasting a whopping capacity of 425.3 million cubic meters.
Despite being built on the Aulencia River, the Embalse de Valmayor receives its largest contribution of water from the Guadarrama River, located several kilometres away and possessing a much higher rate of the flow of than the Aulencia. Water from the Guadarrama reaches the reservoir through a transfer tunnel, about five miles long, which has a small reservoir sensor called Las Nieves, located near La Navata, a residential area near the city of Galapagar.