The Pontón de la Oliva is a Spanish dam, now in disuse, located in the Sierra de Ayllón, northeast of the Community of Madrid and northwest of the province of Guadalajara. It was built in 1857.
It is the sixth and last dam on the course of the Lozoya River, and the oldest of the entire system of dams and canalizations of the Canal de Isabel II, a network that supplies drinking water to Madrid and a large part of the community. The next oldest dam is that of Navalejos, located six kilometers upstream.
In 1848, when Isabel II reigned in Spain, Madrid had 206,000 inhabitants, a number that grew rapidly given its status as the capital of the Kingdom. Except for a minority of privileged people who had water in their residences, the rest of the Madrilenians were supplied with water that flowed from 54 sources and distributed 920 water tanks. This water came from the water journeys, which were a set of qanats or underground water channels built in the Middle Ages (they are mentioned in the jurisdiction of 1202) and subsequently extended until the 19th century.
Since the middle of the 18th century, half a dozen projects were being considered to bring water from the most immediate mountain rivers to Madrid.
On March 10, 1848, the Minister of Commerce, Instruction and Public Works, Juan Bravo Murillo, decided to face the problem. After some preliminary studies he signed a Royal Order appointing a commission to analyze existing projects.
In December 1848 the engineers Juan Rafo and Juan de Ribera presented the "Reasoned Report on the works necessary for the water supply to Madrid". In it, they laid the foundations for a supply project that included a budget and means of financing. They chose the Lozoya River as a source of supply due to the quality and purity of its water.
Rafo and Ribera designed a "Roman-style" supply system, with a 77 km long channel of rolling water. The main part of the project was the construction of a dam in the lower course of the Lozoya. The chosen place was a natural gorge called the "Cerro de la Oliva", a few hundred meters upstream from where the Lozoya River meets the Jarama River.