Alto Eresma River Nature Reserve

Alto Eresma River Nature Reserve protects the upper stretch of the Eresma River, designated for its high natural value, within the province of Segovia in the municipality of Real Sitio de San Ildefonso. The area sits on the north-western slope of the Sierra de Guadarrama, part of Spain's Central System, roughly 50 kilometres from Madrid, and forms part of a wide mountain cirque that opens northward toward the plains of Segovia.

Where does the Eresma River begin?

The Eresma forms at the confluence of three streams fed by numerous springs on the slopes of the Sierra de Guadarrama. Before taking its final name, the river flows as the río de Valsaín — only below the village of Valsaín does it become the Eresma. It is precisely at this point that the river crosses one of its coolest and most mountainous stretches, carving out natural pools and backwaters that have long been used by locals for bathing.

What does the reserve's landscape look like?

Across the middle and lower parts of the valley, the landscape is dominated by forests of Scots pine, oak groves and holm oak thickets, accompanied by typical riverside vegetation that lines the Eresma's banks in bands of shade and moisture. This mosaic of forest and river creates a habitat of considerable ecological importance, setting this stretch of the Sierra de Guadarrama apart from the drier terrain surrounding it.

Which species live here?

The reserve is known for hosting species threatened with extinction that have become emblematic of the region, including the Iberian imperial eagle, the black stork and the black vulture, all of which can be seen circling the valleys in search of prey. Among mammals, the otter stands out as an indicator of clean water, while the rivers and streams of the upper Eresma are home to trout, a species that depends on cold, well-oxygenated water.

What is the wider conservation context?

The upper Eresma lies within the boundaries of the Real Sitio de San Ildefonso–El Espinar Biosphere Reserve, which encompasses the highest peaks of the Guadarrama range along with the surrounding forests and valleys. This combination of high-altitude springs, pine woodlands and river pools makes the area valued both by naturalists and by visitors seeking contact with nature away from the bustle of nearby Madrid.

Frequently Asked Questions

Where exactly is the reserve located? In the province of Segovia, within the municipality of Real Sitio de San Ildefonso, on the north-western slope of the Sierra de Guadarrama.

Which animals are characteristic of this area? The Iberian imperial eagle, black stork, black vulture, otter, and trout living in the waters of the Eresma.

Can you swim here? The stretch of river near Valsaín has traditionally been used by locals for bathing in the natural pools formed by the river.

How far is it from Madrid? The area lies approximately 50 kilometres from the Spanish capital.