The Natural Reserve on the banks of Castronuño-Vega del Duero is a nature reserve situated in the western part of the province of Valladolid, autonomous community of Castile and Leon, Spain.

The Duero is surrounded by forests of Ribera and constitutes a very interesting swamp ecosystem.

It is an area of great plains and absence of high elevations In the 1940s a dam was constructed across the Douro River as part of a hydroelectric plant.

Upstream, in the municipal district of Castronuño, was created the reservoir of San Jose, which was what brought about the formation of this reserve, which constitutes a unique ecosystem with biotic elements and landscape.

Many sediments were flooded and changed the riverbank to fertile fields, which were gradually colonized by large masses of the reedbed, maintained by the constant level of the reservoir.