|Area 8 ha||Established April 11, 2002|
|Location Province of Valladolid,
Castile and Leon,
Nearest city Castronuño, Pollos, Torrecilla de la Abadesa and Tordesillas
Governing body Junta de Castilla y León (Government of Castile and León)
Nearest cities Castronuño, Pollos, Torrecilla de la Abadesa, Tordesillas
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. This is the only natural area protected in this province. Also it is known as the Gran Florida del Duero (the Great Florida of the Douro) .
Is located in an area of moorland and sedimented banks. Therefore this is an area of nesting and migratory water birds. 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 Duero 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 sediment were flooded an changed to fertile field on the riverbank, which were colonized little by little by large masses of the reedbed, maintained by the constant level of the reservoir. The greater was the growing mass of the reedbed attracted more birds specializing in the life of the wetlands. It is a semi-natural wetland.
The nature reserve was considered a Special Protection Area for Birds in 1992 and Strict Nature Reserve in 2002.