The San José Reservoir is a sheet of water created by the dam of the same name on the Duero River, in the municipality of Castronuño, in the southwest of the province of Valladolid. It is a gravity dam linked to hydroelectric exploitation, with an approximate surface area of 192 hectares and a capacity of 5.5 hm³, which has transformed the riverbed into a very recognizable riverside landscape and wetland. From the viewpoints and trails in the area, one can appreciate the great curve of the Duero, the reed beds, the banks of riverside vegetation and the calm waters that attract waterfowl and fauna associated with wetlands. The viewpoint of the reservoir allows a good understanding of the relationship between fluvial engineering and natural landscape, with the meander partially retained by the dam and an open horizon towards the fertile plain. The enclave is part of the Riberas de Castronuño-Vega del Duero Nature Reserve, the only protected natural area in the province of Valladolid, which reinforces its scenic and ornithological interest. In the technical documentation of the Duero it is also described as a very remarkable stretch of the middle meander of the river, partially flooded by the reservoir. The best visit is usually made in soft light, at the beginning or end of the day, and in spring or autumn, when the riverbank is more colorful and birdwatching is more comfortable. In summer the walk can be very hot in the central hours.
