Reservoir of el río Bajoz Nature · Castromonte, Montes Torozos, Valladolid Image of Castromonte, not of the place itself. The reservoir of the río Bajoz, popularly known as the Castromonte marsh or the Santa Espina dam, is a sheet of water set between the hills and the páramo, beside the Cistercian monastery of Santa María de la Espina, in the municipal area of Castromonte. It is an earth-fill dam with a clay core and a capacity of 60,000 cubic metres, built on the upper course of the Bajoz, a river that rises near Castromonte and, after crossing the Montes Torozos and receiving the waters of several streams, joins the Hornija before flowing into the Duero. It is one of the few wetlands in these limestone hills, and its surrounding holm oak and oak woodland contrasts with the dryness of the cereal-growing dry farmland around it. Photo: Lourdes Cardenal (CC BY-SA 4.0) What to see and do in Castromonte