The banks of the Oca river Nature · Valle de Oca, Montes de Oca, Burgos Image of Valle de Oca, not of the place itself. The Oca River rises in the Montes de Oca, near Rábanos, and is the first tributary the Ebro receives after its source; it runs for 70 kilometres and flows from south to north through, among others, the six villages that make up the municipality of Valle de Oca: Cueva Cardiel, Villalmóndar, Villalbos, Villanasur Río de Oca —seat of the town council—, Villalómez and Mozoncillo de Oca. Its banks and tributaries form a Natura 2000 site, and its alder and poplar groves thread together the small hamlets of the municipality. Near its source the river narrows through the La Hoz gorge, once defended by Alba castle, today flooded by the reservoir of the same name, and further on it crosses the Valle de los Ajos amid oak woods. It should not be confused with its homophone, the Oka River in Biscay. Photo: Chebsoler (CC BY-SA 4.0) What to see and do in Valle de Oca