Water and river systems in the Sierra Norte de Sevilla Natural Park
The impermeability of the ground means that practically all rainwater runs on the surface as opposed to disappearing underground. All of the 3 largest rivers in the area, the Viar, Huéznar and Retortillo are tributaries of the Guadalquivir, running gently down to the largest river in the region to the south of the Sierra Norte de Sevilla Natural Park, eventually flowing out into the Atlantic Ocean between the provinces of Huelva and Cadiz. The Viar is 133 kilometres in length and drops some 810 from its highest to its lowest point, passing through the El Pintado reservoir. The Rivera del Huéznar is 65 kilometres long, of which half is within the Natural Park, and whose source is in the village of San Nicolás del Puerto, where the water can be seen to appear from below ground in an attractive pool. Nearby is a waterfall classified and protected as a Natural Monument. Of its total length of 49 kilometres, 16.5 of the river Retortillo run through the Natural Park, ending at the Retortillo reservoir, where it is possible to enjoy water based activities.