The central area of the Seville’s Sierra Morena is drained by the Rivera del Huéznar, one of the three main rivers of the extensive hydrographic network of this mountain range, and the only one that flows entirely within the province, from its source in San Nicolás del Puerto to its mouth in the Guadalquivir, in the municipality of Tocina.
The source of the Rivera del Huéznar takes the form of an upwelling of water in a place known as El Venero or Borbollón. The water, which emerges from the ground at various points, is rich in calcium bicarbonate, due to its prior underground circulation through carbonate rocks. The upwelling, with a flow that has sometimes exceeded 500 litres per second, is the main discharge point of the eastern zone of the karstic aquifer of Guadalcanal-San Nicolás del Puerto, one of the three large aquifers of the Seville’s Sierra Morena .
The area of the source is very shady, surrounded by a large mass of trees covered by climbing plants, forming a dense green cover that provides shelter to a variety of birds. The main tree species are alder, elm and poplar.
The areas near the Rivera del Huéznar have been the site of human settlements since prehistoric times, and there are some archaeological remains that bear witness to this, such as the megalithic site of Huerta Nogales.
The river has traditionally been used for different purposes such as flour mills, cork mills and fulling mills. The latter were used until the mid-19th century for the transformation of wool into a denser fabric by washing it and bludgeoning it in machines driven by waterpower. San Nicolás was home to the Batán de las Monjas (Nun's Fulling Mill), now converted into a rural tourism complex.