Water and river systems
Erosion by water flowing over and through the rock in the area has modelled the countryside. The low impermeability of the ground has led to an extensive river system similar to the branches of a tree. There are three major water basins, flowing into the Guadiana, the Guadalquivir and the Odiel rivers, which flow throughout the year: the remaining rivers in the area are practically dry in summer.
Although just outside of the Sierra de Aracena Natural Park, it is worthwhile mentioning the Río Tinto (“Red River”), so called because of the colouring of the water that passes through the famous mining area, picking up a variety of metals in suspension, and whose oxygen level is so low it has been visited by the NASA authorities for investigation into similarities with Mars.