From a geological point of view, the Malpasillo reservoir is part of the Sub-Baetic unit of the Betic mountain ranges.
Although it is an artificial wetland area, it has become highly naturalised and is of great scenic beauty. The reservoir itself covers 87 ha, although the area protected as a Natural Park extends to 512 ha. Malpasillo is surrounded by very steep slopes, with gradients of up to 60% and is located on gypsum marl soil, which gives its waters a characteristic whitish color. The water comes from both rainfall and surface runoff and from streams and rivers, one of the most important of which is the Genil.
The fact that it suffers hardly any seasonal fluctuation in its water level, together with its level of silting up and shallow depth, means that it has a very well-developed belt of marsh vegetation, where reeds, rushes and bulrushes can be found. You can also find tamarisks and some willows, poplars and elms. The presence of this vegetation makes it a site of great importance for the breeding of water birds such as white-headed ducks, swamphens, red-crested pochards, little grebes, coots, moorhens, common pochards and mallards. During their breeding season, this site is also visited by numerous species of ardeidae, including the purple heron, grey heron, cattle egret and little bittern. The reservoir, surrounded by agricultural land, is used as a wintering site and stopover point by a large number of ducks, and also has a major population of waders that can be observed at different times of the year.
Due to its great ornithological and ecological interest, this area boasts the international protection status of ZEPA zone and RAMSAR wetland.