The Castaños river is the backbone of the basin and the valley of El Regato/Errekatxo. Although several of its sections are channeled or dammed, we can still find a small representation of the riparian forest.
The fluvial system, formed by the river and the riparian forest, is one of the ecosystems most transformed and degraded by humans.
The riparian forest or alder grove favors the oxygenation of the river water and protects the banks from erosion. It is essential for the river to maintain good ecological functions. There are many living beings that depend on the river and its riparian forest to live. In addition to emblematic and endangered species such as the spiny and the eel, we have others as striking as dragonflies and damselflies, small as the kingfisher, large as herons and some stealthy as the polecat.
The alder forests constitute the mature stages of the Euro-Siberian riparian and transitional forest series. This habitat is catalogued as a Priority Habitat of Community Interest by the European Directive. Within the municipality it occupies a small area (1.23 ha) in the Agirre stream, in a small ravine on the northern slope of Mount Argalario.
The alder groves are shady forests, of complex structure, with a great variety of tree and undergrowth species.
The forest is distributed in bands that are distanced from the course of the river according to the lower tolerance to flooding of the tree species, alder being the dominant species. The common willow colonizes the unstabilized banks, slopes and flood or flood zones. Other species that accompany the alder, moving away from the riverbed, are ash and oak. We also find hazelnut, dogwoods, arraclanes, ivy and hops.
The spiny dogfish, a species listed as Vulnerable in the Basque Catalogue of Threatened Species, is found mainly in the section of the river near the Gorostitza neighborhood, between Tellaetxe Park and the Gorostitza reservoir.
Idea and design: Equinoccio Natura, S.C.
Photos: Jon Maguregi
Draughtsman: Oscar Domínguez