The Vallvidrera creek basin, due to its proximity to Barcelona, is a basin that lives under strong anthropogenic pressure. The different urbanizations of the Plains, the Floresta and the Rierada, the tunnels of Vallvidrera, the protection of the Park, the use of leisure... are conditions that denote an aspect specific to this stream.
The current source of the stream is inside a large pipe that was built parallel to the construction of the Vallvidrera tunnels. Thus, with an unknown origin, the first tributary is found: the Les Plaines treatment plant. The low chemical quality of these first meters will irreversibly mark the ecological state of the entire stream, which will improve to an acceptable quality. This improvement, however, will not happen until you reach the Can Busquets area where a riparian forest enables self-purification with the absorption of nutrients, the contribution of primary production and the creation of fluvial habitat for the development of fauna.
Once the stream reduces the initial organic pollution load, a structured food web appears with fluvial species of large predators such as the red-tailed barbel, the water snake or amphibians such as the frog, toad or salamander. Despite this new stability, new factors prevent the correct recovery of the ecosystem. On the one hand, the diffuse pollution that comes from the small urban centers that it passes through, which, despite having a low incidence, is aggravated in times of drought. The drought, however, is the last major problem of the stream since, despite being a temporary river, human extractions have a strong impact reducing its flow to 0.
The passage through agricultural areas reduces the quality of the riparian forest, despite the fact that, in central areas of the park, it enjoys protection. The figure of the Collserola park is important for the development of this forest typology and its connectivity with the adjacent forest. This fact significantly improves, as already mentioned, the ecological quality of the stream.
Finally, the stream reaches its final section, in its alluvial plain where an industrial estate awaits it that will dump its waste water and exterminate the entire riparian forest. The ecological improvement that the stream has achieved after an origin in a sewage treatment plant, is destroyed a few hundred meters before it is lost in the waters beyond the Llobregat