This type of environment includes water bodies with very different characteristics, only united by the presence of running waters. In fact, starting from a tumultuous torrent that breaks out from the glacial front, or from a source of high altitude, and following the path until it becomes a murmure torrent of mountain valley, and then a placid course that plows a main valley, you can check a gradual and profound change in the characteristics of the water flow, the slope and the type of substrate, and therefore of the vegetation of the banks.
In general it can be said that along the way a watercourse faces a progressive enrichment of its biological community, both in terms of fauna and flora.
One type of current water environment that escapes the longitudinal classification is the piedmont resurgence. Its very particular characteristics, such as the brevity of the course or the origin of low altitude sources, prevent it from being included in the source-torrent-river scheme.
The flow of water at the base of large mountain bodies, after long underground journeys, gives these waters some characteristics that strongly influence the biological community.
The water, which usually flows at the base of large rock faces at the intersection with the plain or valley floor, in riverbeds often regularized by the work of man (these are the so-called pits), has a constant temperature and chemical characteristics throughout ‘year.
The dissolved salts are very poor because of the very short distance traveled on the surface. The very high transparency and the constancy of the flow allow the development of a thriving and varied submerged vegetation, which is also the main food source for many invertebrate organisms.
These, in turn, are the essential source of food for the particular fish population of the resurgence.
When the torrents reach the valley bottoms and those of the main valleys, the impetus of the water decreases considerably and so is its ability to reorganize the debris and rejuvenation of the riverbed: on the shore you can thus establish various and fairly stable plant communities .
Although the grains are mostly dry during the summer, the plants that grow there may have water present among the alluvial materials, as the water table is always close to the surface. These phytocoenoses are usually characterized by the presence of narrow-leaved willows, mainly Salix purpurea , S. eleagnos , S. viminalis and S. triandra , all species that usually assume a shrub or low tree bearing.
Among the willows grow various pioneer plants, such as Glyceria plicata , Agrostis stolonifera , Phalarisarundinacea , Calamagrostis epigejos , Petasites hybridus and Tussilago farfara . In water, in stretches with weak current, watercress ( Nasturtium officinale ) is frequent .
The willow willow salix ( Salix eleagnos ) may be considered a precursor stage of the white alder forest, however the phytocenoses of the gravels rarely evolve into arboreal formations, since the floods periodically reshape the river bed by obliterating or almost the vegetation.
The white willow woods ( Salix alba ) represented the typical riparian vegetation of the rivers, but they were almost totally destroyed by capillary interventions of embedding and canalization of the waters; these have not spared even the secondary courses, deeply distorting the characteristics of their precious flora.