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Although freshwater only represents a tiny fraction of the entire hydrosphere, and only part of it flows in watercourses on the surface, rivers are perhaps the most active and forceful of agents modelling the landscape. Aquatic plants resist sometimes strong currents by rooting tenaciously in the riverbed.
Along the banks, sedges and reeds soon make way for woody vegetation dominated by willows, alders or sea-buckthorn. Rivers and their banks provide habitats for many species of animals - aquatic, amphibian and terrestrial – but they are also the route taken by fish like eels and salmon, on their migrations to and from the sea. Riverbanks and riparian woodlands are also popular haunts of both resident and migrating birds.
Levees, dams and weirs, channelling, rerouting and diversion of watercourses and, lastly, a wide array of urban, agricultural and industrial pollutants, all discharged into watercourses, have now profoundly altered Italian rivers and their floodplains, weakening the capacity for self-purification of their waters and affecting the recharging capacity of the water table. The flora and fauna of rivers and riverbanks are also threatened by the continual introduction of often highly competitive non-indigenous species.
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