Clima e attività umane come cause dei cambiamenti fluviali - il caso del Fiume Po

I cambiamenti nella dinamica fluviale possono essere imputati a cause diverse; tra queste le principali sono sicuramente quelle climatiche e quelle antropiche. Spesso i cambiamenti sono repentini anche se la loro preparazione è avvenuta in tempi molto lunghi; di fatto in queste fasi preparatorie, il sistema fluviale si è avvicinato a soglie limite che una volta superate hanno provocato una brusca accelerazione nei processi di erosione o di sedimentazione fluviale. I più conosciuti effetti sull'ambiente fluviale che si manifestano in queste occasioni sono, infatti, inversioni di tendenza tra fasi di sedimentazione e fasi di erosione. L'incisione o la sedimentazione nei corsi d'acqua dipendono dal rapporto tra portata solida e portata liquida nel fiume pur non dovendosi trascurare altri fattori come ad esempio variazioni del livello di base o cause tettoniche. Quando il rapporto è spostato verso la portata solida risulta fondata la possibilità che si verifichino processi di aggradazione, mentre se il rapporto è spostato verso la portata liquida aumentano le probabilità che si manifestino processi erosivi.
Durante l'evoluzione recente della Pianura Padana si assiste a momenti contraddistinti da forte sedimentazione, ad esempio quello verificatosi durante l'Ultimo Massimo Glaciale, e da momenti di intensa erosione, come ad esempio ai nostri giorni oppure durante il Tardiglaciale. Alcuni di questi eventi sono chiaramente influenzati da cause climatiche (fasi glaciali e deglaciazione) altri da cause diverse tra cui quelle antropiche sono spesso preponderanti.
In questo lavoro si cercherà di definire alcuni di questi momenti significativi nell'evoluzione postglaciale della Pianura Padana e di analizzare anche situazioni in cui entrambe le cause si sovrappongono e si mascherano reciprocamente.

Changes of fluvial dynamics in a plain are due to many causes. It is so difficult to evaluate one as the main cause of the environmental changes in a fluvial plain and in its hydrographic network. Rivers, in fact, can suffer rapid and significant changes in their behavior as a consequence of also moderate but extended variations of hydrological parameters that can be recognized only by instrumental measures.
Perhaps the fluvial system, as many other natural systems, can maintain, between characteristic thresholds, a certain energetic inertia in its behavior. When a threshold is exceeded fluvial processes change quickly and fluvial landforms may be strongly modeled.
In the fluvial system events that produce more rapid and disturbing transformations there are those that induce erosion rather than sedimentation. Historically in fact, exaggerated sedimentation problems have been impossible to manage during several decades when man has constrained rivers in fixed tracks between narrow embankments. Before systematic human management of the fluvial system, aggradation phases spread out into large areas while erosion phases are always concentrated along riverbeds. Incision or aggradation depends on ratio solid/water discharge in a riverbed. The more the ratio is unbalanced towards sediment availability, the higher the sedimentation rate is; on the contrary, scarceness sediment availability induces a tendency to erosion. Obviously, total discharge and velocity play an important role. In fact a river inclined to erosion processes but characterized by scarce total discharge and velocity is not able to evacuate large gravels or large amounts of sediment in a short time.
The present day generalized erosion phases in northern Italy are considered the consequence of the growing human impact after the unity of Italy. On the contrary in the past, when human impact was less important, fluvial dynamic was principally controlled by natural causes. Among them, climatic changes have been the most common cause of fluvial modifications.
In the Po Plain, the climatic change at the end of the Last Glacial Maximum (LGM), which caused the changes in fluvial dynamics, was the last largest formative event.
In the northern foothills, Late Pleistocene palaeochannels indicate several cases of underfit streams among the northern tributaries of the Po River. On the other hand, on the southern side of the Po Plain no geomorphological evidence of similar discharge reduction has been found. Here stratigraphic sections, together with archaeological remnants buried under the fluvial deposits, show a reduction in the grain-size of fluvial sediments after the 10th millennium B.C. During the Holocene, fluvial sedimentation became finer, and was characterised by minor fluctuation in the rate of deposition, probably related to shorter and less intense climatic fluctuations.
Given the high rate of population growth and the development of human activities since the Neolithic Age, human influence on fluvial dynamics, especially since the Roman Age, prevailed over other factors (i.e. climate, tectonics, vegetation, etc.). During the Holocene, the most important changes in the Po Plain were not modifications in water discharge but in sediment supply. At present, abandonment of the mountainous region leading to reafforestation and artificial control in the mountain sector of the basins but also in-channel quarrying (now illegal but very intense in the 1960s and 1970s) are causing erosion along rivers and large sectors of the Adriatic coast. These changes are comparable to those occurring in basins of other Mediterranean rivers.