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

Abstract

: Marchetti M., Climate and human activities as causes of fluvial modifications - The case of the Po Plain. (IT ISSN 0394-
3356, 2008).
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.