Evoluzione morfologica di alvei fluviali mobili nel settore occidentale del Bacino Padano (River channel adjustment in the Western Po System (Northern Italy))

The research on channel changes in Italian rivers has pointed out processes of incision and narrowing during the last fifty years. The definition of the causes and the practical implications of such variations has stimulated a wish to know more about this phenomenon. As study cases for analysing the trends in the western sector of the Po system, three Po River tributaries have been selected: the rivers Stura di Lanzo and Orco, both from the Alps, and Trebbia, from the Apennines, flowing into the Po River, respectively at 205 m, 177 m and 43 m a.s.l.. Through GIS procedures (for Orco and Trebbia) and digitizer elaboration (for Stura di Lanzo) and using a topographic map on a 1:10.000 scale as a base layer, the morphological changes of the river channel have been deduced comparing historical maps dating from the end of the 1880s to the 1950s (on a 1:25.000 scale), aerial photographs dating from the 1950s (at scales ranging between 1:45.000 and 1:5.000) and satellite images (70 cm resolution); topographic observation and field surveys have allowed us to verify the present situation. The Stura di Lanzo River develops a braided reach 10.5 km long in the upper alluvial plain. The braided pattern was modelled in coarse gravels. The largest narrowing modifications took place until the 1990s, due to sediment gravel mining. This adjustment caused a width reduction of 80% (1881-1989) along the braided reach that was previously 600 m wide and a 4 m deep channel incision that locally downcuts the fine sediments of buried lacustrine deposits. A local single channel modelling has definitively started since the 1980s, due to the fact that the fine sediments in the bed are unable to maintain the former braided pattern. The studied reach of the Orco River (25 km long, in the middle-lower plain), is incised into a fluvioglacial fan and the Po alluvial plain. Over the 19th century the river channel showed a multi-thread morphology, with a braiding intensity variable in space and time. The progressive simplification of the pattern, already observed since the end of the 19th century, has undergone a diffuse acceleration starting from the 1950s (average reduction of 60% in width over the 1881-1990 period). The analysis of human intervention and floods pointed out two opposing roles: the role of man in starting or accelerating the process of the river pattern simplification; the role of the floods in attempting to restore the previous pattern in the plain. The investigated reach of the Trebbia River, 31 km long, is characterized by a braided morphology. A continuous narrowing of the channel has appeared since 1877 (average reduction of 65% in width over the 1877-1990 period). This narrowing reached reductions of 58% between 1954 and 1990 (during the period of most intense gravel mining) and was accompanied by a channel incision of 2-4 meters. In all studied rivers the channel narrowing, which occurred up to 1990, was characterized by a lengthening of the channel axis. Starting from 1990, the rivers showed a reversal of the previous trend, because channel widening took place, concomitant to a shortening of the channel axis. In the alpine watercourses the opposite trend could be a result of the extreme flood that occurred in 2000; in Trebbia River, the almost total cutback of large-scale gravel mining must be considered.