Variazioni morfologiche recenti di due alvei ghiaiosi appenninici: il Fiume Trebbia ed il Fiume Vara

Recent morphological changes of two Apenninic gravel-bed rivers: the Trebbia and Vara Rivers, Italy

Abstract

The morphological changes that occurred during the last two centuries along the lower reaches of the Trebbia and Vara rivers are analysed, showing similarities with the recent channel evolution of many gravel-bed rivers with an initial braided morphology draining from the Apennines and Alps. From the first decades to the second half of the 19^th century, the two rivers have undergone channel narrowing (between 10 and 25% of the initial channel width) and partial reduction of the braiding characters. These changes are related to a first period of decrease in sediment supply mainly due to changes in land use (reforestation) in the upper portions of the drainage basin. Climate changes, specifically the end of the Little Ice Age with possible effects in reducing sediment supply to the river channels, could be an additional factor in explaining channel changes during this first period of adjustments. This phase of channel narrowing, combined with a quite limited bed incision, continued with approximately the same trend until about the first half of the 20^th century, and was also related to the construction of dams (starting from the 1920s) and of groynes in the alluvial plain (in the case of the Vara R.). A second phase of more relevant channel adjustments occurred during the second half of the 20^th century, as a consequence of intense sediment mining activity (particularly between 1960 and 1980) added to the effects of dams and further reforestation. As a result, a second distinct phase of channel narrowing and a significant channel bed incision (probably higher along the Vara R.) are observed. Finally, recent data relative to the last 10 years suggest the starting of an inversion of the trend of channel changes, with a present phase of channel widening, at the scale of the whole study reaches, combined with an aggradational phase, the latter observed in some reaches along the Vara R. (promoted by the presence of grade control structures) and only locally along the Trebbia R.