The terraces of the Piave River in the Vallone Bellunese (Eastern Alps, Italy)

Abstract

The terraces of the Piave River in the Vallone Bellunese (Eastern Alps) are the focus of this study: they represent the stream evolution after the last glacial period (Wiirm) in this region. This paper deals mainly with three topics: (1) methods and problems commonly faced in the study of fluvial terraces, (2) genesis of the terraces, and (3) geomorphic processes during the Late glacial-Holocene in this part of the alpine region. As for methods, the fundamental value of procedures such as identification of the flood plain, determination of terrace elevation, and correlation of terraces are discussed. In the Vallone Bellunese there are six levels of terraces of the Piave River. The highest terrace is a fill terrace: the end of the valley filling, on the basis of some numerical ages (radiocarbon and thermoluminescence datings), can be ascribed to the Early Holocene. This terrace is a climatic terrace because its formation is due to the transition from a glacial to a non glacial period. The genesis of the lower terraces is more difficult to explain. They could be complox-response terraces, or their formation could be also explained with lateral shifts during the downcutting of the river. In both cases formation of the lower terraces would not require a change of an external variable of the system (climate or tectonics). The dynamics of the Piave River after the last Würm glaciation can be better explained if a paraglacial period is assumed. This implies a revision of previous models which referred to anaglacial and kataglacial period. Changes of sediment yield are probably much more important than changes in precipitation, in terms of both magnitude and their effects on river dynamics. A progressive reduction of sediment yield from deglaciation time to Early Holocene is the main cause of the change, from aggradation to degradation, in the modes of stream operation.