Evoluzione olocenica della pianura alluvionale e dell’alveo del Fiume Sieve nel Mugello (Toscana)

Alluvial plain and channel evolution of the River Sieve in Mugello (Tuscany, Italy) during Holocene

Abstract

In this note the main results of a research concerning the evolution of both alluvial plain and channel of the River Sieve in the Mugello Valley (Tuscany, Italy) are presented. The Authors analyse the relationships between the dynamics of a typical inter-appenninic alluvial plain and the human disturbances linked to land use in the surrounding areas, which have been influenced, in their turn, by the climatic variations occurred during Holocene. In the past centuries, and in particular during a period in which intense deforestation coincided with a prolonged climatic crisis (the Little Ice Age), a considerable aggradation of the alluvial plain took place. Following this phase of continual deposition, a rapid channelbed lowering occurred during last decades, related to instream gravel mining and, in the least, to decreasing of sediment supply caused by reforestation and upland sediment retention. Recent dynamics of both channel and banks is analysed on the basis of historical maps, aerial photographs, topographic surveys and detailed geomorphological mapping. The morphological variations occurred and related processes are interpreted as a result of a sequence of stages of both channel-bed and bank-slope evolution. Degradation of the channel-bed caused bank instability and widening, followed by a progressive stabilization, promoted by establishing vegetation.