BED-LEVEL ADJUSTMENTS IN THE ARNO RIVER, CENTRAL ITALY

Two distinct phases of bed-level adjustment over the past 150 years ar e identified for the principal alluvial reaches of the Arno River (Upp er Valdarno and Lower Valdarno). The planimetric configuration of the river in these reaches is the result of a series of hydraulic works (c analization, rectification, artificial cut-offs, etc.) carried out par ticularly between the 18th and the 19th centuries. Subsequently, a ser ies of interventions at basin level (construction of weirs, variations in land use), intense instream gravel-mining after World War LI, and the construction of two dams on the Arno River, caused widespread degr adation of the streambed. Since about 1900, total lowering of the chan nel bed is typically between 2 and 4 m in the Upper Valdarno Reach and between 5 and 8 m in some areas of the Lower Valdarno Reach. Bed-leve l adjustments with time are analyzed for a large number of cross-secti ons and described by an exponential-decay function. This analysis iden tified the existence of two main phases of lowering: the first, trigge red at the end of the past century; the second, triggered in the inter val 1945-1960 and characterized by more intense degradation of the str eambed. The first phase derived from changes in land-use and land-mana gement practices. The second phase is the result of the superimpositio n of two factors: intense instream mining of gravel, and the construct ion of the Levane and La Penna dams. (C) 1998 Elsevier Science B.V.