Downstream variation in grain size along an Alpine river: analysis of controls and processes

Downstream variation in bed material size was studied along a 115-km reach of the Piave River (Eastern Alps, Italy). This river drains a region producing high sediment yields and has a braided pattern along half of its length. During the last century, the river system has been affected by several human impacts (hydroelectric dams, diversions, gravel mining) which have strongly altered flow regime and sediment dynamics. The aim of the study is to analyse the changes of bed material size in the downstream direction and to explain these changes in the light of both natural and anthropogenic factors. Surface material was sampled at 35 sites using the grid-by-number method. The collected data give a detailed picture of the downstream variation in sediment size, which is characterised by a complex pattern rather than by a simple decreasing trend. An explanation of this variation was found through identification of discontinuities (significant steps in the downstream trend) and factors which can determine changes in sediment texture. Both natural (lateral sediment sources) and anthropogenic factors (e.g. barrages) are significant in this river system, and they provide an explanation for most of the observed discontinuities (seven out of eight discontinuities). Few tributaries disrupt downstream fining, while, as expected, the barrages produce important changes in sediment texture. Fining processes were investigated in the lower part of the study reach where the lateral sediment sources and the barrages have minor effects on the bed material. Exponential models provide acceptable explanations of the downstream variations and give fining coefficients equal to 0.027 and 0.028 km1, respectively, for D50 and D84. Evidence suggests that both abrasion and sorting are important in determining the downstream fining observed, but it is likely that sorting is the dominant process.