The surface sediment of the Barasona Reservoirwas clearly dominated by silts, representing more than 80% for most of the reservoir area (Table 2). The finest silts (i.e. from very fine to medium fractions) were the most abundant, accounting for more than 60% of the total. Clays were the second fraction, with values between 15 and 20% at the outlet of the rivers Isábena and Ésera (S1 to S4), and more than 20% at the samples located close to the dam (S9, S10). Sand was extremely scarce especially during the full-water period, when it was only found at S11 (the Salado tributary). Globally, the average size of particles (i.e., xg) decreased from the tail of the reservoir (S1 to S4) and from S11 towards the dam, obtaining there the minimum average value. Sediments were usually poorly sorted (3 b σg b 4), being very poorly sorted (σg N 4) at S11.