Sedimentological and geomorphological features of Mylopotas and Manganari beaches in Ios Island are investigated in order to determine seasonal changes in the texture of coastal sediments and shoreline position caused by both natural processes and human activities. The fieldwork was conducted in April 2018 and September 2018 including mapping of beach morphology in seasonal scale through topographic monitoring of shoreline, coastline and fixed cross sections in the sites under investigation using a differential GPS. In addition, sediment sampling was carried out along selected cross sections in beaches of Mylopotas (three) and Manganari (two). Grain size analysis and statistical processing were realized to reveal spatial and temporal changes of sediment parameters, such as sorting, skewness, mean and kurtosis. The results indicate a rather homogenous grain size distribution at each study area, with the majority of samples being classified as slightly gravelly sand. DGPS

measurements of shoreline position in Manganari Beach indicate changes that vary between 2.50 m and 4.70 m, with the maximum displacement to be observed in the southern part of the beach, where the shoreline is retreated during spring period. Seasonal variations of shoreline position in Mylopotas Beach are up to 4.50 m with the shoreline to be advanced during spring period in the southern and northern part of the beach, while the minimum changes are measured in the central part, where a beachrock formation occurs. Occasional human interferences, such as small-scale beach restoration and nourishment projects, installation of leisure facilities and reshaping of sand dunes, conducted mainly during spring period, alter the coastal sediment budget and transport reinforcing shoreline retreat.