The Kotychi lagoon is located on the northwest Peloponnese Greece, along a wave dominated and microtidal coast. The lagoon is one of the most important ecological areas in Greece, with international significance as it is protected by the Ramsar International Convention and is listing in the Natura (E.C.) Catalogue. For the present study 59 surficial samples were collected from the bottom of the lagoon, on a grid basis (approximately 300 m spacing) using a grab and were analysed for their grain size distribution as well as grain size parameters, mean, variance and skewness. Using the moment measures, grain-size trend analysis was made and the definition of trend vectors were estimated. Grain-size trend analysis indicates that the sediments of the lagoon are mainly sandy mud. Application of trend analysis indicates that the sediment pathways along the lagoon can be related to a) the sediments source area of the lagoon (small river supplies), b) wind related water circulation and c) the NW and SW wind directions. Moreover, comparing 1990 and 2000 satellite images we have detected three major areas where the shallow waters show a significant increase: area (a) in the northeast part of the lagoon, area (b) in the middle-west part and area (c) in the southern part. All these changes are related to river discharges and to the winnowing effect. Detection of changes in the coastline of the lagoon, showed a total reduction of areas covered by water by up to 13.8 %. The biggest change was detected in the northeast and in the southeast area of the lagoon and is related to the sediment supply from the periphery of the lagoon and to a minor degree to the bottom sediment pathway and winnowing effect. The rate of change of the area covered by water shows a rapid filling up of the lagoon and that additional measures have to applied for the protection of the lagoon ecosystem.