The objective of the research was to examine the effects of the Ásványi estuary sill located in the lower inter-island area of the inter-island wave field water replenishment system on water surface coverage using remote sensing data and the Normalized Difference Water Index (NDWI). The analysis covered the period from 2013 to 2023 and was based on Landsat 8 OLI satellite imagery, processed in QGIS. The research focused on monitoring temporal and spatial changes in water surface area and on comparing conditions before and after the construction of water-level control structures. Based on the results obtained, it can be concluded that the operation of the Ásványi estuary bottom sill and the lower-island water replenishment system had a positive effect on water surface coverage. The NDWI maps clearly show that the change was most noticeable in the tributaries close to the structures, where the riverbed widened. The spatial variations in water surface area observed during the study period indicated that many factors, including climatic, hydrological, riverbed morphological, and hydro-meteorological parameters, can influence changes in water surface area.