Remote sensing is a powerful tool for assessing environmental changes in urban areas and identifying anthropogenic impacts on water bodies. This study evaluated the loss of the surface area of the Ibirité Lagoon, a reservoir of approximately 1.5 km² located in the metropolitan region of Belo Horizonte, Southeast Brazil. We used multitemporal data from the MapBiomas Project to analyze land use and cover classes (natural vegetation, pasture, urban, and agriculture). Using Landsat satellite images to delineate the water surface area, we calculated the NDWI (Normalized Difference Water Index). The analysis covered 1985, 1990, 1995, 2000, 2005, 2010, 2015, and 2022. Findings showed a 38% reduction in the reservoir's surface area, primarily due to siltation and macrophytes presence in the inlet regions of the main tributaries. Over the same period, natural vegetation decreased by 16%, agricultural areas dropped by 28%, pastures declined by 50%, and urbanized areas expanded by 320%. Spearman's correlation analysis showed a strong relationship between land use evolution and the reduction of the water surface area (rho > 0.90). The reduction was positively correlated with vegetation loss, while the expansion of urban areas was directly associated with the decline of reservoirs. Analyzing transitions in land use and cover, we found that the conversion of natural vegetation to urban areas was the primary driver of water surface loss (rho = -0.68) throughout the study period. These results underscore the importance of adopting territorial planning strategies that prioritize green area conservation and mitigate the adverse effects of unplanned urbanization on water resources.