 This research compared the capabilities of Sentinel-2 and Landsat-9 satellite data to map water bodies in the lower Tegas River, Portugal. It found that both satellites provided similar results, but that the accuracy of the mapping was dependent on the specific index used. Therefore, it is recommended that the index-specific classification decision values should be taken into account when making decisions about the quality of the information extracted. This article was authored by Rony Verardo and Isabel P. DeLima.