The urban expansion of Teresina has caused environmental impacts on the Poti River due to the occurrence of eutrophication and proliferation of aquatic plants in the dry season of the year. Considering the characteristics related to water-quality monitoring in the Northeast region of Brazil, from remote sensing, this study aimed to evaluate the performance of semi-empirical algorithms in Sentinel-2 data in the detection and mapping of eutrophication and aquatic plants in the river Poti in Teresina, Piaui, Brazil. The eutrophication detection methodology involved the study of the reflectance of the water surface in the Sentinel-2 images and the respective correlation within situ data of chlorophyll-a, using the MCI, MPH and NDCI indexes. The NDCI index showed superior one-off performance than the MCI and MPH indexes. In this sense, with the NDCI, the spatio-temporal variation of eutrophication in the Poti River was identified broadly and no longer specific. In relation to aquatic plants, the NDVI index proved to be appropriate for the detection and mapping of water hyacinths, demonstrating the location of the areas covered by aquatic plants. At the beginning of proliferation, the maximum expansion rate of 854.7% was evaluated. At the end of the dry period, there was a peak of covered area of 570,145.6 m² and a production of 6,408.4 tons of fresh biomass from water hyacinths. Therefore, in both cases, it was found that the MSI sensor was suitable for the detection, mapping and monitoring of eutrophication and aquatic plants in the Poti River.