###### Abstract
Currently, several online data hubs, such as the Database for Hydrological Time Series of Inland Waters (DAHITI) ([[https://dahiti.dgfi.tum.de/en/](https://dahiti.dgfi.tum.de/en/)]([https://dahiti.dgfi.tum.de/en/](https://dahiti.dgfi.tum.de/en/))) [28], Hydroweb ([[http://hydroweb.theia-land.fr/](http://hydroweb.theia-land.fr/)]([http://hydroweb.theia-land.fr/](http://hydroweb.theia-land.fr/))) [29], Global Reservoir and Lake Monitoring ([[https://ipad.fas.usda.gov/cropexplorer/global_reservoir/](https://ipad.fas.usda.gov/cropexplorer/global_reservoir/)]([https://ipad.fas.usda.gov/cropexplorer/global_reservoir/](https://ipad.fas.usda.gov/cropexplorer/global_reservoir/))) [30], and Thematic Exploitation Platform (TEP) for Hydrology ([[https://hydrology-tep.eo.esa.int/](https://hydrology-tep.eo.esa.int/)]([https://hydrology-tep.eo.esa.int/](https://hydrology-tep.eo.esa.int/))) [31], also provide time series of water levels over large lakes and rivers. The web platform named SAR Versatile Altimetric Toolkit for Ocean Research & Exploitation (SARvatore) ([[http://gprod.eo.esa.int/services/CRYOSAT_SAR/](http://gprod.eo.esa.int/services/CRYOSAT_SAR/)]([http://gprod.eo.esa.int/services/CRYOSAT_SAR/](http://gprod.eo.esa.int/services/CRYOSAT_SAR/))) can also provide Level-2 water level product of Sentinel-3 by exploiting 80 Hz data on line and on demand using the G-POD (Grid-Processing On Demand) service [32]. Numerous studies [33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43] have also focused on middle-sized water bodies. For example, [PERSON] et al. [33] measured the water level of the Mekong River, the width of which varies from 400 m to 1700 m; their results show RMSEs of approximately 44-65 cm for Envisat and 46-76 cm for European Remote-Sensing Satellite-2 (ERS-2). [PERSON] et al. [34] studied water level time series over the Amazon River basin, which exhibits widths ranging from several kilometers to less than a hundred meters, using both Envisat and ERS-2; they showed a RMSE varying from 12 cm in the best cases, 40 cm in most cases, to several meters in the worst cases. Furthermore, [PERSON] et al. [35] employed Envisat to monitor the Zambezi River basin and reported river widths reaching 80 m with RMSEs of 32-72 cm relative to in situ measurements at different locations. In addition, [PERSON] et al. [36] measured the water levels of medium-sized rivers with widths between 100 and 1000 m over the Sao Francisco River, Brazil, with RMSEs lower than 60 cm, and better than 30 cm in some cases using Envisat and Satellite with ARgos and ALtiKa (SARAL) data. Most studies of middle-sized water bodies used mainly Envisat and ERS. In addition, previous results referred to LRM altimeters, which provide accuracies on the order of tens of centimeters. RMSEs over different water targets vary with size and morphology, with smaller size and complexity of the shape bring more uncertainties. With the launch of Sentinel-3, equipped with a SAR Altimeter (SRAL), there is a need to evaluate its performance. Because of its higher spatial along-track resolution, SRAL is naturally better suited to eliminate land contaminations found within LRM altimetry footprint [44]. Our study objective was to retrieve water levels in challenging environments exploiting SAR altimeter data capabilities. We focused on middle-sized and small-sized water bodies whose width varies from hundreds of meters (close to along-track resolution) to a few kilometers, and/or surrounded by rapid changing topography. As the backscattered waveform depends on the surface reflecting the signal, altimeter data are usually applied by different retrackers adapted to different surfaces to better locate the height of the reflective surface. A lot of research has been carried out regarding this, and is still ongoing in the retracking modeling framework, specifically considering the evolution from the LRM to SAR altimetric operation [45].