###### Abstract  
Satellite altimeters have been used to monitor river and reservoir water levels, from which water storage estimates can be derived. Inland water altimetry can, therefore, play an important role in continental water resource management. Traditionally, satellite altimeters were designed to monitor homogeneous surfaces such as oceans or ice sheets, resulting in poor performance over small inland water bodies due to the contribution from land contamination in the returned waveforms. The advent of synthetic aperture radar (SAR) altimetry (with its improved along-track spatial resolution) has enabled the measurement of inland water levels with a better accuracy and an increased spatial resolution. This study aimed to retrieve water levels from Level-1B Sentinel-3 data with focus on the minimization of the land contamination over small- to middle-sized water bodies (130 m to 4.5 km), where continuous clean waveforms rarely exist. Three specialized algorithms or retrackers, together with a new waveform portion selection method, were evaluated to minimize land contamination in the waveforms and to select the nadir return associated with the water body being overflown. The waveform portion selection method, with consideration of the Digital Elevation Model (DEM), was used to fit the multipeak waveforms that arise when overflying the continental water bodies, exploiting a subwaveform-based approach to pick up the one corresponding to the nadir. The performances of the proposed waveform portion selection method with three retrackers, namely, the threshold retracker, Offset Center of Gravity (OCOG) retracker and two-step SAR physical-based retracker, were compared. No significant difference was found in the results of the three retrackers. However, waveform portion selection using DEM information great improved the results. Time series of water levels were retrieved for water bodies in the Ebro River basin (Spain). The results show good agreement with in situ measurements from the Ebro Reservoir (approximately 1.8 km wide) and Ribarroja Reservoir (approximately 400 m wide), with unbiased root-mean-square errors (RMSEs) down to 0.28 m and 0.16 m, respectively, depending on the retracker. altimetry; retracking; Sentinel-3; synthetic aperture radar (SAR) 2019117181060718106 the water levels of inland water bodies to provide early-warning alerts for water shortages or flood predictions. Unfortunately, in situ gauging stations are not always available in many parts of the world, or not publicly available, being maintained by local authorities [3; 4; 5]. An alternative to this is the use of satellite radar altimeters. Satellite radar altimeters are essential tools for monitoring the oceans, a task they have performed for over 25 years [6]. Satellite radar altimetry has also proven to be valuable tools for monitoring the water levels within inland water systems, including lakes and rivers [3; 7; 8]. Satellite radar altimetry can help overcome the lack of data in many parts of the world, and contribute to monitoring the water levels both within inland water systems where no in situ data are available, and also within transboundary river basins. A list of satellite radar altimetry missions and their resolutions is shown in Table 1. Until CryoSat-2 was launched, the satellite radar altimetry was pulse limited to low-resolution mode (LRM). CryoSat-2 is the first altimetry mission with the synthetic aperture radar (SAR) mode available, followed by Sentinel-3. Unlike classical pulse-limited altimetry, SAR altimetry exploits coherent processing of groups of transmitted pulses to make the most efficient use of the power reflected from the surface [9], therefore significantly improving along-track resolution [9; 10; 11], as shown in Table 1. Sentinel-3, the newest in-orbit satellite, has a temporal resolution of approximately 27 days and an inclination angle of approximately 98.6 degrees, meaning it can cover almost all Earth latitudes except the poles.