Fixed side-looking Doppler current profilers (H-ADCP) recently emerged as an innovating technique for the continuous monitoring of river discharges. The discharge can be computed from the flow velocities measured by the H-ADCP along a horizontal profile across the section. This paper reports a field assessment of the quality of velocities and discharges provided by a 3-narrow-beam Teledyne RD Instruments, Inc. (RDI) 300 kHz H-ADCP installed at the Saint-Georges gauging station (Saône river in Lyon, France). Reference velocity and discharge values were established from 18 conventional ADCP river gauging campaigns over an extended discharge range (100–1800 m3/s). The comparison with ADCP data revealed that H-ADCP velocity measurements were reliable (deviations <5%) in a near-field range only (60 m out of a 95 m total section width). In the far field (beyond 60 m), H-ADCP velocity measurements showed negative bias of up to −50% 90 m from the instrument. For section-averaged velocities lower than 0.4 m/s approximately, H-ADCP velocity measurements were found to be significantly underestimated over the whole cross section. The performances of several strategies (index velocity method and velocity profile method) for computing discharge were tested, compared, and discussed. For the velocity profile method, several profile laws and far-field extrapolation methods were implemented. Both methods gave acceptable discharge values (deviations <5% typically) excepted at low-flow conditions. The reasons why H-ADCP velocities were unacceptably biased low in the far field and for low flow conditions require further investigation in order to define correcting measures.