Horizontal acoustic Doppler current profilers (H-ADCPs) are commonly used to monitor flow speeds in rivers and yet the accuracy of their velocity measurements is often taken for granted. Multi-year measurement campaigns on three French rivers have shown that H-ADCPs tend to underestimate velocity when concentrations and backscattered intensities are low. Nine months of data are presented from a study site on the Isère river that was equipped with H-ADCPs operating at 300, 600 and 1200 kHz. The velocity measurements are compared with measurements made with vertical ADCPs during river gauging and with discharge data from an upstream dam. Velocities were underestimated when the backscattered intensity was low and concentrations were less than 50 mg/L. The underestimation was most pronounced in the 300 kHz data and least pronounced in the 1200 kHz data. The relationships that are established between underestimation and intensity (accounting for concentration through the calculation of the sediment attenuation) are used to correct further data. The corrected discharge values agree with reference values with an uncertainty of 10-15% when averaged over multiple cells. Since H-ADCPs can also provide simultaneous measurements of the concentration of suspended sediment, they are a powerful tool for monitoring suspended sediment fluxes.