Horizontal Acoustic Doppler Current Profilers (HADCPs) are a relatively new instrument that can be used to measure cross-river profiles of streamwise velocity and may also provide a relative measure of particle concentration. A number of these instruments are currently installed along three French rivers at locations intermediate to power plants and gauging stations. Unfortunately, during the first year of operation it was found that during low flow conditions at some sites, the horizontal ADCPs often underestimated velocity relative to measurements from gauging with a portable, vertically-oriented ADCP. In an attempt to understand the velocity underestimation that has been observed, studies of velocity and echo intensity profiles have been carried out under a range of flow conditions at various sites. This paper presents an analysis of the echo intensity profiles measured by a 300 kHz horizontal ADCP at the Romans-sur-Isère study site over a ten-day period during November 2009. Time series of intensity data are compared to measurements from two other horizontal ADCPs operating at 600 kHz and 1200 kHz, as well as measurements of optical turbidity. The cross-river intensity profiles are compared to theoretical predictions and it is found that instead of decreasing with range as expected, the echo intensity often diverges from theory, ballooning with distance from the receiver. This increase in signal intensity with distance appears to be a result of acoustic reflection from either the river bottom or roughness at the water surface.