In the last decade, much progress has been made in continuously measuring suspended sediment concentration (SSC) in rivers using horizontal side‐looking Acoustic Doppler Current Profilers. However, these techniques do not provide information on the spatial variability of the suspension. In this study, we explore some new possibilities offered by the down‐looking deployment of a multifrequency acoustic backscatter system (ABS) in order to obtain information on the suspension throughout an entire river cross section. Two sites, with low and high levels of SSC, were investigated. The acoustic signal was processed using multifrequency inversion methods. Both water sample calibration data and modeling were used for retrieving the acoustic properties of the suspended particles. At the low SSC site ( ∼30 mg/L), we successfully inverted the acoustic signal, except in some areas of high acoustic backscatter close to the surface that were probably generated by air microbubbles. At the high SSC site ( ∼10 g/L), estimates of both fine and sand SSCs throughout the river cross section were successfully obtained, except in some areas close to the bottom where the acoustic signal was totally attenuated due to distance and high concentration of fine sediments. This work confirms the capacity of hydroacoustic technology for providing spatial information on river suspensions.

Following the success of the Acoustic Doppler Current Profiler (ADCP) technology for monitoring river discharge, there has been a growing interest in the last decade in extracting information on Suspended Sediment Concentration (SSC) from acoustic backscatter in rivers. One major advantage of using sonar systems such as ADCPs or Acoustic Backscatter Systems (ABSs) for monitoring SSC in rivers is the capacity of these instruments to provide measurements at a much higher spatial and temporal resolution than traditional water sampling techniques. Despite the efforts recently made to find a relation between SSC and acoustic backscatter in rivers (e.g., Gray & Gartner, 2009;Venditti et al., 2016), most studies remain empirical and site-specific. Such calibrations shift when sediment properties change which requires intensive water sampling to limit the uncertainty in SSC. The development of more general, physically based methods applicable in rivers is needed.The sonar response of suspended sediments is determined by sound backscattering and sound attenuation. Both processes are strongly determined by the characteristics of the suspended scatterers. Bimodal Particle Size Distributions (PSDs) are commonly observed in rivers (e.g., Agrawal & Hanes, 2015;Armijos et al., 2017). The first mode is usually composed of silt and clay sediment particles that are often fairly homogeneously distributed throughout the river cross-section. We do not expect these particles to gather in large flocs (Burban et al., 1989;Droppo, 2001) as rivers often show low organic matter, no salinity, and relatively high turbulence during high sediment load events such as floods. The impact of flocculation on acoustic backscattering has been studied in other contexts (

There has been a growing interest in the last decade in extracting information on Suspended Sediment Concentration (SSC) from acoustic backscatter in rivers. Quantitative techniques are not yet effective, but acoustic backscatter already provides qualitative information on suspended sediments. In particular, in the common case of a bi-modal sediment size distribution, corrected acoustic backscatter can be used to look for sand particles in suspension and provide spatial information on their distribution throughout a river crosssection. This paper presents a case-study where these techniques have been applied.

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