•A combined use of the LISST-SL optical sampler and a down-looking 1200kHz ADCP profiler, gave insights in the performances of optical and acoustic methods when investigating suspended sediment in the Danube River.•A combined deployment of those measuring techniques may aid to detect ideal conditions for LISST-SL measurement across a large river channel. For example, a moving ADCP may be used to track regions characterized with acoustic parameters values where LISST-SL could subsequently be deployed for a more accurate and quantitative assessment.•Obtained vertical profiles of sediment concentration, mean size and acoustic parameters from repeated samples by LISST-SL and ADCP recording, respectively, pointed out density anomalies at shallow depths rather than actual concentration values of suspended sediment. These density anomalies were related to the cold water inflow from Hron River.•The LISST-SL required 2min measuring in fixed positions, eventually limiting the standard deviation to 10% of the expected mean value, except for low concentrations (i.e., in the range of 5–10mg/l) of sand that resulted in the same order of the corresponding standard deviations over 2min.•The acoustic device appeared particularly sensitive to suspended sediment from the riverbed. A moving ADCP located regions of sediment entrainment from riverbed, fine sediment fully suspended in the water column and the extent of Hron River inflow dispersion into Danube main streamflow. The use of acoustic and optic devices has become more and more common for estimating suspended sediment loads in rivers. The echo intensity levels (EIL) recorded by means of an Acoustic Doppler Current Profiler (ADCP) have been applied in different methods, which provided relationships between scattering particles features derived from samples (i.e., concentration and grain size) and corresponding backscattering strength and sound attenuation. At the same time, the laser diffraction was applied by an in-stream sampler (LISST-SL) to measure suspended sediment concentration and the corresponding particle size distribution (PSD). These two techniques exhibited different limitations in terms of the measured range of concentration, sensitivity to a certain spectrum of particle sizes, and instruments deploy feasibility especially in large rivers, in a way that the use of sampled PSD by LISST-SL to validate ADCP methods may not be trivial. The aim of this study was to combine the vertical profiling of EIL by an ADCP with results from LISST-SL, eventually demonstrating the possibility of using moving ADCP measurements to detect different suspended matters along a Danube River section characterized by a small tributary junction. At the same time, this work elucidates optical to acoustic method deviations that hinders an actual validation of ADCP methods based on LISST-SL rather than with physical samplings.