[en] Estimation of the mobile layer thickness in sediment transport study is always considered to be important for quantitative derivations of transport, whether for bed load transport rates or for percentage return and settling percentage rates in suspension transport. In view of application of the tracer technique, the mobile layer thickness is defined as the depth at which the tracer particles can intrude - the mixing depth. In this paper, the authors present a new approach to determine the mixing depth based on the principle that the scattering to peak ratio in gamma spectrum from the radioactive tracer-native sediment mixture is a function of the mixing depth because of scattering and absorption interactions of the radiation with the media. It is shown both from theoretical considerations and experiments that there is a rather clear relation between ratio and mixing depth. In this method. the mixing depth is derived from comparison of the ratio of counts in the spectrum obtained in the field with the calibration curve built in the laboratory by reproducing the geometrical conditions existing in the field. A case study of the mixing depth using Sc-46 tracer material in the experiments at Haiphong Navigation Channel is also presented. The mixing depth determined by the sampling and this method shows a rather good agreement.The practical capability of the method and the ability of application for common tracer isotopes such as Ir-192, Sc-46 are estimated. These isotopes are suitable to the range of mixing depth up to 30 cm