Trough-based compound channels are not only the most common morphology of alluvial rivers, they are also the most important content of river dynamics research. In this paper, based on the generalization experimental data of compound channel and regional characteristics of water-sediment movement, and in combination with four regions of trough-based compound sections, i. e. the main channel equilibrium region, trough interactive region, floodplain equilibrium region and side wall region, the trough interactive region with the most complicated river flow morphology is further divided into two parts, i. e. the logarithmic velocity zone (inner zone) and non-logarithmic velocity zone (external zone). The S-shaped curve describing the animal and plant growth process is introduced so as to propose the determination method of the inner-external zone boundary. Based on the lateral distributional formula of the trough interactive region through linear assumption and integral transform, the calculation equations of sectional average velocity and sediment content in the trough interactive region are given in order to perform comparative analysis on the differences in the velocity and sediment content in different regions. In this paper, the proposed trough region boundary and calculation method of water-sediment distribution will provide technical support for trough planning and governing of compound channels.