Experiments were conducted in a straight channel that widens from a single channel to a compound channel based on a real navigation channel with a similar shape in a mountainous region of China. Flow entered the single channel and then the compound channel with vegetated floodplains. The channel widened at the interface between the single and compound channels, resulting in different flow depths between the single and compound channels. An equation was proposed to estimate the percentage that the flow depth decreased from the single channel to the compound channel. Vegetation increased the floodplain resistance, causing more water to flow in the main channel and yielding a greater mean velocity in the main channel relative to the nonvegetated case. The velocity reached a maximum and gradually decreased until reaching a constant in the compound channel. Sediment motion (bedload vs. deposition) was determined by the local bed shear and the critical bed shear. Sediment was deposited in the compound channel at the position where the local bed shear was smaller than the critical bed shear. When the floodplains were vegetated, the increase in the main channel velocity enhanced the local bed shear, which surpassed the critical bed shear; thus, deposited sediment was reinitiated and moved downstream, and no deposition was observed.