While the water plants in small rivers through city play an important roll on the environment of rivers and have a function to purify the water quality, the water plants increase the roughness of a water channel. In Nikaryo channel passing through Kawasaki city, we surveyed the distribution of water plants and measured the velocity profiles at cross-section with water plants. It was revealed that the states of water plants varies with the passage of time and the area occupied by water plants becomes the dead water zone. And it was showed that the longitudinal water level distribution could be evaluated, applying the sheer stress model using the boundary mixing coefficient to the flow in the open channel with the water plants, and Manning's roughness coefficient N of the flow with water plants is about 0.08.