To evaluate accurately velocity and discharge in rivers from observed results at discrete points in a cross section, we present a new method for data assimilation in which observed velocities are properly incorporated into a river-flow computation. In the present method, we assimilate observed velocities into a shallow-water flow model, and then we can incorporate spatially the observed velocities in a cross section by satisfying dynamic principles. The present method is therefore referred to be here as a dynamic interpolation method. To confirm the fundamental performance of the dynamic interpolation method, we apply the present method to the spatial interpolation of velocities in a cross section using the simulation data for a river-flow computation and observed data. The interpolated results indicate that the present method can have higher accuracy for the evaluation of river velocity and discharge than the simple interpolation method.