It is required a continuous measurement of the flow speed and flow rate in the river at the flood, but there is yet no effective measurement technique. The purpose of this study is the quantitative examination of the measurement defect and sound propagation characteristics for measuring river flow velocity and rate in the flood using ultrasound. We have continuously observed the sound propagation characteristics in various river situations. In this paper, the sound propagation characteristics and the defect factor of flow velocity measurement in the snow melting flood were examined quantitatively. We found the impulsive noise which rapidly increased with flow velocity obstructed the velocity measurement and the center frequency of the impulse noise was over 150 kHz.