Abstract
It is well known that a gravitational wave caused by a tidal change propagates upstream with a velocity of √gH (H: depth). According to a field observation conducted by the author on the Teshio River, the surface level of water at the distance of 5 km from the mouth changes periodically with the same phase of the tidal change. This can be explained by a fact that the distance of 5 km is too short for the wave to travel from the mouth to the station with a velocity of √gH. Velocity of surface flow, however, is observed to change with a phase difference of about 3 hours at the upper station and at a station near the mouth. This phenomenon can be analyzed by a theory of internal wave. Except the period of flood, the Teshio River has a stratified layer of fresh water and salt water which is called as a salt-wedge. An internal wave is generated at the interface by any disturbance as wind, atmospheric depression, tidal change, etc., and is propagated with a velocity of about √ (1-γ)gH1+U1 (γ: density-ratio of fresh water to salt water, U1: vertically mean velocity of a surface-flow, H1: thickness of a fresh layer). Then, by computing a travelling time of the internal wave, the phase-difference of the surface-velocity between the two stations can be estimated as about 3 hours.
