Several of the mixing processes at density interfaces in highly stratified flow at the mouths of rivers flowing into the ocean bear close resemblance to mixing processes at the density interface seen in freshwater bodies, and can provide valuable insights into them. This paper addresses the mixing phenomena at the interface in the very highly stratified flow seen at the mouth of the Ishikari River during the dry season. It is shown that the chief factor influencing mixing across the interface is the wind, while factors disturbing the steadiness of the flow such as tide have but little influence. Furthermore, when the entrainment coefficient is examined as a function of wind speed, it is found that the surface salinity at any given river station can be accurately predicted. Turbulence structures are also discussed.