why does a clear stream always appear to be shallower than it actually is

A clear stream appear shallower than actually it is , because of the refractive  effect of light. since there are two medium ( air and water) with different densities , ray coming out from the stream bend away as they cross the boundary before reaching the observers eye. since the stream is a liquid medium, more dense than the air ( where the observer is seeing) the ray of light coming from the inside of the stream bend away from the normal thereby lifting up the object( giving it an apparent depth). whereas if the observer will find himself inside the stream , he will see object outside  faraway from him.