Stratification, when a light body of water rests on a heavier body of water in layers, is a stable condition. However, when there is a current speed difference things aren't quite that simple. When the current speed difference exceeds a certain level, the current becomes unstable, and the stratification starts to fall apart. This is called Kelvin-Helmholtz instability. Let's have a look at this situation in an experiment. We will pour saltwater colored with ink and clear freshwater in a long narrow water tank. When the boundary surface has settled, if you tilt the tank, the saltwater flows down and the freshwater flows upward, causing a current speed difference at the boundary of the two types of water.