The Baltic Sea and the Northern Sea meet but their waters do not mingle is the difference in their density, which is connected with the salinity. This chemical difference is known as a halocline. The North Sea has more saline water as it comes from the Danish Straits while the Baltic Sea has fresh and clean water that comes from the drainage basin. From time to time, the temperature of the bodies of water alters, which causes surface mixing. Still, it happens only in particular regions where the seas are rather shallow, so they remain mostly separate.