This happened because fresh water glaciers melted and flowed to join the ocean water. Because of the difference in the salinity and densities of these two water bodies, a surface tension developed between them that acts like a thin wall which prevents them from mixing. The boundary between the two is outlined by a thin layer of foam.
It’s kind of poetic isn’t it? Two powerful bodies of water, relentlessly standing together, but unable to become one. They’re like the Romeo and Juliet of aquatic bodies. But fear not, hopelessly romantic weirdos, according to scientists this story will get its happy ending; given enough time, the differences between these two bodies will disappear and they will merge together.