When the human queen gave birth, the kingdom of Arendelle rejoiced. Magic stirred at the birth of the new young Spirit, the Daughter of Ahtohallan. When Her Daughter was born, Ahtohallan's flow froze completely. The Life that once swam in her currents now dwelt in the tiny Spirit of Elsa of Arendelle. Instead of running water stood pristine, perfect ice, the Memories safely kept within. Waiting for when their Spirit would come and wake them once more. Or, my explanation on what was going on in Frozen 1 and 2.