Apsu, sweet water, refers to the fertilizing groundwater and its potentiated mud, and in some cases its male personification. In the Enuma Elish and other creation accounts, there was first chaos, a primordial state where Apsu mingled with Tiamat, the salty water. This reflects the natural southern marshes where brack and fresh waters met. Apsu and Tiamat had offspring, which vary in different genealogies: Mummu, who by parthogenesis produced the young gods; or Lahmu and Lahamu, who produced Anshar and Kishar, who conceived the young gods; or in later accounts, Apsu and Tiamat's primordial soup just progressively differentiated.
Eventually, the younger gods lead a rebellion against a quick-tempered Apsu; the elder gods Apsu, Tiamat and Mummu lose to the younger gods led by Apsu's son or grandson Enki. Apsu becomes merely an underworld place where Enki establishes his residence, taking on Apsu's characteristics and functions. Though Apsu was no longer a personification, Mesopotamian religion still fixated on the abzu. It was magical, used to carry spells and for ritual purification; and it was ejaculated by Enki to irrigate and fertilize. Also, abzu referred to the shrine of Eridu, the holy mountain, the imago mundi in the city.
Leick 2001, p 20-21