He is father to Bhishma by the goddess of the river Ganga. After Bhishma's birth, Ganga leaves him.
When Bhishma is adult, Santanu sees Satyavati, a ferryman's daughter, and falls in love with her. She agrees to marry him on condition that any children she bears him would inherit the throne. Bhishma agrees to this.
Santanu and Satyavati have two sons, Chitrangada and Vichitravirya. After Santanu's death, Satyavati continues to rule the kingdom with his three sons.