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Gangaa is the elder daughter of Himvaan (Himaalaya), and elder sister of Paarvatee. 

Once she bore a son for Shiv Jee through Agni Devtaa, that was Kaartikeya. Thus Kaartikeya is the son of Gangaa too.

How did She come to Prithvi?

Later Bhageerath (Raajaa Sagar's son was Asmanjas, his son was Anshumaan, his son was Dileep and his son was Bhageerath) brought Gangaa on Prithvi to wash the sins of Sagar's' 60,000 sons who were burned by Kapil Muni, because they blamed Muni for stealing their father's Ashwamedh Yagya horse. 

Gangaa is also known as 'Divine Gangaa', 'Bhaageerathee' (because Bhageerath brought her from Swarg), 'Tripathagaa' (since she flows in three worlds - Swarg Lok, Bhoo Lok and Paataal Lok', 'Jaahnavee' (as Devtaa gave Var to Jahnu Muni that Gangaa will be known as his daughter "Jaahnavee")  when Gangaa flowed all Yagya materials with her flow, and Jahnu Muni drank her all waters and then released it through his both ears on the prayer of Devtaa. 

She is very holy because (1) she doesn't have the 'Rajswalaa' Dosh (sin), while other ordinary rivers have it for four months of rainy season. Gangaa is beyond this Dosh. (2) Besides its water doesn't get spoilt even after storing for years together, (3) Because it has its origin from Vishnu's feet.