Introduction

The holy river Ganga(Ganges) is a 1,000 km long river that stretches from the Himalayas in the north to the Bay of Bengal in the south. The Ganga basin has a population of over 1.3 billion people and it is the world’s second-largest river after the Nile. Ganga is a sacred river and worshipped as Ganga mata. It is said that Ganga descended from heaven. The story of Ganga coming to earth from heaven is as follows:

Ganga ki avataran katha

Long time ago. There was a war between the gods and the demons. The Asuras made a plan to defeat the gods. During the day they would hide in the sea and attack the gods at night.
Gods realised that they will lose if this keeps going on so they went to Sage Agastya for help.

Sage Agastya solved this problem by drinking all of the sea water

Since sage Agastya drank all water the Ashuras could not find a place to hide, so the gods easily defeated them. But when gods asked sage Agastya to return the sea water,Agastya told them that he already digested the water.
Now that sea water was gone,people on the earth didn't have enough water to survive. Then the gods went to Vishnu and asked for help. Vishnu assured them that the Ganga would descend from heaven to earth.

Meanwhile, king Sagara on earth was performing a yajna to conquer the whole world. Sagara had two queens named Keshni and Sumati, Sumati had sixty thousand sons and Keshni had one son. King Sagara performed Ashwamedha Yagya. According to that yagya, the king of the land where the Ashwa(horse) will stop have to fight with king Sagara.

Indra(the king of Heaven) got scared after hearing about this. So in order to break the yagya, Devraj Indra stole the horse left by the king and tied it in the ashram of Kapil Muni.

The king sent his sixty thousand sons in search of the horse for the sacrifice. While looking for the horse, all the sons reached Kapil Muni's ashram, seeing the horse tied there, all the sons started calling Muni a thief and hypocrite and also abusing him. Which made him angry and all sixty thousand sons of king Sagara were burnt to ashes by his anger.

Anshuman(son of his second wife)

When there was no information of any son even after a long time, after getting the permission of the father, Anshuman(son of Keshni) reached the sage's hermitage while searching for his brothers.

There he met Garud, who witnessed the whole incident himself and narrated the whole story of his brothers being consumed by Kapil muni's anger. Then Garudji told Anshuman that if you want his salvation, then Gangaji will have to be brought from heaven to earth.
But first complete your father's yagya, then do the work of bringing Goddess Ganga to earth, only through Goddess Ganga can the souls of all your brothers be liberated.

Anshuman reached the Yagya Mandap with the horse and narrated the whole story to King Sagara. For the peace of the soul of his brothers, Anshuman did penance to bring Goddess Ganga to earth, but he failed.
After the death of Anshuman, his son was Dilip, Dilip also did penance to bring Goddess Ganga to earth but he also did not get success.

The story of Bhagirath

Finally, Dilip's son Bhagirath went to the Gokarna shrine and did severe penance to please Brahma. After many years, Brahmaji, pleased with Bhagirath's penance, granted a boon to take Gangaji to the earth.

Now the problem was that after the release of Brahmaji from the kamandal, no one on earth has this power to handle the speed and power of the Ganga. Brahmaji told that only Lord Shankar is in the earth, who can handle the Goddess Ganga. So you should pray to him.

Bhagiratha started worshiping Lord Shankar by standing on one thumb. Pleased together with his severe penance, Shiva agreed to handle Ganga in his hair. When Goddess Ganga moved from the kamandal of Brahma to the earth, Shiva covered the stream of Ganga in his hair.

For many years, Ganga could not come out of Shiva's hair, then Bhagirath requested Shiva to free Goddess Ganga from the hair. After this, Shiva untied his locks and Ganga start to flow. 

Goddess Ganga was moving towards the plain after coming out of the locks in the Himalayan valleys, on the same path was the ashram of sage Jun.

The noise of the Ganga was disturbing the penance of Sage Jun, due to which Sage Jun drank the Ganga in anger. When Bhagirath found about this he requested Sage Jun to let Ganga go. After that Sage Jun cut his thigh from where the Ganga started flowing again. Since then Ganga came to be called Jahnavi or Jahuputri.
Crossing many places, the Ganga reached Kapil Muni's ashram and freed the ashes of Sagara's sixty thousand sons. Brahma ji blessed Bhagirath's hard penance that Gangaji's name will be Bhagirathi in your name.