Why Ganga is a holy river

Ganga was originally in heaven, the Swargaloka.
You must have heard about the Vamanavatara of Lord Mahavishnu.
When he measured the whole universe with three steps, his thumb pierced the top of the universe and through that hole, Ganga descended upon Swargaloka.
She remained there for a long time.
There was a king called Sagara.
He had two wives. Sumati and Keshini, but they didn't have children.
They prayed to Lord Shiva for a long time.
Shiva became pleased and blessed them.
Sumati gave birth to 60,000 sons and Keshini gave birth to just one son.
His name was Asamanja.
The 60,000 sons of Sumati were all brave and good.
Asamanja turned out to be wicked.
Once, king Sagara was conducting an Ashwamedha yaga.
In Ashwamedha yaga a horse is let loose to roam around freely for one year.
It is accompanied by soldiers for its protection.
Wherever the horse goes, the rulers of those places should become subservient to king Sagara or they have to fight and defeat him.
That is the rule of Ashwamedha yaga.
Indra, the king of devas, took away the horse.
Sagara's 60,000 sons went in search of the horse.
They searched everywhere and finally reached Pathalaloka.
There they found a Muni in meditation and the horse standing near him.
They thought that the Muni had taken away the horse.
It was Indra who hid the horse there.
The Muni's name was Kapila.
He was very powerful.
The princes insulted the muni by calling him a thief.
The Muni got so angry that he turned all of them to ashes with one look.
Since they didn’t return after a long time, Asamanja's son Amsuman went looking for his uncles.
He was a very brave and righteous boy.
Finally, he also reached Pathalaloka.
There he found Kapila muni, the horse, and a heap of ash by his side.
Amsuman paid respects to the Muni and humbly told him the purpose of his visit.
The muni was impressed with his humility and told Amsuman that he can take the horse with him.
Amsuman prayed to the Muni for the salvation of his uncles.
Muni said: if you bring Ganga from heaven and make her to flow over the ashes, that will wash away their sins and they will all attain heaven.
Amsuman thanked the Muni and went back with the horse.
The yaga was completed.
King Sagara appointed Amsuman as his successor.
After ruling for a few years, Amsuman decided to do tapas to get Ganga to earth.
He did not get success in his lifetime.
Amsuman's son Dilipa also tried hard but did not succeed.
Dilipa's son was Bhagiratha.
Bhagiratha took a vow.
I will become king only after I get Ganga to earth.
He did intense tapas and pleased Brahmadeva.
Brahmadeva blessed him but said: Ganga is very naughty.
It is very difficult to control her.
She goes wherever she wants.
She destroys whatever she wants.
The impact of her falling on earth will be huge.
You pray to shiva.
Only he will be able to absorb that impact.
Bhagiratha did tapas again and pleased lord shiva.
When Ganga fell from heaven, Shiva caught her in his jata and kept her locked there.
Slowly, he allowed her to descend upon earth.
Bhagiratha led her to Pathalaloka and made her flow over the ashes of his uncles.
They all were purified and attained heaven.
So we have three Gangas now.
The one that we see at Gangotri, Prayagraj, Kashi - that is Ganga on earth.
Then we have Akashaganga in Swargaloka and Pathalaganga in Pathalaloka.
Goddess Ganga is so divine, so great that even thinking about her can purify you.