Since Lord Shiva held Ganga as she descended on Earth, he is known as Gangadhar, and the river is also referred to as Bhagirathi as King Bhagiratha worshipped for her appearance on the Earth.
The story dates back to the times of Sage Kapila and King Sagara. The King had performed as many as 99 Ashwamedha Yagnya, a ritual to showcase ones' supremacy and expand the geographical boundaries of the empire by sacrificing a white horse that would wander randomly. Interestingly, the King wanted to perform another Ashwamedha Yagya to show his winning streak. But Indra, the head of the Devas, felt insecure after seeing Sagara growing more powerful than him. Moreover, Sagara was also blessed with 60, 000 sons. Hence to disrupt Sagara's Yagya, Indra stole the horse and hid it at Sage Kapila's hermitage.
After learning that the horse has gone missing, King Sagara asked his sons to go in search of it. After exploring every nook and corner, they found it at Sage Kapila's ashram, who had been in a deep state of meditation and oblivious of its presence.
Disturbed by the commotion caused because of Sagara's army, an enraged Sage Kapil opened his eyes from the deep state of penance for the first time. He got infuriated after being accused of stealing the horse, and hence, he burnt all the 60,000 sons of Sagara at one glance. He also cursed that they wouldn't attain Moksha.
Years passed, and the souls of Sagara's sons wandered in the Patala Loka (nether land) awaiting Moksha or liberation. Many of their successors attempted to please Brahma but failed. In the end, it was Bhagiratha, who succeeded in getting the blessings of Brahma, the creator of the Universe.
Thus, Brahma, pleased by Bhagiratha's devotion, asked Ganga to visit the nether lands where his ancestors were wandering to attain Moksha.
Ganga didn't want to visit the nether lands and felt humiliated by the order, and hence, she decided to destroy the Earth by her massive force.
After learning that Ganga has been offended, Bhagiratha worshipped Lord Shiva and requested him to intervene. And on the day when Ganga descended, Lord Shiva held her with his matted locks to lessen the velocity of the descent. Thus, Ganga landed on the Earth as well as the Patala Loka to liberate the sons of Sagara.
And since Shiva held Ganga as she descended on Earth, he is known as Gangadhar, and the river is also referred to as Bhagirathi as King Bhagiratha worshipped for her appearance on the Earth.
Ganga is considered the holiest river on the surface of the Earth because she has descended from heaven and has divine powers to heal. Right from Gangotri to Dev Prayag in Uttarakhand to her merger with the Yamuna and Saraswati rivers at the Sangam in Prayagraj and the Dashashwamedh Ghat in Kashi, Uttar Pradesh, to the amalgamation with the ocean at Gangasagar in West Bengal, Ganga is worshipped by millions.
