I heard this fact that actual origin was Ganga is Lord Vishnu's lotus feet, and it held on shiva's head by the brahma? please share me the actual source from scriptures. Thanks
Here is what Vishnu Purana says about Ganga:
Vishnu Purana 2.8
From that third region of the atmosphere, or seat of Vishńu, proceeds the stream that washes away all sin, the river Gangá, embrowned with the unguents of the nymphs of heaven, who have sported in her waters. Having her source in the nail of the great toe of Vishńu's left foot, Dhruva receives her, and sustains her day and night devoutly on his head; and thence the seven Rishis practise the exercises of austerity in her waters, wreathing their braided locks with her waves. The orb of the moon, encompassed by her accumulated current, derives augmented lustre from her contact. Falling from on high, as she issues from the moon; she alights on the summit of Meru, and thence flows to the four quarters of the earth, for its purification. The Śítá, Alakanandá, Chakshu, and Bhadrá are four branches of but one river, divided according to the regions towards which it proceeds. The branch that is known as the Alakanandá was borne affectionately by Mahádeva, upon his head, for more than a hundred years, and was the river which raised to heaven the sinful sons of Sagara, by washing their ashes. The offences of any man who bathes in this river are immediately expiated, and unprecedented virtue is engendered. Its waters, offered by sons to their ancestors in faith for three years, yield to the latter rarely attainable gratification. Men of the twice-born orders, who offer sacrifice in this river to the lord of sacrifice, Purushottama, obtain whatever they desire, either here or in heaven. Saints who are purified from all soil by bathing in its waters, and whose minds are intent on Keśava, acquire thereby final liberation. This sacred stream, heard of, desired, seen, touched, bathed in, or hymned, day by day, sanctifies all beings; and those who, even at a distance of a hundred leagues, exclaim "Gangá, Gangá," atone for the sins committed during three previous lives. The place whence this river proceeds, for the purification of the three worlds, is the third division of the celestial regions, the seat of Vishńu.
Here also Bhagvagt-Puran says about Ganga
Bhavat Purana canto 5 chapter 17
Sukadeva Gosvämi said: My dear King, Lord Vishnu, the enjoyer of all sacrifices, appeared as Vämanadeva in the sacrificial arena of Bali Mahäräja. Then He extended His left foot to the end of the universe and pierced a hole in its covering with the nail of His big toe. Through the hole, the pure water of the Causal Ocean entered this universe as the Ganges River. Having washed the lotus feet of the Lord, which are covered with reddish powder, the water of the Ganges acquired a very beautiful pink color. Every living being can immediately purify his mind of material contamination by touching the transcendental water of the Ganges, yet its waters remain ever pure. Because the Ganges directly touches the lotus feet of the Lord before descending within this universe, she is known as Vishnupadé. Later she received other names like Jähnavé and Bhägérathé. After one thousand millenniums, the water of the Ganges descended on Dhruvaloka, the topmost planet in this universe. Therefore all learned sages and scholars proclaim Dhruvaloka to be Vishnupada
Ganga is also called as Visnupadi. The story about the origin of Ganga is present in the Nagara kanda of the Skanda Purana.
When Bali was bound by Visnu, the powerful one, the three worlds, including the mobile and immobile beings were covered by the three steps (of his).
The first step was placed on the Hatakesvara Ksetra. The second step was placed on Maharloka then by the noble-souled one. When the Discus-bearing Lord attempted the third one, O excellent Brahmanas, the whole Cosmic Egg became split up and too small. When the Cosmic Egg became split up by the tip (toe) of the foot, pure water dripped down to the earth, slowly through the tip of the big toe.
The sacred water, clear like crystal, flooded the entire Brahmaloka. Its lustre resembled that of a jasmine flower and the moon. Fishes, tortoises and herds of crocodiles abounded therein. Ever since then she is remembered as Ganga, VisnupadI, in the world. She rendered the spot already sacred, still more sacred. It was thus, that the foot of Visnu came to be present there, O excellent sages. Then VisnupadI is remembered as destructive of all sins.