Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 43: Gaṅgā Descends to the Earth
Text 1.43.30

तत्र देवर्षिगन्धर्वा वसुधातलवासिनः।
भवाङ्गपतितं तोयं पवित्रमिति पस्पृशुः॥

tatra deva-rṣi-gandharvā vasudhā-tala-vāsinaḥ
bhavāṅga-patitaṁ
toyaṁ pavitram iti paspṛśuḥ

tatra = there; deva-ṛṣi-gandharvāḥ = the demigods, sages and gandharvās; vasudhā-tala-vāsinaḥ = as well as residents of the earth; bhava-aṅga-patitam = had fallen from the body of Lord Śiva; toyam = since the water; pavitram iti = considered the water to be purifying; paspṛśuḥ = and touched it.

Since the water had fallen from the body of Lord Śiva, the demigods, sages and gandharvas there as well as residents of
the earth considered the water to be purifying and touched it.

The demigods and others understood that because Gaṅgā was held by Lord Śiva on his head, it was purifying. Therefore, they touched it.

Gaṅgā purified Lord Śiva and others. Śrīmad-Bhāgavatam (3.28.22) states:

yac-chauca-niḥsṛta-sarit-pravarodakena
tīrthena mūrdhny adhikṛtena śivaḥ śivo ’bhūt

“The blessed Lord Śiva becomes all the more blessed by bearing on his head the holy waters of the Ganges, which has its source in the water that washed the Lord’s lotus feet.”

The Mahābhārata states:

tad ambu patitaṁ dṛṣṭvā dadhāra śirasā haraḥ
pāvanārthaṁ jaṭā-madhye yogyo ’smīty avadhāraṇāt

“Noticing that the water had fallen, Lord Śiva held it on his head to purify himself, for he understood that he is capable of holding it within the matted locks of his hair.”

In Vālmīki Rāmāyaṇa (2.5.24), Gaṅgā is described as viṣṇu-pāda-cyutāṁ divyām, “divine and having fallen from Lord Viṣṇu’s lotus feet.” She is divine because she fell from Lord Viṣṇu’s lotus feet.

The demigods thought, “Because Lord Śiva has held her on his head, we should also hold her on our heads,” and so touched her.