In the episode of Vamana dev coming to the assembly of Bali Maharaj, He understands that it is Lord Vishnu and he washes the feet of Lord Vamana dev. He takes that water and puts it on his head remembering that this water is none other than Ganges.

Bali Maharaj, follows in the footsteps of great devotees like Lord Shiva who wore the Ganges on his head.

For what reason did Lord Shiva accept Ganges on his head ?
Did he want some material things from the Lord?

No.
He is deva devon. By mentioning Lord Shiva as deva devo it is indicated that Lord Shiva doesn't need anything material because He is devon ke dev Maha dev.

The demigods have all the material opulences but still He is sitting under a tree renouncing everything. He doesn't need anything material then why is he taking the Ganges on his head?

He is living on a mountain and his wife is the daughter of the king of mountains. On the mountains there are many springs, many waterfalls, all the rivers originate from the mountains more or less.

Is he feeling hot after drinking that poison and hence he wants to cool ? No.

He is having the moon on his forehead. Moon is very cooling. He has already got something to cool his head. So why does he take the Ganges on his head?

Because he wants the mercy of Lord Vishnu. He wants to get purified of being in contact with tamoguna. He has to do that duty. He wants to get the blessings of the Lord.

So you can see that that is the potency of bathing in the Holy rivers.