Hare Krishna devotees,
We often find Lord Krishna associated with the river Yamuna. He performed all His childhood pastimes on the bank of Yamuna. In contrast, when Lord Krishna appeared as Lord Chaintanya in Kaliyuga, He performed His childhood pastimes on the bank of Ganga. So we find Krishna more associated with Yamuna whereas Chaitanya associated with Ganga.
Now we can think, how Lord Krishna and Ganga are related to each other? Did Ganga get the chance to server Lord Krishna like her sister Yamuna? For this question, let's analyze the following points:
1. Chaitanya Mahaprabhu says
“In this Age of Kali, Kṛṣṇa is manifest in two forms — wood and water. Thus, by enabling the conditioned souls to see the wood and bathe in the water, He helps them become liberated.Lord Jagannātha is the Supreme Lord Himself in the form of wood, and the river Ganges is the Supreme Lord Himself in the form of water."[CC 2.15.134-135]
2. Lord Krishna says in BG 10.31 and SB 11.16.20 "...Among all the rivers, I am Ganga."
So from the above, we can find Ganga is none other than Lord Krishna Himself in the form of water (river). Therefore we should never treat mother Ganga in undignified way like calling her polluted river or her water to be ordinary etc. While taking bath in her or drinking her water, we should consider we are directly interacting with Lord Krishna in liquid form just like Lord Jagannatha is never considered just wood..
Besides the above, we can find river Ganga did manage to serve Lord Krishna in the form of Manasi Ganga. In fact before starting Govardhana parikrama, we should purify ourselves by bathing in Manasi Ganga and after the parikrama, again we should take bath in Manasi Ganga. Only then our parikrama would be complete.