Can you explain the analogy of the River Ganges used in the opening of Chapter 47?

In Chapter 47, an analogy is used to highlight the supreme purity and power of saints. It describes the River Bhagirathi (Ganges) as a force that washes the feet of the world, thereby purifying others of their sins. However, in this process, the river herself becomes soiled by the accumulated impurities. The chapter poetically states that the river then desires to be purified and longs for the day when saints will come to bathe within her, knowing that the dust from their feet is the only thing that can grant her deliverance from her own collected sins. This illustrates that the cleansing power of a saint like Sai is superior even to that of the holy Ganges.