By the end of the Brahma-vaivarta Purana, we find the narration of the departure of Lord Krishna from this world and how all the holy rivers headed by the personified Ganges approached Him:
he natha ramana-srestha yasi golokam uttamam
asmakam ka gatis catra bhavisyati kalau yuge
Ganga Devi said : “O Lord, best of lovers, now that You are going to the supreme abode, Goloka, what will be our situation here in this age of Kali?”
sri bhagavan uvaca
kaleh panca sahasrani varsani tistha bhutale
papani papino yanti tubhyam dasyanti snanatah
The Supreme Personality of Godhead said : “Sinful people will come to you and upon bathing will give you their sins. You must remain like this on earth for 5,000 years of the age of Kali.”
man-mantropasaka-sparsad bhasmi-bhutani tat-ksanat
bhavisyanti darsanac ca snanad eva hi jahnavi
O Ganga, then there will be many devotees worshiping Me by My mantra, and merely by their touch, glance and bathing all those sins will be immediately burnt.
Thus, Sri Krishna predicted the appearance of a powerful devotee who would spread the chanting of His names worldwide.
Reference: Brahma-vaivarta Purana (Krishna-janma-khanda, Chapter 129, 49-51)