Evolution of earlier melas to Kumbh Melas
There are several references to river-side mela (festivals) in ancient Indian texts including at the places where present day Kumbh Melas are held, both the earliest exact age of those melas and when they came to be called the Kumbh Mela is uncertain.
Earliest mention of any type of mela held at the the current location of Kumb Mela is by Xuanzang in 644 CE. The earliest extant mention of the name “Kumbha Mela” are Khulasat-ut-Tawarikh (1695 CE) and Chahar Gulshan (1759 CE) which describe the fairs held at Haridwar, Prayag and Nashik, among those the magh fair at Prayag might be oldest without being called Kumbh Mela at those time and the fair at Haridwar appears to be the original Kumbh Mela which is held according to the astrological sign “Kumbha” (Aquarius), and the renaming of fairs held at other places to Kumbh Mela is more recent.
The earliest mention of the such riverside melas at the current locations of Kumbh Mela, [without mentioning the word Kumbh mela], is by the Chinese traveler Xuanzang (Hiuen Tsang) who describes a ritual organized by Emperor Shiladitya (identified with Harsha) at the confluence of two rivers, in the kingdom of Po-lo-ye-kia (identified with Prayaga), where half a million pilgrims of various faiths took a bath at the confluence to wash away their sins. According to some scholars, this is the earliest surviving historical account of a mela held at Prayag in 644 CE where the present day Kumbh Mela is also held. However, Australian researcher Kama Maclean notes that the Xuanzang reference is about an event that happened every 5 years (and not 12 years), and might have been a Buddhist celebration (since, according to Xuanzang, Harsha was a Buddhist emperor).