How did the city of Pataliputra emerged?

Prior to Mauryan Empire Pataliputra was not a city as we do not find it being mentioned among cities in any of the early Buddhist text. Rather we find a place named Pataligram. Pataliputra's recorded history began with Ajatshatru, the second of the Magadhan kings, who established a small fort in 490 BCE at Pataligram where the Ganga and Sone Rivers met in order to better fight his enemies, the Licchavis.
In later half of 5th century BCE we find that Magadha rulers shifting their capital from Rajagaha to there. But even then it was not a big city but they renamed it as Pataliputra. However when Mauryan overthrew Nanda dynasty at the fourth century BCE and become the capital of Mauryan empire, Pataliputra became one of the major cities. Which described fondly by Megasthenes in his work Indika. But with the decline of the Mauryan empire we also see gradual decline of the city too.