If pubbe is taken as adj. modifying 'nagare', then it would mean 'In the former saavatthi city'. Here, pubbe is not used as an adj.
'Pubbe' is a nipaata but used in the sense of locative: 'in front', 'in the past', 'in the past life'.
'Pubba' can also mean 'pus'. As an adj.,' pubba' means former, earlier, eastern, first, foremost; eastern; earlier, former, preceding; ancient, customary.