Just saw a programme on the History Channel about Roman engineering that mentioned that german tribesmen finally sabotaged the aqueduct system in either the fifth or sixth century (apologies for my failing mental faculties) with the knock on effect that without the man made water supply Rome depopulated quite rapidly.
Don’t have my text books in front of me, but I recall that several of the aqueducts continued working in reduced capacity until medieval times (maybe 12th century?). I do know that several Popes contributed funds to restore/maintain Rome’s working aqueducts. The principal problem behind these efforts was that the central authority which ancient Rome once commanded was lost and the ability to organize and maintain a large bureaucracy to oversee public works like the aqueducts was lost. Small scale works to “maintain” an aqueduct happened, but an official office with effective funding and authority which could pass through generations did not exist. Without such a central power/bureaucracy it was just a matter of time before Rome’s aqueducts would slit over and decay.