'However something around the 4th c. CE happens and the Aqueducts are destroyed and the population plummets.
I'm not sure about the 4th century but in about 535 something happened across the world to cause untold damage and suffering. Some say it was a comet or asteroid, some a massive volcanic explosion that blotted out the sun.
At the same time that England went into a prolonged winter - 'In the year 536 the sun dimmed and the world shivered, leading to famine, plague and the fall of empires. New clues point to an double-whammy apocalypse'http://www.newscientist.com/article/mg2 ... ended.html
- Puma punku in Bolivia was destroyed.http://www.ancient-wisdom.co.uk/boliviapumapunka.htm
Procopius tells of problems in the Mediterranean area
'… during this year a most dread portent took place. For the sun gave forth its light without brightness...and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear.,History of the Wars: The Vandalic War
'http://www.ancient-origins.net/unexplai ... ard-001360
Meanwhile back in Blighty
'This is an extract from the Chronicles of the Kings of Britain who observed this Comet as described in the writings.
At this time a star of amazing size appeared. It had one beam,
and on the head of the beam was a ball of fire resembling a dragon ;
and from the jaws of the dragon two beams ascended, the one towards
the extremity of France, and the other towards Ireland, subdividing
itself into seven small beams.'
Taken from a quote in: http://www.thunting.com/smf/myths_legen ... html;wap2=
The site discusses possible reasons for the catastrophe of this period.
Even the Daily Mail gets involved with an article about Dallas Abbott's research and theory of an asteroid strike.http://www.dailymail.co.uk/sciencetech/ ... e-age.html
In the opposite corner is the argument that a massive volcanic eruption was responsible, possibly Krakatoa.http://www.relfe.com/07/super_volcano_ice_age.html
Between the two possibilities I imagine there would be enough to cover many a fine aqueduct.