Indeed the river is still flowing but in a tunnel under the street After only a few years of Roman occupation, when the town had no protective walls a local British Queen "Boudicca", from East Anglia, burnt the place to the ground!
After Boudicca, over the next 200 years the Romans built "Londinium" into a fortified city of considerable wealth and comfort covering an area of some 330 acres surrounded by a wall 20 feet high.
When the Romans left (to defend Rome against the German Vandal tribe) the German Saxons moved in.
Being basically rural peoples they had no use for towns and London was abandoned.
The Saxons chose Winchester just north of present day Southampton as their base.
It was not until some 200 years after the departure of the Romans (about AD 670) that the London area was reoccupied (by the Saxons) to any effective level and it was in a new area around present day Charing Cross and Trafalgar Square that was to be their base for their new city and port.
Roman London eventually stretched from the Tower (built 1000 years later) in the east to the Fleet River in the west.
London Wall was the northern boundary and over a bridge (on the site of present day London Bridge) to present day Southwark described the city limits in the south .
London would have had all the trappings of Rome at that time.
An easy passage by ship down the river Medway and up the Thames.
The Romans used London to bring in all their vital goods from the rest of their empire particularly of course military equipment.
This little cosy scene was catastrophically changed with the arrival of 350 Danish Viking ships in the Thames in about AD 842.
And in AD 865 a massive Danish Viking army assembled in East Anglia, destroyed Saxon rule in the East and North of England and made London their winter HQ.