The issue of water supply was mentioned, and I thought at the time, it would have taken a lot of work to use water from the river, rather than from a spring. This point was brough home on a later programme on BBC2. Boris Johnson was at the Pont du Gard ,Roman Aqueduct in France. It was said that the prime purpose of this massive piece of engineering, was to supply water to the baths and fountains.
As well of course, to show the power of the Emperor.