Since Roman was expanding and it didn't have control of their lands and couldn't govern most of it. It kept expanding until the western part of Roman was conquered by Barbarians.
IT was estimated about 800,000 Vietnam .
The answer to the question above is water-powered machinery requires continuous running water for it to work. While steam-powered machinery requires water to produce steam, it is still different and advantageous compared to water-powered because it does not require continuous running water.
