Short answers, pulled from the story.
Philo of Byzantium wrote a technical treatise describing the watermill in the year 200 BC. This document detailed a machine capable of turning the force of a flowing river into mechanical power to grind grain.
The Domesday Book of 1086 recorded 5,624 watermills in England alone. Historians believe the true count likely exceeded 6,000 when unrecorded northern reaches of the country are considered.
The largest known concentration of mechanical power in the ancient world was located at the mill complex at Barbegal in southern France. This site operated with sixteen overshot waterwheels and ground enough flour to feed twelve thousand people daily.
The Tang dynasty Ordinances of the Department of Waterways were written in 737 AD to regulate the use of watermills. These regulations ensured that mills did not interrupt riverine transport and allowed the government to demolish non-compliant mills.
The overshot wheel was two and a half times more efficient than the undershot wheel. This design utilized both the impulse of the water and the weight of the water in the buckets to turn the wheel.
An estimated 200,000 watermills operate in India today. These mills are widely used for processing grain in developing countries where electrical energy is less available.