Hydro Power in the Middle Ages:
ca. 500 CE–1500 CE

Between 500 and 1500 CE, the use of hydro power exploded across much of Europe and Asia. The foundational waterwheel designs (horizontal, undershot, overshot) remained the same, but were modified, refined, and at times adapted for new environments. Written records for the Middle Ages are often fragmentary and incomplete, so it is difficult to fully detail the spread and extent of water power usage in this period. The records that do exist, however, paint an impressive picture. One of the most comprehensive sources available for the study of this period is the Domesday Book, an extremely thorough survey of land use and ownership launched by the Crown of England in the late eleventh century. The Domesday Book indicates that there were over 5,600 watermills in England in the 1080s, situated in over 3,000 different locations. Southern France and northern Italy were also regions where hydro power was used extensively. The Bazacle, a dam and mill complex on the River Garonne in southern France, was likely the largest dam in Europe (and possibly the world) at the time it was built.

Other evidence points to the growing importance of tide mills in this period—mills built along coastal areas, powered by undershot waterwheels that drew their energy from the rising and falling of the tides rather than from the steady flow of rivers. Archaeological excavations indicate that such mills were built along coastal Ireland as early as the seventh century, and became increasingly common on the shores of the British Isles and France over the next several hundred years. The use of hydro technology in the Middle Ages was not confined to Europe; indeed, the available evidence points to a flourishing of water-powered technology in other parts of the world as well. Dozens of archaeological sites, most particularly in present-day Iraq, Iran and the Arabian Peninsula, point to the extensive use of water power throughout the Islamic world after the seventh century. Similarly, the use of water power grew in China in this period. By the late tenth century, the Chinese government owned and operated water mills to ensure the supply of flour to the imperial capital.

There is significant debate regarding the diffusion of water-powered technology in this period, and whether particular innovations originated in Europe or Asia. What is clear, however, was that water power in this period was harnessed for a wide range of industrial purposes. Though the most important function of water power remained the milling of grain, it was increasingly harnessed for a variety of industrial purposes as well—for example, sawing wood, processing food, forging metals or manufacturing textiles. This was made possible through the addition of cams and cranks, which converted the rotary motion of the

waterwheel into reciprocal vertical motion (such as a hammer moving up and down) or horizontal motion (such as a saw moving back and forth). Many of the technologies that made these applications possible have their roots in the Roman period—for example, waterwheels had been used to power sawmills during the late Roman Empire. In the Middle Ages, however, these technologies were improved, refined, and applied on a much larger scale, allowing for an unprecedented ability to harness water power. In Europe, and particularly Great Britain, this growth of hydro power set the stage for the Industrial Revolution.