How common and important were watermills in the late Western Empire? Did they become less common immediately after the dissolution of the Western Empire? And if so, did they become more common suddenly in the high middle ages? How important was mechanical hydropower (for, I dunno, processing large amounts of metal or something) at this time, was it a major factor contributing to manufacturing productivity, and was that productivity in any way a causative factor in the "Renaissance"?
I have a hard time understanding why using watermills for manufacturing would become a "lost art" in Europe, presumably the Eastern Empire would still have plenty of use for them. And presumably watermills were built not far from where people actually lived and could use them. Was there less demand for manufactured goods in the early middle ages vs late antiquity?
To be clear, this claim was from a random European travel show that seemed marketed towards retired Americans, so I don't think fidelity to contemporary academic opinions on late medieval manufacturing and the Renaissance is high on their list of priorities.
Thank you!
Watermills continued to be common in western and northern Europe after the fall of the western Empire, and they were common beyond the former borders of the Empire. For example, watermills were common in Ireland by 600, if not before (it is at about this time that law codes, previously transmitted orally, were written down, including laws related to watermills). The fundamental technology of water-power was not lost.
Most Roman and medieval watermills were grain mills, e.g., for making flour. Industrial mills were far fewer. The main Roman industrial mills appear to have been for (a) sawmills, for sawing wood or stone, and (b) ore crushing mills. Most late medieval industrial mills in NW Europe are fulling mills (for processing wool for clothmaking), next most common are water-powered hammers for forging iron, and then sawmills and water-powered grinding wheels for grinding metal. Ore crushing mills were also used, in mining areas.
There is very little evidence for industrial watermills between 500 and 1000. Four mills are recorded in the Domesday Book as paying rents in the form of iron, and it has been suggested that they were working in the iron industry; this is quite possible, but far from certain. One possible conclusion is that industrial milling disappeared from NW Europe between 500 and 1000, which would mean a post-Roman disappearance (there had been Roman industrial mills in NW Europe). The same applies to Italy. The case of Spain is less clear, but there doesn't appear to be any certain evidence of industrial post-Roman mills before 1000. It has been suggested that the sugar and paper industries used water power, but it isn't certain. One important point: the purpose of many mills is not known, and they could have been industrial mills. This is the case for both mills known from literary sources and archaeology. A text might just say it's a "mill", and if the archaeological remains consist of a millrace (a channel dug to provide water for the mill) and nothing of the mill itself remains, we can't be sure of the function. The written evidence for 500-1000 is rather sparse, so we should assume too much.
In any case, it seems that either (a) industrial mills were absent from NW and SW Europe between 500 and 1000 or (b) they were rare.
When clear evidence for fulling mills appear in England (in the 12th century), fulling mills seem to be both much less important and less profitable than grain mills. If the available water could support either a grain mill or a fulling mill, but not both, a grain mill would be there. The cloth industry in regions using fulling mills didn't outcompete the cloth industry were fulling was still done by hand (which often meant done by foot). Even when watermills were being used for industrial purposes, they didn't always become common. The English iron industry only began making common use of waterpower after 1500. Grain milling was important because grain was the staple food, but the greater profitability of grain mills was not always purely a result of the free market: in many cases, the lord of a manor had a monopoly on milling, and it illegal for his tenants to hand-mill grain at home (or travel to a cheaper neighbouring mill).
This tells us why industrial mills might have been quite rare, even if the technology was still available. A labour-saving device only saves labour if the labour needed to build and maintain it is less than the labour it saves in use, and a cost-saving device only saves money if the cost of building and maintaining it are less than the money it saves in processing. Mills were expensive to build and maintain, and represented a significant investment. A small settlement might find it easier and cheaper to hand-mill their grain, or use animal power, rather than to build and maintain a watermill (or a windmill, when windmill technology spread). Where cloth is made for local use, water-power for fulling isn't likely to be useful. Where there is a profitable export trade, fulling mills become more attractive. Increased trade, and a growing money economy, could provide both the motivation for industrial water-power and the capital to provide it. This is sufficient to explain the growth in industrial milling that we see, and the non-visibility of industrial milling between 500 and 1000 is plausibly explained by few industrial mills combined with less literary and archaeological evidence in general.
In summary, we don't know whether industrial milling technology was lost in western Europe or not. Water-power technology was certainly not lost.
References:
P. Squatriti (ed), Working with water in medieval Europe: Technology and resource-use Brill, 2000.
Lucas, A. (2005), "Industrial Milling in the Ancient and Medieval Worlds: A Survey of the Evidence for an Industrial Revolution in Medieval Europe", Technology and Culture 46(1), 1-30. http://www.jstor.org/stable/40060793