In the early Middle Ages, in northern Europe, spelt was one of the primary grains for human diets. In the ninth century in what is today Belgium, spelt comprised between sixty and eighty percent of grain production. This staple grain was milled into flour and made into bread. (Hugh of St. Victor tells us there are two types of food: bread, and the stuff you eat with bread.)
The watermill was developed sometime around the first or second century C.E. It spready slowly, although plenty of classical historians argue that it didn't spread nearly as slowly as many medievalists would have you believe. Anyways, by the ninth century, watermills were increasingly well-ensconced as the way to grind grain, trumping the slower methods of grining by handmill or animal power.
Unlike its relative wheat, a "naked grain", spelt has extra protective layers. Those extra layers made it a bit fiddly to grind into grain, and it was processed inefficiently - inadequately - by the mechanically-powered mills. It was better processed by hand. Thus, by the tenth century, spelt production was falling rapidly - a major part of the early Medieval diet killed off by the watermill.