hunger stone

noun1

1. •
   A stone, either naturally exposed or intentionally embedded in a river during a drought, which is dated and often inscribed to mark the water level as a warning to future generations that they may face famine if the water reaches that low level again. Such stones are common in Central Europe.

   • Being here in Frankfurt reminds me of this region's famous »Hungersteine«, or hunger stones, as the Main River boatmen used to call the rocks that became visible when the water level was low. — 1993