Inversely our local river, the Trent, has markings for historic height and floods. Perhaps it has something similar to this but we've not had drought as yet in my living memory. A lot of Britain is apparently in that territory right now. Most areas have a hosepipe ban (a semi annual tradition here from the water companies) but thats about it so far. I hear Germany has it fairly badly.
Thanks to modern technology we can just release some long-sequestered pollutants, moving the immediate localized danger these stones warn of into a long-term globalized hellscape. Woohoo go humans!