E: as the Danube laters flows through the once independent state of Württemberg, and the Rhine through the once independent state of Baden, there was also a nice court battle between them because of the lost water in the 1920s - the "Donauversinkungsfall"  ("the court case of the swallowing of the Danube")
It should be noted that this only the first few hundred meters of the Danube and it's relatively flat there so the exact way the water sheds is probably always subject to changes up there (at least across large spans of time). There are relatively small masses of water that flow up there, so things are not set in stone as much (literally).
There is a group of hobby enthusiasts who are digging into the cave system since 1990 , expecting to find a cave containing an enormous underground lake, as the amount of water disappearing into the underground is much larger than the amount of water later carried by the Aach. AFAIR, they managed to finally reach the underground river ~15 years ago. arte did a documentary about them, called "Die schwarze Donau" ("The black Danube"). You find it on YouTube, but it's in German.
E: here is an image of the underground Danube found by the hobbyists mentioned above after 13 years of digging: http://www.wolfgang-bauer.info/pages/reportagen/schwarze_don...
E2: if you are interested in the subject, I highly recommend visiting the Skocjan cave in Slovenia, which was also formed in a karst region by an underground river. I found the visit to be an almost surreal experience .
You can see the source of the Loue on the Wikipedia page. The river is basically coming out of the mountain with already a very high flow rate. This is one of the nicest river in the east part of France, it is a pleasure to walk along or navigate it.
Edit: Found videos where you can see the holes sucking the water in the river bed.
Suits my definition of drinking water.
Even so, sinkholes can be subtle enough to miss, and still fatal. Karst is some dangerous geology for boating. Although riverbeds are generally quite smooth, which is pleasant enough if you go for a swim.
There is very likely a sizable cave that exists beneath the river bed, the entrance to which had heretofore been clogged by sediment, which has been washed away by recent rains. Could also be that a choke deep in the cave itself has been washed out, joining it to another cave, which provided enough throughput to drain the river entirely underground.
Sometimes this can be affected by human activities(such as mining, as has happened with aquifer withdrawals and phosphate mining in Central Florida and Clear Springs
After reading, it is true but indeed needlessly alarming. Changing underground water levels are influencing the rotation of the Earth, but it's mainly happening in India and East Europe, way far from the east of France.
"The researchers found the answer in Eurasia. "The bulk of the answer is a deficit of water in Eurasia: the Indian subcontinent and the Caspian Sea area," Adhikari said.
The finding was a surprise. This region has lost water mass due to depletion of aquifers and drought, but the loss is nowhere near as great as the change in the ice sheets.
So why did the smaller loss have such a strong effect? The researchers say it's because the spin axis is very sensitive to changes occurring around 45 degrees latitude, both north and south. "This is well explained in the theory of rotating objects," Adhikari explained. "That's why changes in the Indian subcontinent, for example, are so important.""
Well, allmost all the water that gets consumed by people still end up in the rivers afterwards ...
(Sinkholes occur when an already-significant underground cavity, typically formed by water dissolving the rock, gets close to the surface and the roof falls in.) https://en.wikipedia.org/wiki/Sinkhole
> geologists say the river disappearance is due to large cracks in the karstic riverbed, which empty the river underground like a siphon. It is perhaps the important spring floods that washed away the sediment patches that blocked the holes and fissures.
Talks about a very visual thing .... doesn't have any photos.
It's crazy because all those media companies fired their photographers and in the age of the internet ... PHOTOS AND VIDEOS ARE IMPORTANT!
I don't know how common seasonal variation is (per the other comment about the Danube doing a similar thing to the French river).
 https://www.floridastateparks.org/park/Oleno https://www.youtube.com/watch?v=0JiXOk1pXus
Worst case scenario, the water permanently goes elsewhere, either to an aquifer or to another river or lake. People adapt. End of history.
So, you are saying this is an Armageddon level event?