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Am I understanding correctly that the Danube always seeps into the ground near Immendingen (since 300 years), but if the conditions are dry enough there is not enough water to flow past this sink hole, so all of it drains into the European watershed?

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).




This is correct. The main sinkhole is roughly 3 km downstream from the source. Disappearing water was first noticed 300 years ago, and the first total disappearance was in the 1870s. It is expected that this process will eventually make a small creek [1] the new source river of the Danube over the next 1,000 to 10,000 years.

There is a group of hobby enthusiasts who are digging into the cave system since 1990 [2], 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 [3][4][5].

[1] https://de.wikipedia.org/wiki/Kr%C3%A4henbach_(Donau)

[2] http://www.wolfgang-bauer.info/pages/reportagen/schwarze_don...

[3] https://cache-graphicslib.viator.com/graphicslib/thumbs674x4...

[4] https://cdn.tourismontheedge.com/wp-content/uploads/2011/02/...

[5] http://www.pension-silvia.si/images/activities/the_skocjan_c...


interesting. You can visit a similar cave system with an underground river (and go on a boat ride) in Czech Republic https://en.m.wikipedia.org/wiki/Punkva_Caves




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