The European river system is complex and extensive. Two of its main rivers, the Rhine and the Danube, despite not having their sources relatively close to each other, meet in a famous example of natural escapism that has fascinated poets and geographers alike for centuries.
The Danube (Donau in German) originates not far from the city of Donaueschingen in the German Black Forest, at the confluence of two small rivers. In contrast, the sources of the Rhine are located in the heart of the Swiss Alps.
The Danube flows eastward, while the Rhine flows northwest, passing through Switzerland and entering Lake Constance from its southeast end. From the lake, it exits from its eastern end, flowing south of the Danube for about 120 kilometers until it reaches the Swiss city of Basel, where it turns 90 degrees to head north.
The dry bed of the Danube in the sinkhole near Immendingen. Photo: Markus Schweiß/Wikimedia
It is Lake Constance that connects the waters of both rivers. Although the Danube never enters the lake, a part of its water does. This is because in its upper course, just 23 or 24 kilometers after its source, the Danube disappears.
The place where this happens is appropriately called Donauversickerung (which in German means Danube sinkhole) and is near the town of Immendingen at an altitude of about 673 meters. There, the river water filters through the caverns of the karst system below, flowing through them in a southerly direction.
The first time the complete disappearance of the Danube was documented was in 1874. Since then, "disappearances" have been recorded ranging from just 29 days a year to 309 (the latter in 1921). The annual average of days when the river completely disappears is around 155 days, mainly in the summer months.
When it doesn't disappear completely, a part of its water continues to seep into the sinkhole and the rest continues its course, crossing half of Europe until it empties into the Black Sea in Romania. The "disappeared" water flows south through numerous cracks and small fissures, and 12 kilometers later reappears in Aachtopf at an altitude of about 475 meters.
Schematic of the sinkhole locations and the route to Aachtopf. Image: Kreuzschnabel/Wikimedia
Aachtopf is a spring that was originally thought to have a thermal origin, but it is actually the "disappeared" Danube water that emerges from a karstic cavity. With an approximate amount of 8,500 liters per second, Aachtopf is the spring from which most of Germany's mineral water is obtained.
Until 1877, the relationship between the sinkhole and the Aachtopf spring was suspected but could not be proven. On October 9th of that year, geologist Adolf Knop from the Karlsruhe Institute of Technology poured 10 kilograms of sodium fluorescein (an organic dye) along with 20 tons of salt and 1,200 kilograms of oil shale (obtained from rocks with high organic content) into the Danube just before the sinkhole.
After 60 hours, water colored with fluorescent green and with a tar-like taste emerged in the Aachtopf spring, demonstrating that it was indeed the "lost" water from the Danube.
A sign in Immendingen, that reads “Sinkhole – Here the Danube sinks dry on about 155 days per year.” Photo: Holger Gruber/Wikimedia
At Aachtopf, the water from the Danube becomes a new river called Radolfzeller Aach, which flows into Lake Constance. As we saw earlier, the river that exits the lake is none other than the Rhine. In this way, a part of the Danube's water also flows into the Rhine, a curious and striking feature of the great European hydrographic basin.
But there's more, because according to experts, it's likely that in the future, the current upper course of the Danube will completely divert towards the Radolfzeller Aach, and therefore towards the Rhine. If that were to happen, the new sources of the Danube would have to be located in small tributaries that flow into the river past the sinkhole, specifically the Krähenbach and Elta.
The Donauversinkung and the dry riverbed are a popular natural attraction, where you can walk between mid-May and mid-September. Entire families spend these months hunting for Jurassic fossils dragged along by the current that are in sight and within reach for the delight, especially of the little ones. There are even guided tours for this purpose.
The dry river bed of the Danube. Photo: Wikimedia
This article was originally published in La Brújula Verde. It has been translated from Spanish and republished with permission.
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