Aachtopf (Aach)

 spring, river source, karst

The Aachtopf is Germany's biggest natural spring, producing an average of 8,500 liters per second. Most of the water is derived from the Danube River and is obtained where the latter river disappears underground at the Donauversickerung (Danube Sink) near Immendingen and Fridingen. Strangely, the Danube flows eastwards into the Black Sea, whereas the Rhine flows northwards to the North Sea. Therefore the water of the Aach flows under the European continental divide.

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Although the headwaters of the Danube are relatively small today, geologically, the Danube is much older than the Rhine, with which its catchment area competes in today's southern Germany. This has a few interesting geological complications. Since the Rhine is the only river rising in the Alps mountains which flows north towards the North Sea, an invisible line beginning at Piz Lunghin divides large parts of southern Germany, which is sometimes referred to as the European Watershed.

However, before the last ice age in the Pleistocene, the Rhine started at the southwestern tip of the Black Forest, while the waters from the Alps that today feed the Rhine were carried east by the so-called Urdonau (original Danube). Parts of this ancient river's bed, which was much larger than today's Danube, can still be seen in (now waterless) canyons in today's landscape of the Swabian Alb. After the Upper Rhine valley had been eroded, most waters from the Alps changed their direction and began feeding the Rhine. Today's upper Danube is but a meek reflection of the ancient one.

Since the Swabian Alb is largely shaped of porous limestone, and since the Rhine's level is much lower than the Danube's, today subsurface rivers carry much water from the Danube to the Rhine. On many days in the summer, when the Danube carries little water, it completely oozes away noisily into these underground channels at two locations in the Swabian Alp, which are referred to as the Donauversickerung (Danube Sink). Most of this water resurfaces only 12 km south at the Aachtopf, Germany's wellspring with the highest flow, an average of 8500 litres per second, north of Lake Constance—thus feeding the Rhine. The European Water Divide thus in fact only applies for those waters that pass beyond this point, and only during the days of the year when the Danube carries enough water to survive the sink holes in the Donauversickerung.

Since this enormous amount of underground water erodes much of its surrounding limestone, it is estimated that the Danube upper course will one day disappear entirely in favor of the Rhine, an event called stream capturing.