Origin of the Danube-Aach system

The Swabian Alb formed by an Upper Jurassic carbonate sequence is the most extensive karst area of Germany. The western part is crossed by the upper Danube River. It represents an old, mainly Pliocene, drainage system that is now restricted by the young Rhine system. The low base level of the upper Rhine graben causes a strong headward erosion. Since the Upper Pliocene, the Danube has lost more than 90% of its headwaters. The underground Danube-Aach karst system of the western Alb represents the last capture of the Rhine, leading periodically to a complete loss of water in the upper Danube. The seepage of this water, together with the huge karst catchment area, supplies the strong discharge of the Aach Spring, forming the largest spring of Germany with an average discharge of 8.5 m³ s^–1.