River experience between Lake Constance and Aach spring

Starting from the old bishop’s town (Radolfzell was indeed founded by a bishop, whose diocese was however Verona), the route leads through the hilly Hegau to the Aachtopf, Germany’s most water-rich spring. The water of the Aach rises from an 18-meter deep underground cave system and forms a small atmospheric lake. In the cave system beneath the karst mountain range, the water of the Danube collects, which seeps away near Tuttlingen and Immendingen, then flows under the European watershed within 20 hours and emerges here again. Instead of flowing into the Black Sea, the water now flows into Lake Constance and through the Rhine to the North Sea.

Overall, the river meanders 32 kilometers more or less through the unique cultural landscape of the Hegau. Rich animal and plant wetlands (Weitenried, Aachried) are crossed. Picturesque villages line the banks of the river, and the volcanic town of Singen is lively. The volcanic neck of Hohentwiel towers steeply above the banks of the Aach. A detour to the summit is worthwhile not only because of the fortress – with nine hectares, one of the largest fortress ruins in Germany – but also because of the wide view of the Lower Lake, the Hegau region, Switzerland, and, in clear weather, the Alpine panorama. Notably, at the slopes of the Hohentwiel, a uniquely tasty Lake Constance wine grows due to the especially mineral-rich soil.

Along the short stretch from the spring to the mouth into Lake Constance, the Aach passes about 30 islands. Their water power drives nearly a dozen hydroelectric plants, both records for the Lake Constance region. Due to its abundant water power, the Aach was an important engine for the industrial development of the Hegau. Today, numerous river sections have been renaturalized. Special river experiences are ensured by renaturation projects along the Aach banks carried out for 20 years, such as in southern Singen, between Worblingen and Bohlingen, at "Hausen Beach," or since summer 2020 at the "Promenade Beuren," where, sensibly, an "E-bike charging station" has even been installed.

The fourth-largest inflow to Lake Constance also forms an extensive delta, an important breeding and resting place for numerous migratory birds.