The Radolfzeller Aach (also known as Hegauer Aach) is a right or north

tributary A tributary, or an ''affluent'', is a stream or river that flows into a larger stream (''main stem'' or ''"parent"''), river, or a lake. A tributary does not flow directly into a sea or ocean. Tributaries, and the main stem river into which they ...

of the

Rhine The Rhine ( ) is one of the List of rivers of Europe, major rivers in Europe. The river begins in the Swiss canton of Graubünden in the southeastern Swiss Alps. It forms part of the Swiss-Liechtenstein border, then part of the Austria–Swit ...

in the Hegau region, southern

Baden-Württemberg Baden-Württemberg ( ; ), commonly shortened to BW or BaWü, is a states of Germany, German state () in Southwest Germany, east of the Rhine, which forms the southern part of Germany's western border with France. With more than 11.07 million i ...

(

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

). It is approximately long and drains into Lower Lake Constance. Most of its waters derive from the

Danube The Danube ( ; see also #Names and etymology, other names) is the List of rivers of Europe#Longest rivers, second-longest river in Europe, after the Volga in Russia. It flows through Central and Southeastern Europe, from the Black Forest sou ...

river through the Danube Sinkhole.

Course

The source of the

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

is the '' Aachtopf'' in Aach, the largest spring in

, at a height of 475 m above

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

. The water emerges from a

karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...

cave. About two thirds of the water originates in the headwaters of the

. Between

Immendingen Immendingen is a municipality in the district of Tuttlingen (district), Tuttlingen in Baden-Württemberg in Germany located on the Upper Danube Nature Park, Upper Danube. It is famous for the Danube Sinkhole. Geography Immendingen is located on ...

and Möhringen an der Donau and in Fridingen, water from the Danube disappears underground (this is known as the Danube Sinkhole or ); this water reappears in the Aachtopf. The river first flows through a number of nature reserves in the Hegau area. This segment of the river is known as ''Hegauer Aach''. It then flows South to

Singen Singen (; Low Alemannic German, Low Alemannic: ''Singe'') is an industrial city in the very south of Baden-Württemberg in southern Germany and just north of the German-Swiss border. Location Singen is an industrial city situated in the far sout ...

, where it is known as ''Radolfzeller Aach''. The last six kilometers of the lower course flow through a nature reserve called ''Radolfzeller Aach Ried''. It flows into the northwestern part of Lake Untersee (the ''Zellersee'') between Radolfzell and Moos. At the mouth, the average discharge is 10 m³/s, making it the fourth largest contributor to Lake Constance.

Catchment area

The Radolfzeller Aach drains a watershed of 255 km². If one includes the catchment area of the Danube upstream of the Danube Sinkhole (), then the Radolfzell Aach drains an area of 1560 km² on days when all of the Danube's water disappears into the Danube Sinkhole.

Hydrology

In the past, the fast flowing water of the Radolfzeller Aach drove many

watermill A watermill or water mill is a mill that uses hydropower. It is a structure that uses a water wheel or water turbine to drive a mechanical process such as mill (grinding), milling (grinding), rolling, or hammering. Such processes are needed in ...

s. Today, it drives several

hydropower Hydropower (from Ancient Greek -, "water"), also known as water power or water energy, is the use of falling or fast-running water to Electricity generation, produce electricity or to power machines. This is achieved by energy transformation, ...

stations. In some areas, nature restoration projects are being carried out, for example in Beuren an der Aach, where the borough of

has set up a

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

catching facility, catching sediments and gravel carried by the river. Plans to release the sediments upstream of Volkertshausen were abandoned.

Fauna

Several endangered species of waterfowl can be found at the Radolfzeller Aach, including snipe, sandpiper and green sandpiper. In the estuary, beavers established themselves in 1998, after having been absent for 200 years.International Symposium on Beavers in Lake Constance
Hydromorphologically, the Radolfzeller Aach is the lower end of trout and mullet territory. The dominant fish species in the Aach is the brown trout, as grayling are less shy and consequently more exposed to the feeding pressure of the large

cormorant Phalacrocoracidae is a family of approximately 40 species of aquatic birds commonly known as cormorants and shags. Several different classifications of the family have been proposed, but in 2021 the International Ornithologists' Union (IOU) ado ...

colony in the Radolfzeller Aach Ried. There are also noticeable stocks of some other endangered fish species, such as

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

and European bullhead, who enjoy their protection close to the bottom of the river. In the lower reaches, we also find other species from Lake Constance.

Communities

Villages on the Radolfzeller Aach include Aach, Volkertshausen, Beuren an der Aach, Friedingen, Hausen an der Aach,

, Rielasingen-Worblingen, Bohlingen and the Rickelshausen district of Radolfzell.

Water sports

Canoeing is possible all year round, due to the strong Aachtopf spring. Canoeing in the section between Aachbad Singen and Bohlingen, however, is not advisable on account of the many water discharges and heavy industrialization. The lower reaches of the Aach are a nature preserve and closed to water sports year round.

References

{{Authority control Rivers of Baden-Württemberg Tributaries of Lake Constance Geography of Hegau Rivers of Germany