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

tributary A tributary, or affluent, is a stream or river that flows into a larger stream or 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 drain the surrounding drainage ...

of the

Rhine ), Surselva, Graubünden, Switzerland , source1_coordinates= , source1_elevation = , source2 = Rein Posteriur/Hinterrhein , source2_location = Paradies Glacier, Graubünden, Switzerland , source2_coordinates= , so ...

in the south of

Baden-Württemberg Baden-Württemberg (; ), commonly shortened to BW or BaWü, is a 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 inhabitants across a ...

(

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

). It is approximately 32 km long.

Course

The source of the

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

is the ''

Aachtopf The Aachtopf () is Germany's biggest karst spring, south of the western end of the Swabian Jura near the town of Aach. It produces an average of 8,500 litres per second. Most of the water stems from the River Danube where it disappears undergro ...

'' in Aach, the largest

spring Spring(s) may refer to: Common uses * Spring (season) Spring, also known as springtime, is one of the four temperate seasons, succeeding winter and preceding summer. There are various technical definitions of spring, but local usage of ...

, at a height of 475 m above

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

. The water emerges from a

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

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

Danube The Danube ( ; ) is a river that was once a long-standing frontier of the Roman Empire and today connects 10 European countries, running through their territories or being a border. Originating in Germany, the Danube flows southeast for , pa ...

. Between

Immendingen Immendingen is a municipality in the district of Tuttlingen in Baden-Württemberg in Germany located on the Upper Danube. It is famous for the Danube Sinkhole. Geography Immendingen is located on the Upper Danube. On the municipal area are part ...

and

Möhringen an der Donau Tuttlingen (Alemannic: ''Duttlinga'') is a town in Baden-Württemberg, capital of the district Tuttlingen. Nendingen, ''Möhringen'' and ''Eßlingen'' are three former municipalities that belong to Tuttlingen. Tuttlingen is located in Swabia eas ...

and in

Fridingen Fridingen () is a town in the district of Tuttlingen, in Baden-Württemberg, Germany. It is situated on the Danube, 10 km east of Tuttlingen, and 23 km west of Sigmaringen. A large hoard of Bronze Age jewellery (mostly armlets and brac ...

, water from the Danube disappears underground (this is known as the ''

Danube Sinkhole Sinkhole in the Danube near Möhringen Schematic of the sinkhole locations and the route to Aachtopf Completely dry Danube riverbed right , Sink hole on the southern bank of the Danube, at the main sinkhole site below Immendingen Sign i ...

''); this water reappears in the Aachtopf. The river first flows through a number of nature reserves in the

Hegau The Hegau is an extinct volcanic landscape in southern Germany extending around the industrial city of Singen (Hohentwiel), between Lake Constance in the east, the Rhine River in the south, the Danube River in the north and the Randen (mountain r ...

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 very south ...

, 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 Radolfzell am Bodensee is a town in Germany at the western end of Lake Constance approximately 18 km northwest of Konstanz. It is the third largest town, after Constance and Singen, in the district of Konstanz, in Baden-Württemberg. Rado ...

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 ''Donauversinkung'', 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 milling (grinding), rolling, or hammering. Such processes are needed in the production of ...

s. Today, it drives several

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

stations. In some areas,

nature restoration Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...

projects are being carried out, for example in

Beuren an der Aach Beuren may refer to: *Beuren, Esslingen Beuren is a municipality in the district of Esslingen in Baden-Württemberg in southern Germany. It is a spa and resort. With the majority of its district (95.2%), Beuren is part of the Swabian Alb bi ...

, where the borough of

has set up a

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

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 A snipe is any of about 26 wading bird species in three genera in the family Scolopacidae. They are characterized by a very long, slender bill, eyes placed high on the head, and cryptic/camouflaging plumage. The ''Gallinago'' snipes have a near ...

sandpiper Sandpipers are a large family, Scolopacidae, of waders. They include many species called sandpipers, as well as those called by names such as curlew and snipe. The majority of these species eat small invertebrates picked out of the mud or soil ...

and

green sandpiper The green sandpiper (''Tringa ochropus'') is a small wader (shorebird) of the Old World. The green sandpiper represents an ancient lineage of the genus ''Tringa''; its only close living relative is the solitary sandpiper (''T. solitaria''). They ...

. 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 IOC adopted a consensus taxonomy of seven ge ...

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 an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

and

European bullhead The European bullhead (''Cottus gobio'') is a freshwater fish that is widely distributed in Europe, mainly in rivers. It is a member of the family Cottidae, a type of sculpin. It is also known as the miller's thumb, freshwater sculpin, common bu ...

, 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 Volkertshausen is a town in the district of Konstanz in Baden-Württemberg, Germany. Twin towns Volkertshausen is twinned with: * Bolsena, Italy * Schönau-Berzdorf Schönau-Berzdorf (IPA:ˈʃøˌnauˈbɛɐtsˌdɔɐf) is a municipality i ...

, Friedingen, Hausen an der Aach,

Rielasingen-Worblingen Rielasingen-Worblingen is a municipality in the Konstanz (district), district of Konstanz, in Baden-Württemberg, Germany. It is situated on the border with Switzerland, 4 km south of Singen, and 20 km east of Schaffhausen. This town i ...

, Bohlingen and the Rickelshausen district of

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.

Footnotes

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