The Aa is a 20.6 km long,

orographic Orography is the study of the topographic relief of mountains, and can more broadly include hills, and any part of a region's elevated terrain. Orography (also known as ''oreography'', ''orology'' or ''oreology'') falls within the broader discip ...

left tributary of the

Nethe Nethe is a river of North Rhine-Westphalia, Germany. It flows into the Weser near Höxter. See also *List of rivers of North Rhine-Westphalia A list of rivers of North Rhine-Westphalia, Germany: A * Aa, left tributary of the Möhne * Aa, l ...

river in

North Rhine-Westphalia North Rhine-Westphalia (german: Nordrhein-Westfalen, ; li, Noordrien-Wesfale ; nds, Noordrhien-Westfalen; ksh, Noodrhing-Wäßßfaale), commonly shortened to NRW (), is a state (''Land'') in Western Germany. With more than 18 million inhab ...

Germany Germany, officially the Federal Republic of Germany (FRG),, is a country in Central Europe. It is the most populous member state of the European Union. Germany lies between the Baltic and North Sea to the north and the Alps to the sou ...

. The Aa belongs as a tributary of the Nethe to the river system of the

Weser The Weser () is a river of Lower Saxony in north-west Germany. It begins at Hannoversch Münden through the confluence of the Werra and Fulda. It passes through the Hanseatic city of Bremen. Its mouth is further north against the ports o ...

Geography

The Aa's source is located on the eastern slope of the Rehberg (427.4 m), part of the

Egge Hills The Egge Hills (german: Eggegebirge, ), or just the Egge (''die Egge'') is a range of forested hills, up to , in the east of the German state of North Rhine-Westphalia. Geography The Egge extends from the southern tip of the Teutoburg Forest ...

, at a height of 350 m, 2 kilometers east of

Altenbeken Altenbeken () is a municipality in the Paderborn (district), district of Paderborn, in North Rhine-Westphalia, Germany. Geography Altenbeken is situated in the Eggegebirge, approx. 15 km northeast of Paderborn. To the west of the town is th ...

and over the eastern entrance of the ''Rehbergtunnel'' in Langeland, Bad-Driburg. The creek flows 2 kilometers east before it passes beneath the Herford-Altenbeken train line. Shortly after the Aa turns primarily south-easterly and passes the eastern side of the Reelsen community. The ''Rautebach'' creek joins the Aa on the left at Alhausen. The ''Schwallenbach'' creek joins the Aa on the right east of

Bad Driburg Bad Driburg () is a town and spa in Höxter district in North Rhine-Westphalia, Germany, situated on the river Aa and the Altenbeken–Kreiensen railway. Geography Bad Driburg lies on the eastern slopes of the Eggegebirge which is roughly 20&nb ...

. After passing beneath the Altenbecken-Kreiensen train line, the Hilgenbach also joins on the right side. At Herste the ''Katzbach'', the 8.2-km longest tributary of the Aa, joins the former on the right hand. West of

Istrup Istrup is one of 19 villages belonging to the city of Blomberg, North Rhine-Westphalia. It is about 4 km away from Blomberg and has a population of about 1600, making it the second-largest village in Blomberg. Infrastructure Istrup is l ...

the ''Escherbach'' joins the Aa on the left side. After having flown through the village ''Riesel'' the Aa joins the

on the left southwest of

Brakel Brakel () is a municipality in the Belgian province of East Flanders in the Denderstreek and the Flemish Ardennes. The name is derived from a Carolingian villa ''Braglo'' first mentioned in 866 and located in the center of Opbrakel. Since 1970, ...

at an elevation of 133 m. The Aa is 20.6 km long and drops 217 m, which corresponds to an average gradient of 10.5%. The Aa's basin is 80.153 km² large. With large rainfall in the

Eggegebirge The Egge Hills (german: Eggegebirge, ), or just the Egge (''die Egge'') is a range of forested hills, up to , in the east of the German state of North Rhine-Westphalia. Geography The Egge extends from the southern tip of the Teutoburg Forest ra ...

quickly large amounts of water will flow through the Aa valley.

Tributaries

* ''Rautebach'' - 2.6 km long, left tributary at 193 m elevation.Topografische Map 1:25.000 * ''Schwallenbach'' - 5.0 km long, right tributary at 181 m elevation. * Hilgenbach - 5.5 km long, right tributary at 172 m elevation. * ''Katzbach'' / 8.2 km long, right tributary at 155 m elevation. * ''Escherbach'' - 4.9 km long, left tributary at 149 m elevation. One of its particularities is the unnamed, left tributary, only a few hundred meters long, at the southernmost point of the ''Johannes-Gelhaus'' path at Reelsen, where one can observe the creek well. As a

Tufa Tufa is a variety of limestone formed when carbonate minerals precipitate out of water in unheated rivers or lakes. Geothermally heated hot springs sometimes produce similar (but less porous) carbonate deposits, which are known as traverti ...

source and under the influence of moss the creek forms small

Calcareous sinter Calcareous sinter is a freshwater calcium carbonate deposit, also known as calc-sinter. Deposits are characterised by low porosity and well-developed lamination, often forming crusts or sedimentary rock layers. Calcareous sinter should not be conf ...

. During the summer the creek dries out.

Mill Digs

Reelsen, Alhausen, Istrup, and Riesel still have old mill digs. Herste Katzbach

Ashlar bridges

Some old bridges of

ashlar Ashlar () is finely dressed (cut, worked) stone, either an individual stone that has been worked until squared, or a structure built from such stones. Ashlar is the finest stone masonry unit, generally rectangular cuboid, mentioned by Vitr ...

are still preserved over the Aa. * At the ''Josefmühle'', an arched bridge (replaced by a simple concrete one). * In Herste, an arched bridge over the ''Katzbach.'' * In

, an arched bridge with four arches at the ''Istruper Strasse''.''Deutsches Gewässerkundliches Jahrbuch Weser-Ems 2008''
Niedersächsischer Landesbetrieb für Wasserwirtschaft, Küsten- und Naturschutz, retrieved 22 January 2016 (PDF, German, 6.18 MB).

Pictures

File:Holzbrücke über die Aa in Reelsen.jpg, South of Reelsen a wooden bridge connects the Johannes-Gelhaus path over the Aa. (5 August 2011) File:Trockengefallene Sinterterrassen eines Nebenstroms der Aa bei Reelsen.jpg, Dried out Calcareous sinter File:Aa-Brücke - In der Stiege.jpg, Bridge of the 'In der Stiege' road from Alhausen to Bad Driburg. Water flows here even though the Aa had dried up beneath Reelsens and the Rautebach also lay dry at the time.

References

{{reflist Rivers of North Rhine-Westphalia Rivers of Germany