| Aa | |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | North Rhine-Westphalia |
| Subdivision Type3: | Reference no. |
| Subdivision Name3: | DE: 4526 |
| Length: | 20.6 km |
| Source1 Location: | On the east slope of the Eggegebirge, east of Altenbeken, above the Rehbergtunnel |
| Source1 Coordinates: | 51.7686°N 8.9761°W |
| Source1 Elevation: | ca. |
| Mouth Location: | Southwest of Brakel into the Nethe |
| Mouth Coordinates: | 51.7014°N 9.1703°W |
| Mouth Elevation: | approximate |
| Basin Size: | 80.153 km2[1] |
| Discharge1 Location: | at Rustenhof gauge |
| Discharge1 Min: | Record low: 102 L/s (in 10.08.1996) Average low: 212 L/s |
| Discharge1 Avg: | 1.41 m³/s |
| Discharge1 Max: | Average high: 30.6 m³/s Record high: 71.2 m³/s (in 28.10.1998) |
| Custom Label: | References |
| Extra: | The weak river dries out during a hot summer leaving behind the rubble of the creek floor. (5 August 2011) |
| Pushpin Map: | North Rhine-Westphalia |
| Pushpin Map Size: | 300 |
| Pushpin Map Caption: | North Rhine-Westphalia |
| Pushpin Map Alt: | Relief Map of North Rhine-Westphalia |
The Aa is a 20.6 km long, orographic left tributary of the Nethe river in North Rhine-Westphalia, Germany. The Aa belongs as a tributary of the Nethe to the river system of the Weser.
The Aa's source is located on the eastern slope of the Rehberg (427.4 m), part of the Egge Hills, at a height of 350 m, 2 kilometers east of Altenbeken 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. 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 the Escherbach joins the Aa on the left side. After having flown through the village Riesel the Aa joins the Nethe on the left southwest of Brakel 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 quickly large amounts of water will flow through the Aa valley.
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 source and under the influence of moss the creek forms small Calcareous sinter. During the summer the creek dries out.
Reelsen, Alhausen, Istrup, and Riesel still have old mill digs.
Some old bridges of ashlar are still preserved over the Aa.