Radolfzeller Aach

Radolfzeller Aach
Aachtopf, view from the Radolfzeller Aach source in the direction of the flow
Map, with place names in Alemannic
Country	Germany
State	Baden-Württemberg (District of Constance)
Cities	Radolfzell, Singen
Basin features
Main source	Aachtopf in Aach 475 m (1,558 ft) 47°50′48″N 8°51′29″E / 47.84667°N 8.85806°E / 47.84667; 8.85806
River mouth	into Untersee between Radolfzell and Moos 395 m (1,296 ft) 47°42′57″N 8°56′28″E / 47.71583°N 8.94111°E / 47.71583; 8.94111Coordinates: 47°42′57″N 8°56′28″E / 47.71583°N 8.94111°E / 47.71583; 8.94111
Progression	Rhine → North Sea
River system	Rhine
Basin size	261 km2 (101 sq mi)
Physical characteristics
Length	32 km (20 mi)
Discharge	Average rate: 10 m3/s (350 cu ft/s)
TVC	DE/2172

The Radolfzeller Aach (also known as Hegauer Aach) is a right or north tributary of the Rhine in the south of Baden-Württemberg (Germany). It is approximately 32 km long.

The source of the river is the Aachtopf in Aach, the largest spring in Germany, at a height of 475 m above sea level. The water emerges from a karst cave. About two thirds of the water originates in the headwaters of the Danube. Between Immendingen and Möhringen an der Donau and in Fridingen, water from the Danube disappears underground (this is known as the Danube Sinkhole); 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, 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.

The Radolfzeller Aach drains a watershed of 261 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.