| Seerhein | |
|---|---|
 Seerhein between Obersee (above) and Untersee (below) | |
 | |
| Location | |
| Country | Germany |
| State | Baden-Württemberg (Konstanz) |
| Country | Switzerland |
| Canton | Thurgau (Kreuzlingen) |
| Physical characteristics | |
| Source | |
| • location | Upper Lake Constance (Obersee) in Konstanz |
| Mouth | |
• location | Lower Lake Constance (Untersee) west of Gottlieben |
| Length | 4.3 km (2.7 mi) |
| Basin features | |
| Tributaries | |
| • left | Grenzbach, Dorfbach |
The Seerhein (lit. 'Rhine of the lake(s)'; c.f. Rheinsee) is a river about 4.3 km (2.7 mi) long, in the basin of Lake Constance (German: Bodensee). It is the outflow of the Upper Lake Constance and the main tributary of the Lower Lake Constance. The water level of the Lower Lake is about 30 cm (12 in) below the level of the Upper Lake. It is considered part of the river Rhine, which flows into Lake Constance as the Alpine Rhine and flows out of the Lower Lake as the High Rhine.
The Seerhein arose after the last ice age (the Würm glaciation, about 9650 BCE). Some time after this period, the water level of Lake Constance gradually dropped by about ten metres and the shallow parts fell dry. Some parts of the Seerhein still have a lake-like character.
The border between Germany and Switzerland runs along the river within its lower stretch; the German city of Konstanz (Constance), the largest community on the river, is situated on both sides of the upper part of the Seerhein.