The Rench is an eastern tributary of the Rhine in the Ortenau in Central Baden, Germany. It rises on the southern edge of the Northern Black Forest at Kniebis near Bad Griesbach im Schwarzwald. The source farthest from the mouth is that of the Schöngrundbächle which rises at a height of around 915 m above NN in the parish of Zuflucht. After flowing through its steep mountainside klinge it is joined by other streams to form, first, the Old (Alter), then the Wild Rench (Wilder Rench), which first becomes the Rench below Bad Griesbach. The Rench runs in a prominent southerly arc through the Central Black Forest and crosses the hilly region of Ortenau with its orchards and vineyards before breaking out into the Rhine Plain. After just under 57 km it, together with the Mühlbach, confluences with the Rhine at 314.7 km between Helmlingen (Rheinau) and Lichtenau.
The B 28 federal road, long an important route through the Black Forest, runs through the Rench valley linking the Strasbourg area with the upper Neckar region. It climbs steeply above Bad Griesbach in hairpin bends to the Alexanderschanze (968 m). The Rench Valley Railway starts at Appenweier and runs as far as Bad Griesbach. It has also contributed to the development of the thermal baths in the Rench valley, which had already been established in the 16th century in Bad Griesbach, but which soon extended to Bad Antogast near Oppenau (especially for the nobility), Bad Peterstal and Bad Freiersbach (today part of Bad Peterstal-Griesbach).
The "Rench Flood Canal" was built between 1936 and 1953 at the Acher Rench Correction to divert floodwaters. It branches off the Rench at Erlach, picks up the Acher Canal at Rheinau and merges with the actual Rench at Memprechtshofen.
