The source region of the Acher lies on the northeastern mountainside of the Vogelskopf (1,055.8 m above sea level (NN)), an area divided into cirques formed by the ice age.

From the point where the Unterwasser valley joins from the south, the Acher continues in a northwesterly direction, flowing through the municipality of Kappelrodeck, to the Upper Rhine Plain, which it reaches in Achern, the largest town of the northern Ortenaukreis.

Even in the Black Forest foothill zone above Oberachern, the Acherner Mühlbach was cut short by the Acher at a weir, which has an average discharge here of around 5 m³/s.

A section of the Rhine Plain Canal used to pass under it coming from the west, being joined by the water-rich Sulzbach, the lower reaches of the Sasbach and Laufbach as well as the Acherner Mühlbach.

[5] Under the name Oberes Achertal, a protected area was created by the county council of Ortenaukreis around the municipalities of Ottenhöfen im Schwarzwald and Seebach covering 3,600 hectares.