The Black Ernz (Schwaarz Iernz in Luxembourgish) is a tributary of the Sauer on the right bank and forms part of the river basin of the Rhine. It comprises two watercourses taking their source in Grünewald.
The Black Ernz weaves across Little Switzerland in the north east of Luxembourg and flows through Junglinster.
It flows into the Sûre north of the village of Grundhof, which lies close to Echternach.
The White Ernz weaves through the medieval village of Larochette and joins the Sauer at Reisdorf.